On High Energy Cosmic Rays from the CREAM Instrument. Dissertation

Transcription

1 On High Energy Cosmic Rays from the CREAM Instrument Dissertation Presented in Partial Fulfillment of the Requirements for the Degree Doctor of Philosophy in the Graduate School of The Ohio State University By Theresa J. Brandt, M.S., A.B. Graduate Program in Physics The Ohio State University 2009 Dissertation Committee: Prof. James J. Beatty, Adviser Prof. John F. Beacom Prof. Richard J. Furnstahl Prof. Brian Winer

2 c Copyright by Theresa J. Brandt 2009

3 Abstract The Cosmic Ray Energetics And Mass (CREAM) experiment is a balloon-borne, high energy particle detector designed to measure cosmic ray nuclei from protons through Iron at energies up to ev. It has succeeded in measuring this broad range of charge and energy through multiple Antarctic flights, data from the first of which will be presented here, using complementary charge and energy detectors. These included a Timing Charge Detector (TCD), a Transition Radiation Detector (TRD), a Silicon Charge Detector (SCD), and a Calorimeter. The TRD and Calorimeter provide both tracking and an energy determination. The TCD and SCD provide excellent charge resolution, of order 0.2 e. Together, these have enabled us to construct absolute spectra for individual primary nuclei, Carbon, Oxygen, Neon, Magnesium, Silicon, and Iron, as well as the less abundant secondary, Nitrogen. Our spectra agree well with previous measurements, and for several nuclei extend to the highest energies yet measured. The well-resolved charge species have also permitted us to form the secondary to primary ratios of Boron to Carbon and Nitrogen to Oxygen, also up to the highest energies measured and in agreement with previous data. Since charged particles like cosmic rays bend in magnetic fields which permeate our galaxy, traditional pointing astronomy is not possible. Instead, we use the spectra and ratios to provide us with clues to cosmic rays origins, acceleration mechanism, and propagation history. In particular, the CREAM I Boron to Carbon ratio fits a propagation model with index of δ = while the CREAM II primary nuclei spectra all have an index of 2.66 ± This last suggests that they all have the same acceleration mechanism, and after accounting for propagation energy loss consistent with the Boron to Carbon ratio, that the mechanism is likely Fermi first order acceleration. Finally, Nitrogen serves as a particularly useful test bed for these findings. Its ratio with Oxygen is consistent with a small amount of Nitrogen existing in the cosmic ray source, 10% with respect to the source s Oxygen content, given propagation conditions again based on the Boron to Carbon ratio. At the highest energies, this source flux is seen, as expected, to emerge over the secondary flux in the Nitrogen spectrum itself. ii

4 To my mother, who taught me to think scientifically. iii

5 Acknowledgements This work would not have been possible without the support of many people, not least of whom is James Beatty, my advisor, who encouraged the diversity of my research and my travels to present it. Thanks to Nick Conklin, Isaac Mognet, and Taylor Childers, who patiently shared their knowledge of CREAM, programming, cable-making, and other assorted tidbits with me, and to Scott Nutter, particularly for the conversations on interaction fractions and the skiing and recovery fellowship. Simon Swordy, Scott Wakely, Michel Buénerd, and the other members of the CREAM collaboration were helpful and supportive throughout this process. I appreciate the willingness of John Beacom, Richard Furnstahl, and Brian Winer to serve on my committee. Without the support and suggestions of Debra Elmegreen and Fred Chromey my first year at Vassar and my summer s research with Charles Foster at IUCF, I may well have not pursued research. My years at OSU have further been enriched by my fellow grad students friendship, from the years in the stacks over problem sets to the time over beers. Thanks are also due for the assistance and friendship of my fellow group members, not limited to Kimberly J. Palladino, Matthias Leuthold, Ryan Nichol, Patrick Allison, Chad Morris, Michael Sutherland, Brian Baughman, and Eric Grashorn, and to Kyler Kuehn who gently held me to deadlines. I am also grateful for the shelter he, Rebecca Stigge, Chris Bernard, and my family have shared with me for the last few months of the writing of this document. Thanks are due to Robin Post and Kim Palladino, Krzysiek and Mariah Sakrejda- Leavitt, Laura and Sirius Fuller-Usher, Amalia Dray, and my other Vassar and adopted-vassar friends for the long conversations on any subject in the universe. To Betty Cremmins, not least for her advice: Sprint! It s the end of your marathon. Thank you. I know how to work through that one! And to all the Ice Crew, who made such a memorable season, thanks. Thanks to Emily Warmann and Amber Hanna for the reliably awesome ultimate and friendship, and to Nathan Salwen, iv

6 who believed in my potential and helped me realize it. To Lauren MacDade and Luke DeGroote for sharing birds and walks: I will never forget holding the ruby-throated hummingbird and thinking of string theory! To Jessica Miesel and Jane and Jotham Tausig: this would not have happened without your care and friendship. To everyone mentioned here, and anyone I have inadvertently forgotten, Thank you. v

7 Vita Atholton High School A.B. Physics, Vassar College Physics Graduate Teaching Assistant and Graduate Webpage Development, The Ohio State University 2005-present...Physics Graduate Research Assistant, The Ohio State University M.S. Physics, The Ohio State University Publications Ahn, H. S. et al. Energy Spectra of Cosmic Ray Nuclei at High Energies. Accepted for publication by ApJ, Oct Ahn, H. S. et al. Measurements of Cosmic-Ray Secondary Nuclei at High Energies with the First Flight of the CREAM Balloon-Borne Experiment. Astropart. Phys. 30: 133, Seo, E. S. et al. Approaching the Spectral Knee in High Energy Cosmic Rays with CREAM. J. Phys. Soc. Japan A. 78: 63-67, Wakely, S. P. et al. First Measurements of Cosmic-Ray Nuclei at High Energy with CREAM. Adv. Space Res. 42: 403, Brandt, T. J. et al. Charge Identification in the CREAM Experiment. Proc. Int. Cosmic Ray Conf. 2: 337, vi

8 Brandt, T. J. et al. The CREAM Experiment: Towards Primary-to-Secondary Ratios. Proc. Am. Phys. Soc. B11.7, Fields of Study Major Field: Physics Experimental Particle Astrophysics with a focus on Cosmic Ray Astrophysics vii

9 Table of Contents Page Abstract Dedication Acknowledgments Vita List of Tables List of Figures ii iii iv vi xii xiii Chapters: 1. Introduction 1 2. Cosmic Rays in the Galaxy Cosmic Rays: the Basics Cosmic Rays Origins Abundances Cosmic Ray Propagation The Boron to Carbon Ratio Energy Dependence Residence Times and Volumes Observations and Models Measuring Cosmic Rays: The CREAM Instrument Introduction to the CREAM Flights Flight Constraints viii

10 3.1.2 Albedo The Timing Charge Detector Detector Material Photomultiplier Tubes TCD Electronics Initial Verification TCD Improvements The Transition Radiation Detector Detector Physics Flight Constraints Calibration The Cherenkov Detector Detector Material and the Cherenkov Response CD Electronics The Cherenkov Camera Detector Materials Validation The Silicon Charge Detector Detector Material Inherent Noise Validation CREAM II and beyond: the Dual-layer SCD Hodoscopes and S The Calorimeter Detector Materials Calibration and Validation Beyond CREAM I Calorimeter Trigger Master Trigger CREAM on Ice Integrated TCD and CD Ground Test Initial TCD and CD Tuning Flight Recovery ix

11 4. Analyses of CREAM Cosmic Ray Data Charge Determination Tracking: CREAM I TRD Charge Identification: TCD Tracking: CREAM I, II Calorimeter Charge Identification: SCD Energy Calibration The Cherenkov Detector The TRD The Calorimeter Energy Cross-Calibration Simultaneous Charge and Velocity Determination First Analysis Second Analysis Final Analysis Spallation in the CREAM detector Charge Comparison Interaction Fraction Systematic Error Calculation Comparison to Monte Carlo Discussion of Selected CREAM Results Primary Nuclei Absolute Spectra Corrections to the Top of the Instrument and Atmosphere Spectral Derivation Spectral Indices Discussion of Sources of Error Secondary to Primary Ratios Charge, Tracking, and Energy Improvements Top of the Instrument and Atmosphere Corrections Ratio Derivation Boron to Carbon Systematic Errors Nitrogen to Oxygen The Nitrogen Spectrum x

12 6. Conclusion 156 Bibliography 161 Appendices: A. CREAM III TCD and CD Flight Documentation 168 xi

13 List of Tables Table Page 3.1 Power converter map Map of PMT taps to electronics and detected charges TCD initial flight threshold settings S3 initial flight threshold settings TCD and S3 flight HV settings Initial CD flight settings SCD Charge Resolution Interaction fraction Monte Carlo-determined Interaction Fraction A.1 TCD/CD LPT A.2 TCD/CD CPT A.3 LOS TCD, CD and S3 tuning A.4 TCD, CD and S3 fine tuning xii

14 List of Figures Figure Page 2.1 All-particle cosmic ray spectrum High energy all-particle cosmic ray spectrum First order Fermi acceleration Relative Abundances of cosmic rays Propagation of cosmic rays Lower energy Boron to Carbon ratio High energy Boron to Carbon ratio CREAM I Instrument Schematic Timing Charge Detector diagram PMT voltage divider circuit PMT peak circuit diagram PMT anode and dynode pulse traces PMT timing circuit diagram TCD trigger logic TCD electronics box TCD charge spectrum from CERN Indium beam fragments TRD simulated response agreement with CERN beam test Representative Cherenkov response versus incident particle energy Schematic diagram of the Cherenkov Camera CherCam aerogel radiator and imaging (PMT) plane CherCam signal: simulated and beam test Diagram of the SCD PIN Diode Photographs of the assembled SCD Dual-layer SCD xiii

15 3.18 Photograph of the Hodoscope The CREAM II Calorimeter and associated read out materials Calorimeter response to increasingly energetic Indium beam fragments CREAM III during launch Pre-launch LPT muon data: TCD ADC Pre-launch LPT muon data: TCD ADC Pre-launch LPT muon data: S3 ADCs Pre-launch LPT muon data: Sum of CD ADCs Tuned, CREAM I TCD ADC1 values Tuned, CREAM I TCD ADC1 versus the CD ADC sum CREAM I flight path CREAM I altitude CREAM III after landing CREAM III during recovery TCD charge reconstructed with TRD tracking SCD charge (B - Si) reconstructed using calorimeter tracking SCD charge (S - Ni) reconstructed using calorimeter tracking Flattened Cherenkov Detector response to Oxygen Effect of Finite Energy Resolution on a Falling Spectrum Calorimeter energy deposition agreement between flight and MC data TRD, CD, and Calorimeter energy cross-calibration TCD scintillator peak detector signal versus the CD signal Charge versus β determined from the approximate Bethe-Bloch eq Theoretical scintillator to Cherenkov signal ratio versus β Charge and velocity determined from the exact Bethe-Bloch equation Monte Carlo simulation of CD response including δ-rays Uniformity of TRD tracks projected to the TCD and SCD Charge measured with the SCD and TCD SCD charge for TCD-identified Carbon events SCD charge for TCD-identified Oxygen events Verification of random systematic error Systematic error determination for survival probabilities Measured and Monte Carlo-estimated survival probabilities agree Measured and MC-estimated interaction fractions agreement xiv

16 5.1 Top of the instrument corrections Top of the atmosphere corrections CREAM II primary nuclei spectra Spectral indices for CREAM II primary nuclei Final charge distribution for secondary to primary ratios B:C Ratio N:O Ratio The Nitrogen Spectrum xv

17 Chapter 1 Introduction This document explores the evidence for the origins and acceleration of galactic cosmic rays (CRs) as well as their indications of propagation conditions through the galaxy. In Chapter 2 we review the current case for CRs originating in stellar material and undergoing first order Fermi acceleration in supernovae and their remnants. We also discuss methods for parameterizing CR propagation and extracting information thereon from secondary to primary ratios, notably the Boron to Carbon ratio. The Cosmic Ray Energetics And Mass or CREAM experiment was designed specifically to allow us to gather further information to aid in solving the mysteries of CR origins, acceleration, and propagation. Chapter 3 details the detectors and their physics which enable us to measure the charge and energy of CRs during several Long Duration Balloon (LDB) flights of the instrument around Antarctica. The author has been involved with CREAM hardware since joining before the second flight, including detector preparation, integration, and verification as well as tuning and monitoring during flight, through the latest incarnation of CREAM (V). During the third CREAM flight, the author was the local (on-ice) lead responsible for the Timing Charge and Cherenkov Detectors, two of the five detectors aboard the instrument, and assisted with the instrument recovery following landing. She also participated 1

18 in the implementation of improvements to those detectors electronics, making them more robust to unusual failure modes. Chapter 4 reviews the general analysis procedures for producing accurate charge and energy measurements from the CREAM detectors as well as two analyses in which the author took particular part: a simultaneous charge and energy determination and a study of spallation within the CREAM I instrument (Sections 4.3 and 4.4, respectively). The latter is of particular note in light of the lack of data on high energy nuclear interaction cross-sections. The author also contributed substantially to a timing analysis found in [1] and participated in the development of the collaboration s general analysis procedures. With well-resolved charge and energy data from the CREAM I and II flights, we construct the primary CR nuclei spectra and primary to secondary ratios which permit us to both confirm and expand our understanding of the CR origins, acceleration mechanisms, and propagation introduced in Chapter 2. The author participated in the collaboration s interpretation of these results, covered in Chapter 5. The CREAM data covers a broad range of energies, consistent with previous low-error measurements, and extends to some of the highest energies yet measured. This is particularly true for the Nitrogen spectrum, which permits an excellent case study for combining the average primary spectral index gleaned from CREAM II spectra and the propagation index determined via the CREAM I Boron to Carbon ratio with the Nitrogen to Oxygen ratio information. In the final chapter, we summarize our findings and discuss future directions for CREAM, which continues to gather data in nearly yearly flights, and our understanding of CRs. 2

19 Chapter 2 Cosmic Rays in the Galaxy In 1912 Victor Hess made a discovery which opened the field of high energy particle physics long before the first large-scale accelerators. He found that, contrary to popular notions, the charged particles seen in the earliest detectors did not mainly come from radioactive decays in the earth, but rather appeared to be bombarding the planet from outer space. Since then scientists have worked to understand the sources and propagation of these cosmic rays (CRs) by measuring their composition and energy spectrum at various locations around the earth and extending into interplanetary space [2]. 2.1 Cosmic Rays: the Basics Protons comprise the majority of all cosmic ray nuclei (90%); ionized Helium accounts for 9% and all heavier ionized nuclei the remaining fraction [3]. Electron plus positron flux is approximately 1% of the protons, while the positron flux is about 1% of the electron s [4]. Antiproton flux is four orders of magnitude below the proton s around 10 20GeV [4]. No evidence of heavier anti-nuclei has yet been found, though experiments such as PAMELA [5] and BESS [6] are still investigating this. 3

20 Since cosmic rays are charged particles, bending in magnetic fields (galactic, solar, and geo-) between their source and observation location isotropizes the flux. This prevents the directional measure of any given cosmic ray from indicating its source. As interactions during propagation further complicate traditional astronomical sourcefinding techniques, the energy spectrum has become a primary investigative tool. Figure 2.1 shows the all-particle differential flux spectrum over twelve orders of magnitude in energy along with the main features as seen by various detectors. Above 10 20GeV/nucleon, the spectrum follows a power law: df de E α (2.1) with a differential spectral index α of about 2.7 for E ev, below the so-called knee, and about 3 for E ev, between the knee and the ankle. Below the knee cosmic rays are numerous enough ( 1 particle per m 2 -yr) to allow direct detection by sensitive balloon-borne experiments like CREAM; above the knee low fluxes of 1 particle per km 2 -yr, require a larger instrumented area, so indirect, ground-based techniques become most effective (e.g. [12]). The dearth of direct experiments in the CREAM energy range ( ev) is obvious. Even selected for quality, the fairly substantial spread in indirect measurements above the knee remains. Figure 2.2, the high energy all-particle spectrum from indirect experiments multiplied by E 2.7, shows this spread, most likely caused by systematic errors, even more clearly. By extending direct measurements up to ev, into the domain of indirect detection, CREAM will help clarify the present welter of indirect measurements. 4

21 T e v a t r o n L H C Figure 2.1: The all-particle cosmic ray differential flux spectrum spanning over 12 orders of magnitude in energy with data selected for quality by [7]. The knee and ankle indicate changes in the spectral slope and conveniently demonstrate the rapidly decreasing cosmic ray flux which necessitates indirect, ground-based measurements above the knee. Below it, balloon-borne experiments such as CREAM [8], ATIC [9], and BESS [6] measure fluxes in the indicated energy range. Arrows locate the energies attainable by the highest energy accelerators on Earth: the currently operating Tevatron [10] and the LHC, coming online at the end of 2009 [11] (in the reference frame of a relativistic particle incident on a stationary target). Cosmic rays remain the highest energy particle beams. 5

22 10 5 Knee E 2.7 F(E) [GeV 1.7 m 2 s 1 sr 1 ] Grigorov JACEE MGU TienShan Tibet07 Akeno CASA/MIA Hegra Flys Eye Agasa HiRes1 HiRes2 Auger SD Auger hybrid Kascade 2nd Knee Ankle E [ev] Figure 2.2: The high energy, all-particle cosmic ray spectrum multiplied by E 2.7 for indirect experiments, demonstrating the rather substantial spread in the data. The grey band shows the reach of direct detection experiments as of [4] Cosmic Rays Origins Power Energy arguments strongly suggest that supernovae may be responsible for the majority of cosmic rays accelerated up to about ev (the knee). Taking the local cosmic ray energy density, ρ E 1eV/cm 3, as typical of the galaxy, the galactic cosmic ray luminosity is L CR = V Dρ E τ R erg sec (2.2) where the disc volume V D is about cm 3 and the residence time τ R in the accelerating region is about yrs [3]. As early as 1964, Ginzburg and Syrovatskii 6

23 recognized that a type II supernova ejecting 10M at speeds u cm/s every 30years produces a luminosity of about erg/s [13]. Thus, they would only need to be a few percent efficient to supply the observed cosmic rays [13]. More recently Berezhko et. al. suggested smaller mass ( 1.4M ) supernovae with slightly higher energy output (E SN erg) could accelerate cosmic rays, based on a comparison of data with a hot reacceleration model (see also Section 2.2) [14]. Multiwavelength observations of potential cosmic ray sources, mainly in X- and γ-rays from energetic electrons (and described further in Section 2.1.3), imply the necessary efficient acceleration mechanisms (e.g. [15]). Fermi Acceleration Observations point to Fermi acceleration, most likely of the first order, characterizing supernova shocking. Charged particles in such a model diffuse in a turbulent, (approximately) planar magnetic field, resulting in a net acceleration [3]. In Fermi acceleration, whether of first or second order, the fractional energy gain per encounter with the shock, E = ηe 0, naturally produces the power law spectrum observed in cosmic rays. Following Gaisser, the number of particles left after n encounters with an escape probability P esc is N( E) (1 P esc ) m = (1 P esc) n (2.3) P esc m=n [3]. A particle injected with energy E 0 will, after n encounters, gain energy E n = E 0 (1 + η) n (2.4) 7

24 [3]. Thus, combining Equations 2.3 and 2.4, the number of particles left with energy more than E N(> E) 1 ( ) γ E where γ = ln(1 P esc) P esc E 0 ln(1 + η) P esc η (2.5) and γ is the spectral index [3]. A higher escape probability leads to a steeper spectrum; a larger energy gain per encounter flattens the spectrum. Figure 2.3: (First order) Fermi shock acceleration across a planar front moving with velocity u 1. The particle enters with energy E 1 and angle θ 1 with respect to the shock velocity and exits with energy E 2 and angle θ 2. One encounter is defined as one trip back and forth, returning to the original side. [3] To find the fractional energy gain, we trace the particle through its journey across the shock and back, defined to be one encounter, as illustrated in Figure 2.3. The planar shock travels with velocity u 1 while the incident particle receives a kick in the opposite direction and of magnitude u 2 < u 1. The kick derives from collisionless scatterings off irregularities in the magnetic field, which is assumed to be perpendicular to the shock front (parallel to u 1 ) and carried in the shock plasma 8

25 [3]. Thus, having crossed the shock once, the particle is seen downstream in the lab frame to have a velocity V = u 1 +u 2. One may then define the Lorentz factor and velocity of the shock (downstream) as γ and β = V and assume relativistic particles c such that the mass may be neglected (E pc) [3]. The particle crosses the shock front at some angle θ relative to the shock velocity. In the downstream (moving) frame, the particle appears to enter the shock with energy E 1 = γe 1(1 β cosθ 1 ) (2.6) and leave it with energy E 2 [3]. Transforming the outgoing particle energy back into the lab (rest) frame yields: E 2 = γe 2 (1 + β cosθ 2) (2.7) [3]. For elastic scatterings off the magnetic field irregularities, E 1 = E 2 so E 2 = γe 1 (1 + β cosθ 2) = γ 2 E 1 (1 β cosθ 1 )(1 + β cos θ 2 ). (2.8) Defining the fractional energy gain, η = E E 1 = E 2 E 1 E 1 = E 2 E 1 1 gives η = γ 2 (1 β cosθ 1 )(1 + β cosθ 2 ) 1 (2.9) recalling that γ β 2. Particles may enter and leave the shock at any angle so the net result of many crossings leads to an average over both incoming and outgoing angles. In both the downstream-going and upstream-going crossings, the isotropic flux appears projected onto the planar shock such that the number crossing the shock for a given d cosθ 9

26 is dn d cos θ = 2 cosθ; the 2 comes from normalizing. Recalling that the average of a quantity x is x = x dx, the average angle is cosθ = 2 B A (cosθ) 2 d cosθ = 2 3[ cosθ ] A B (2.10) where 1 cosθ 1 0 gives 2 3 for the entrance angle and 0 cosθ 2 1 gives 2 3 for the outgoing angle. Thus, we have η = for non-relativistic shocks where β = u 1 u 2 c 1. ( ) 1 [ β 2 3 β 4 9 β2 ] 1 4 β. (2.11) 3 In this first order Fermi acceleration, the fractional energy gain is first order in the shock plasma velocity β. The angular averages for second order Fermi acceleration cosθ 1 = 1 3 β and cosθ 2 = 0 arise from the physical situation of a particle entering and leaving a magnetic field-carrying plasma cloud and yield a non-relativistic fractional energy gain of order 4 3 β2. A particle undergoing second order Fermi acceleration, unlike first order, can gain or lose energy in a given crossing, but has a net gain when averaged over many crossings. Proceeding with first order Fermi acceleration allows completion of the estimate for the spectral index by determining the escape probability (see Equation 2.5), by comparing the relative rates of particles entering and leaving (for good) the shock. For an isotropic population ρ CR of relativistic cosmic rays, the rate of encounters, again projected onto the shock plane, is: 1 cos θ=0 2π φ=0 cρ CR 4π cosθ d cosθ dφ = cρ CR 4 (2.12) [3]. These particles convect away from the shock in the downstream region with a rate of ρ CR u 2, giving an escape probability of P esc = 4u 2 c, where u 2 is again the 10

27 downstream (convective) velocity. Thus, the spectral index from Equation 2.5 with this escape probability becomes γ 1 4u 2 4 β c 3 = 3 u 1 u M 2. (2.13) The last form, written in terms of the mach number M u 1 c 1, where the shock speed u 1 must exceed c 1, the speed of sound in the gas for a shock to actually form, gives the classic (differential) spectral index, assuming a monatomic gas and continuous mass flow across the shock [3]. For strong shocks, M 1 and γ 1 plus a little bit, as observed (in more detail in Section 2.1.3). A similar analysis for second order Fermi acceleration using the galactic disk as the acceleration region containing the magnetic-field-carrying clouds has P esc = T cycle T esc, where the typical escape time is of order 10 7 yrs and the cycle time is the inverse of the relativistic cosmic ray s velocity convolved with the clouds spatial density and cross-section [3], giving: γ 10 7 yrs 1 4. (2.14) 3 β2 cρ cloud σ cloud Thus, second order Fermi acceleration is disfavored because the spectral index depends in a complicated way on clouds properties and tends to be larger than observed (> 10 for typical cloud parameters) [16], [3]. Maximum Energy Acceleration mechanisms with a finite lifetime T max will have an associate maximum energy per particle. The timescale for a shock is generally considered to be the time it takes the shock to sweep through an amount of matter equal to its own mass. A spherically expanding shell will travel a radial distance l through the intervening 11

28 material (the interstellar material, for example) in this time: π 2π l θ=0 φ=0 r=0 ρ ISM r 2 sin θ dr dθ dφ = 4 3 π ρ ISM l 3 = M SN = mm (2.15) Solving this for l and a shock moving with speed u, the maximum acceleration time is simply T max = l u m1/ yrs (2.16) where the fractional solar mass m for supernovae may be anywhere from 1.4 to more than 10, for a lifetime of order one thousand years. Models by Berezhko et. al. suggest smaller type Ia supernovae accelerate the majority of cosmic rays at energies around the knee early in the supernova shock s evolution (see also Section 2.2) [14]. A particle will make at most n max = T cycle /T max cycles through the acceleration region, gaining an amount of energy given by Equation 2.4 if the cycle time is independent of energy. To estimate the cycle time, we first examine diffusion and convection in the upstream region. For a diffusion coefficient D 1 with units m 2 /sec and convection velocity [u 1 ] = m/sec, the number of particles per unit area is ρ CR D 1 u 1. The average time spent in the upstream region is found by dividing this by the number of particles leaving the region per unit area and time, as found earlier for the number of encounters with the shock (Equation 2.12). The equivalent analysis holds for the downstream region, so the total time for one cycle is T cycle = 4 c ( D1 + D ) 2 u 1 u 2 (2.17) One may approximate the diffusion coefficient as D = 1 3 λ Dv, where v is the particle velocity and λ D the characteristic diffusion length [3]. For a diffusion mechanism of scattering off magnetic field irregularities, the particle s trajectory curvature is limited by its Larmor radius, r L = pc, which one may then take as a lower limit for the ZeB 12

29 diffusion length [17]. Thus, for a relativistic particle, D min = 1 3 Ec ZeB, and the minimum cycle time becomes T cycle = 4 ( E ) = 4 E 5 (2.18) 3 ZeB u 1 u 2 3 ZeB u 1 for a strong shock with u 1 = 4u 2. (Note that fixing this does not violate the earlier non-relativistic shock assumption, which depends on the order ( u c) 1.) Since the cycle time does depend on the particle s energy, we use for the fractional energy gain per time de dt = ηe 3 T cycle 20 ZeBu2 1 c (2.19) rather than Equation 2.4 and employ the first order, non-relativistic approximation for strong shocks for the energy gain η (Equation 2.11) [3]. Integrating this to obtain the maximum energy demonstrates the importance of the energy cancellation in Equation 2.19 with cycle time s energy. E max = 3 20 ZeBu2 1 c T max Z ev (2.20) for a magnetic field of similar order to the galactic one ( 3 µg) [17], [3]. Relaxing some assumptions can allow an increase in E max (e.g. [18]). More recent estimates raise this to E max Z ev or higher if they include other (re)acceleration mechanisms [19], [20], [14]. As suggested by Figure 2.1, the CREAM experiment is perfectly situated to test this maximum energy charge dependence Abundances While recent observations improving the understanding of other astrophysical high-energy emitters suggest that they may contribute a portion of cosmic rays (e.g. [21]), any potential acceleration mechanism (e.g. [22], [23]) should be associated with 13

30 a stellar-like source of material. Figure 2.4 illustrates this by comparing the cosmic ray abundances to those in our solar system, normalized to silicon Z = 14. Figure 2.4: The absolute flux of cosmic rays by elemental charge at E 1 TeV/nucleus. Boxes represent the measured solar system abundances; the two are normalized to silicon (Z = 14) [21]. The even-odd effect, where evenly charged elements are more abundant relative to their odd counterparts, arises in the solar system from stable (end) products of stellar nucleosynthesis. The Pauli exclusion principle under the nuclear shell model allows evenly charged nuclei to fully fill all the available energy levels with protons pairs (and separately, neutrons) of opposite spin, analogously to orbital electron shell filling. This results in a more strongly bound, and thus more stable, nucleus than one with a half-filled valence shell. The even-odd effect resulting from varying stability 14

31 is seen in a variety of energy ranges, including 1TeV as in Figure 2.4. That cosmic rays exhibit this general trend is a strong indication of stellar origin. The relative underabundance of some common stellar nucleosynthesis end products with respect to the solar system s, notably Hydrogen, Helium, and the Carbon, Nitrogen, and Oxygen group, could be due to their high first ionization potential or from their high volatility, such that they do not condense (as easily) on interstellar grains [24]. The latter has been supported with recent evidence from the TIGER heavy cosmic ray detector, whereby the refractory elements, having condensed into grains which experince more numerious shock crossings, sputter off the grains at higher energies, which are then preferentially accelerated to still higher energies than their volatile counterparts (e.g. Equation 2.18), and thus appear more populous in the TIGER data (relative to an 80%/20% solar system/integrated massive star outflow mixture) [25], [26]. It will be useful to see if this trend carries to lower charges and higher energies; TIGER results covered 26 Z 38 and included HEAO-3-C2 results down to Nitrogen at an energy around 2.5GeV/n. The other main feature apparent in Figure 2.4, the overabundance of elements such as the Lithium, Beryllium, Boron group, Fluorine, and the Scandium, Titanium, and Vanadium group, which are uncommon stellar nucleosynthesis products, comes from spallation of primary cosmic rays during their propagation through the galaxy Cosmic Ray Propagation Diffusion Cosmic rays undergo some fairly substantial travels and tribulations before arriving at our detectors. Figure 2.5 depicts the possible interactions cosmic rays (labeled p, He, CNO) might have after their release from their acceleration source. For 15

32 Figure 2.5: Pictograph of cosmic ray (labeled p, He, CNO ) propagation in the galaxy, from source to observation instrument [27]. Cosmic rays undergo spallation in the gas, producing secondaries, notably the LiBeB group. See Section for further details. energies below the knee, cosmic rays wander around the galaxy, contained and diffused (through a random walk-type process) by the galactic magnetic field. Density changes with changing temperature at the galactic boundary allow convective escape into intergalactic space, as shown in the inset figure. Primaries, those cosmic rays accelerated at the source such as Carbon nuclei, interact with the interstellar gas (ISG), and produce secondaries such as Boron nuclei. This process, referred to as spallation, approximately conserves the energy per nucleon. 16

33 The transport equation for the number density of a given particle i at a given energy and point in spacetime N i (E, x, t), describes these processes mathematically: N t = Q i(e, x, t) + (D i N i ) u N i E [b i(e)n i (E)] p i N i + vρ dσi,k (E, E ) N k (E ) de (2.21) m de k i [13], [3], [28]. The first term (Q i ) denotes the source, allowing evolution in spacetime and energy. The second term describes the diffusion of particles through the galaxy with diffusion coefficient D i (see also Section 2.1.1); convection with velocity u also occurs. b i (E) de dt describes energy losses or gains such as through ionization or acceleration, respectively while p i represents the loss of nuclei from collision or decay. For particles travelling with a speed v in a medium of density ρ and collisional crosssection σ i, nuclei loss may be expanded as p i = vρσ i m + 1 γτ i = v λ i + 1 γτ i, (2.22) where the last term in both equations is the Lorentz-dilated lifetime and λ i the interaction length (assuming the intervening material is all of mass m, typically hydrogen). The last term describes the cascade of particles for all nuclei k producing i in the medium; primes indicated k s quantities. The most basic leaky box model assumes that diffusion is a constant in spacetime equal to N τ esc and sources only exist as Q i (E, t) = N 0 (E)δ(t) [3]. Integrating this transport equation gives N(E, t) exp[ t τ esc ]. (2.23) Thus τ esc appears as the mean time cosmic rays spend in the galaxy and λ esc ρβcτ esc may be understood as the escape length for particles moving with a Lorentz velocity β in a medium of density ρ. 17

34 If for primaries we instead require equilibrium (such that N t = 0) and allow sources with a constant output in time (Q i (E, t) = Q i (E)), leaky diffusion as before, and interaction losses dominating all remaining terms in the transport equation, the observed particle flux at a given energy is N p (E) = Q p(e)τ esc 1 + λesc λ p Q p (E)τ esc. (2.24) The last approximation holds for escape lengths shorter than the primary s interaction length. Given the quantities energy dependencies, one may equate their spectral indices: for an escape time proportional to E δ and δ = 0.6 (see Section 2.2), a source energy dependence of E (γ+1) (see Equation 2.3), and an observed spectral index of α 2.7 (Figure 2.1 and Equation 2.2), α = (γ +1)+δ. Thus, γ = 1.1 as expected from Section for first order Fermi acceleration in strong, non-relativistic shocks. While the leaky box model has been sufficient for describing cosmic ray propagation over a limited energy range below the knee, as experiments obtain improved particle fluxes over an increasing energy range, the diffusion model has proven more accurate (for instance, [14], [19], [24], [28], [29], and [30]). It allows a spatial density gradient and defines the escape time τ H H2 D in terms of a characteristic halo height H and the diffusion coefficient [D] = m2 s. This implies D E δ for an escape length proportional to the (halo) escape time. As indicated in Figure 2.5, accelerators of nuclei will most likely also accelerate electrons and positrons, and a small fraction of the spallation secondaries will be (charged) pions which decay to electrons and positrons. Neutral pions from spallation decay to a pair of γ-rays. The electrons and positrons may interact with a magnetic field, producing synchrotron X-rays to be measured by, e.g. Chandra [31]. They may 18

35 also produce γ-rays via Inverse Compton scattering off of the interstellar radiation field (ISRF) or bremsstrahlung off of gas in the galaxy. Thus, models such as Galprop [29], have had some success in evaluating various acceleration and interaction schemes by integrating multiwavelength observations with associated sources. Long-running experiments such as Chandra have expanded our understanding of possible sources, for instance through the study of the youngest discovered galactic SNR [32]. New experiments such as the Fermi Gamma-Ray Space Telescope (labeled GLAST in the diagram) are also adding to our understanding of possible sources through studies of supernovae remnants like W51C [33], and of propagation by measuring the diffuse γ-ray flux. The latter was found to be consistent with spectra expected from local cosmic ray fluxes for energies of 100MeV - 10GeV [34]. Likewise, observing the nearby Large Magellanic Cloud, a galaxy similar to our own, in γ-rays is shedding light on emission features, including from propagation of cosmic rays in such galaxies, e.g. [35]. The Knee A phenomenological fitting of data covering the knee [24] strongly suggests the change in the differential flux spectral index, as seen in Figure 2.1, is best described by a rigidity dependent cutoff R pc Ze (2.25) where p is the particle s momentum, Z its charge in units of the electron charge e, and c is the speed of light. R then has units of GeV e GV. This holds for the source-dependent cutoff described in Section and for a rigidity-dependent escape length, effectively increasing δ with increasing energy (and charge). Though the large 19

36 and increasing (with increasing energy) anisotropy suggested by a rigidity-dependent escape length is not observed (cosmic rays appear isotropic to an order 10 4 near knee energies), the escape time energy proportionality (τ esc E δ ) also breaks down: the associated escape length is of order the disk depth ( 300pc) [19]. Equivalently, the escape time is of order one cycle through the disk (the acceleration region). (See also Section 2.2.) Clearly, a better understanding of δ at higher energies, such as those accessible to CREAM, will help resolve this issue. 2.2 The Boron to Carbon Ratio Separating propagation conditions from source considerations will help to more completely understand galactic conditions. As suggested by Equation 2.24 and the discussion thereof, a given propagation energy dependence helps determine the source energy dependence (γ). Secondary to primary ratios, in particular Boron relative to Carbon, are useful tools for this task Energy Dependence The secondary flux represents propagation conditions for a given source spectrum. With a few basic assumptions, the primary flux may be used to represent the source spectrum, so their ratio reduces the source dependence, particularly for a secondary derived directly from a given primary. One can construct a simplified form for the secondary to primary ratio s energy dependence by returning to the transport equation (2.21). For the primary, acceleration sources may be considered to be the main source and diffusion the main transport mechanism, with losses negligible. Spallation from the primary (or from heavier primaries with the same energy dependence) with the same diffusion defines the secondary s flux. Thus, the secondary to primary flux 20

37 ratio becomes N s N p Γ psn p Q p E (γ+1) δ E (γ+1) = E δ (2.26) for secondaries and primaries in equilibrium and the spallation cross-section Γ ps (approximately) independent of energy. Carbon, a relatively abundant (as shown in Figure 2.4), stable, common endproduct of stellar processes, is an ideal primary for this purpose. Likewise, Boron makes a good secondary as it is stable but rarely synthesized in stellar environments, its big bang nucleosynthesis abundance is low and relatively well known, and it is both a direct spallation product of Carbon and has similar properties. The last helps reduce experimental systematic errors. Figure 2.6 shows the Boron to Carbon ratio for energies 10 8 to ev/nucleon. Above 10 20GeV/nucleon, the higher energy data fit a power law in energy: E δ with δ 0.6. The curves suggest solar modulation in a diffusion model provides a better fit to the low-energy data [36], and that the more complete diffusion model fits the data, so our approximations are reasonable. The Boron to Carbon ratio demonstrates that cosmic rays are generally accelerated before their propagation occurs. If the two processes were approximately simultaneous, secondaries would be made and escape at a constant ratio to their primaries for all energies up to the cutoff. Should secondaries be created before acceleration, the ratio would be at least constant and possibly even increasing with energy. Instead, we see an abundance of Boron relative to its primary which decreases as Carbon escapes from the galaxy before interacting at the highest energies. Such behavior strongly suggests a point source origin for cosmic rays such as supernovae. 21

38 Figure 2.6: The Boron to Carbon ratio as measured by various experiments. The curves represent diffusion models with reacceleration and two levels of solar modulation [36]. See Section for more details Residence Times and Volumes The decreasing secondary-to-primary ratio with increasing energy may also be translated into a decrease in the effective grammage (or containment lifetime) seen by the high-energy primaries on their journey through the galaxy. Around a GeV/n, secondary to primary ratios indicate cosmic rays traverse an average grammage of 5 10g/cm 2 equivalent Hydrogen, three to four orders of magnitude greater than the galactic disk thickness [3], [20]. Since the majority of astrophysical objects reside in the disk, it is reasonable to assume that cosmic ray accelerators also live in the disk (see also Section 2.1.2). Thus, the ratio indicates that cosmic rays diffuse through the galaxy for a while. If we assume a disk density of order 1 proton per cm 3, following 22

39 the escape length defined by Equation 2.23, (relativistic) cosmic rays reside in the galaxy for approximately yrs. By examining other ratios in a similar manner to the Boron-to-Carbon one (Figure 2.6), we can, for instance, determine the approximate propagation volume. The ratio of such spallation secondaries as β-decay unstable 10 Be to stable 9 Be, which have welldefined relative production cross-sections, gives a direct measure of the propagation time. The observed decrease in the unstable relative to stable element flux indicates decay and thus propagation times (at least) of order the unstable element s halflife. 10 Be, with a half-life of years, is an ideal element in comparison with the disk residence time. Various analyses of Beryllium, Aluminium, Chlorine, and similar isotopes show the galactic (ordinary, not dark matter) halo size consistent with 5 10kpc (versus a disk thickness of order 300pc) (e.g. [27], [37], [38]). For a diffusion coefficient of order cm 2 /sec, the halo residence time becomes 10 8 yrs (see Section 2.1.3) Observations and Models Figure 2.7 shows models that fit the observed Boron to Carbon ratio for energies from 10 8 to ev, but diverge at higher energies. Berezhko et. al., for instance, favor a diffusion model with significant, possibly multiple, reacceleration in the sources. While they find the lower energy data able to rule out their model given a hot, dilute interstellar medium (ISM), they are unable to distinguish between those with hydrogen number densities 0.3 to 1 cm 3 [14]. Higher energy observations will be key in distinguishing between various scenarios. 23

40 Figure 2.7: The Boron to Carbon ratio from several experiments [28]. The dashed line indicates a simple power law fit to the data: 1.4E 0.6, while the solid lines show a diffusion model calculation for δ = 0.3, 0.46, 0.6, 0.7, 0.85 (from top to bottom) [28]. See Section for more details. (Experiments referenced in plot: [11]HEAO (90); [7]Simon (80); [9]Mahel (77); [6]Lezniak (78); [46]Juliusson (74); [47]Caldwell (77); [8]Orth (78); [17]CRN (90); [5]Dwyer (87).) Figure 2.7 illustrates the relatively broad range of the propagation energy index δ (see Section 2.1.3) still consistent with data, for a relatively simple diffusion model. The dashed line represents the Boron to Carbon ratio as a simple power law equal to 1.4 E 0.6 ; the solid lines show the model calculations for δ = 0.3, 0.46, 0.6, 0.7, 0.85 (upper to lower, respectively) [28]. With excellent charge and energy resolution in 24

41 the TeV to PeV range, CREAM sits in an ideal position to better determine the propagation index and thereby further constrain the models. 25

42 Chapter 3 Measuring Cosmic Rays: The CREAM Instrument 3.1 Introduction to the CREAM Flights Since charged, high energy cosmic rays bend in the galaxy s magnetic field, the energy spectrum provides one of the most powerful tools for discovering their origins and conditions of propagation. Thus, the CREAM detector is designed to measure the charge and energy of cosmic rays before they interact in the Earth s atmosphere. To measure cosmic rays prior to interaction, the instrument flies on a long duration balloon near the top of the atmosphere. The Antarctic continent provides an ideal location for this as the circumpolar winds reliably carry the balloon payload around the continent several times during the course of an austral summer. CREAM I circumnavigated the continent three times, for a record-breaking 42 days, from 15 Dec 2004 to 27 Jan 2005, while the following flights of CREAM II through IV (2005-6, , and ) have increased the total flight time to well over 100 days Flight Constraints The flight conditions put several constraints on the instrument. All detectors must be able to operate for extended periods in near vacuum and in widely varied temperatures, from the Antarctic ground-level cold ( 10 C) to the summer s continuous 26

43 solar heat near the top of the atmosphere, nearly doubled by the continent s albedo, to 35 C. The balloon payload itself must be relatively light ( 5500lbs, excluding ballast [39]) and have low power consumption, as solar panels are its main power source, with batteries as a rechargeable backup source. The CREAM I instrument, typical of the flights, uses about 380 W [40]. The compact cubic design minimizes non-instrumented area and allows easy handling of the payload. Data is retrieved from the instrument in two ways: via satellite downlink during flight and directly from the flight hard drives following landing and recovery of the instrument. The first, over NASA s Tracking and Data Relay Satellite System (TDRSS) with Iridium satellite backup, has limited available bandwidth ( 85 kbps [41]) but also allows in-flight commanding of the instrument, permitting near real-time tuning of the instrument. The second, recovery, is possible because the circumpolar winds generally bring the balloon back to its starting point, and at least carry it near enough a populated base to send a recovery team to retrieve the payload and associated data recording devices. The CREAM instrument employs complementary cosmic ray charge and energy detectors: a Timing Charge Detector (TCD), a Transition Radiation Detector (TRD) for the first flight, a Cherenkov Detector (CD), a Cherenkov Camera (CherCam) for flights 3 and 4, a Silicon Charge Detector (SCD), and a sampling Tungsten Calorimeter. Figure 3.1 shows the layout of the instrument for the first flight. The TRD and Calorimeter measure the incident particle s energy while the TCD and SCD, as well as the CherCam in later flights, primarily provide charge measurements. The CD is designed to ensure only relativistic particles trigger the instrument via the TCD, rather than the more plentiful lower energy ones. A scintillator ribbon layer known 27

44 Figure 3.1: CREAM I Instrument Schematic as S3 provides this function for charge 1 and 2 particles, as well as maintaining a similar geometry for all TCD-triggered events. High energy Calorimeter events can also trigger the instrument. For all flights following the first, the SCD had a second layer of silicon added, increasing the sensitivity of the detector. In the third and fourth flights, the CherCam was located in the position of the TRD Albedo Backsplash or albedo from the Calorimeter is the largest source of noise in triggered events. Segmentation of the detector elements in the SCD and CherCam allows the signal to be distinguished from albedo, particularly when coupled with accurate tracking from the TRD and Calorimeter. In the worst-case scenario for the TCD, backsplash creates detected light in the TCD at least 3 ns after the incident particle s 28

45 photons reach the PMT. Thus, by measuring the signal within the first 3 ns of detection, the TCD avoids substantial contamination. The detectors are described in more depth in the following sections. Unless otherwise noted, configurations and detector specifics did not change between the various CREAM flights. The completeness of sections reflects the author s hardware work, particularly on CREAM III, when she was the TCD and CD on-ice expert and subsequently participated in the instrument s recovery. 3.2 The Timing Charge Detector The Timing Charge Detector (TCD) uses scintillator material read out with photomultiplier tubes (PMTs) and fast electronics to measure the charge of relativistic cosmic ray nuclei from protons through Iron. As the incident cosmic ray passes through the scintillator, it deposits a fraction of its energy, primarily through inelastic collisions with electrons, causing ionization and atomic excitations. Since the ionization energy threshold is relatively high, and low energy interactions are most probable, excitations tend to exceed ionization. The energy deposition from these processes is proportional to the square of the incident charge and inversely proportional to the square of its velocity (β). The Bethe-Bloch equation more fully characterizes the mean energy deposition per unit length as: ( de dx = K Z2 Z mat 1 β 2 A mat 2 ln 2m ec 2 β 2 γ 2 T max β 2 δ ) I 2 2 (3.1) where K is a constant proportional to the electron radius (r e ) and mass (m e ) and Avogadro s number (N A ): K = 4πN A r 2 e m ec 2 (3.2) 29

46 and Z mat and A mat are the atomic number and mass of the absorbing material; T max is the maximum kinetic energy transferable to a free electron in a single interaction; I is the mean excitation energy; and δ is the density correction to the ionization energy loss [4]. As the electrons and atoms recombine or de-excite they scintillate, producing visible photons proportional to the energy deposition in the material. This light propagates through the scintillator to a wave guide which matches the scintillator geometry to the PMT face and shifts the wavelengths range to better suit the PMT response. Fast electronics (of order a few nanoseconds) convert the signals from four PMT dynodes, allowing a greater range of linear response prior to saturation of a given dynode. The remaining subsections describe these and testing and verification measures in greater detail Detector Material The TCD uses 8 paddles of fast scintillator with vacuum-rated fluors (Saint- Gobain BC-408V) oriented in two orthogonal layers and read out on both ends by fast photomultiplier tubes (PMTs). Figure 3.2 shows the TCD layout. Each slab is 120 cm 30 cm 0.5 cm. With four slabs in a layer, the detector area is 120 cm 2. Ultraviolet-absorbing light guides pipe the photons from the scintillator to the PMTs. By absorbing UV photons, the Saint-Gobain Lucite material minimizes the Cherenkov photons reaching the PMT. The constant cross-sectional area of the twisted strip, adiabatic light guides minimizes the probability that a scintillation photon will escape the light guide before reaching its PMT. Light guides are glued to 30

47 Figure 3.2: Timing Charge Detector (TCD) diagram, as viewed from the top. [42] the scintillators using Saint-Gobain BC-600, a two part optical cement. Air bubbles, which could cause scattering and photon loss out of the system, are prevented from forming in the glue. The cement s brittleness at low temperatures ( 20 C) requires care and mechanical support pre-launch, but is ideal for recovery, when it allows a clean break between the scintillator paddle and light guide, easing recovery of the parts. The rectangular end of the light guide is attached to a Lucite block which tapers slightly to the PMT s circular cross-section. For CREAM I, the light guides were directly coupled to the PMTs using GE Silicones SS4120 primer and RTV 615. The following flights employed a more modular coupling, allowing damaged PMTs to be replaced without removing an entire paddle assembly. Thin Lucite cylinders were attached to both the PMT s face and the end of the light guide. Aluminum rings are secured around the Lucite cylinders with set 31

48 screws and then the PMTs are joined by screwing the two rings together. Optical coupling is further enhanced by a layer of Saint-Gobain BC-630 optical grease. The primary photon loss in this system, via transmission through the scintillator boundary [43], is minimized by wrapping the entire paddle assembly first in crinkled aluminum foil and then with DuPont Tedlar. The crinkled foil diffusively reflects stray photons back into the scintillator/lucite system allowing collection of over 100 photo-electrons at each PMT for one minimum-ionizing particle (MIP) normally incident at the center of the paddle [42]. Two layers of Tedlar create a light-tight boundary between the TCD system and the outside world, and are affixed with 3M Y966 adhesive transfer tape. Further details may be found in [42] and [44] Photomultiplier Tubes 12-stage Photonis XP2020URFL photomultiplier tubes selected for their ultrafast timing characteristics, good linearity, and low noise occupancy are attached to the ends of each light guide. For CREAM II and beyond, the less expensive XP2020FL model had sufficient performance characteristics. Magnetic fields can decrease PMT efficiency; while CREAM does not employ a magnet, the Earth s magnetic field can negatively impact PMT performance. Therefore the PMTs are wrapped in two layers of 0.004in thick Mu-metal separated by six layers of in thick Tedlar as a dielectric. The wrapping extends from the base of the PMT to 2inches beyond its face, and the whole is again covered over by the two layers of Tedlar also wrapping the scintillator and light guide. The gain and timing characteristics depend on the exact voltage divider powering the PMT, potted and attached to the base of the PMT. It was discovered that simply 32

49 maximizing the anode gain resulted in instabilities in the deeper dynodes, so CREAM employed the active voltage divider shown in Figure 3.3. The circuit design minimizes the power consumption while balancing the requisite gain, linearity, dynamic range, and timing constraints. Large resistor values in the divider limit the quiescent current, keeping noise and power use low. A common collector configuration of NPN transistors allows a much larger current gain for a small change in input current, allowing sufficient flow with a linear response to the PMTs dynodes. Reverse-biased and 2.3V Zener diodes protect the transistors from being loaded and turn off the transistor should current begin to flow from the dynodes back towards the voltage source. The 10 nf capacitors stabilize inter-dynode voltages [42] and provide easily accessible charge reservoirs, ensuring fast response during the cascade. The anode and five of the deeper dynodes are tapped and capacitively coupled to the readout electronics to protect them from high voltage differences. As a last safety measure, the large resistor following these capacitors prevents excess high voltage accumulation following shutoff. The selection of the anode and dynodes 7, 9, 10, 11, and 12 provides sufficient dynamic range for a good system response to all charges from Hydrogen (deepest dynode: 12) to Iron (shallowest: 7). The PMTs are powered by Ultravolt 4A12-P4-F power supplies, which convert the input 12VDC to the required kV. This model is equipped with a Ripple Stripper which reduces output voltage ripple by greater than a factor of 10 [45] and has a 0 5VDC control voltage, allowing the output voltage to be adjusted during flight. For CREAM I, one Ultravolt supplied power for all four PMTs on each side of the instrument; the following flights paired the PMTs together, improving fault tolerance and allowing better gain-matching of the PMTs. For all flights, series resistors were 33

50 Figure 3.3: Voltage divider circuit used to power the PMTs. [42] included in the junction boxes holding the power supplies to better balance the gains between coupled PMTs. All high voltage connections were made using vacuum-safe coaxial Reynolds cable (type L) and all high voltage electronics were encased in Cumming Stycast 4640 White potting compound with required Catalyst 50 to prevent coronal discharge or arcing in the near vacuum conditions encountered at the top of the atmosphere. Charge can also build up on the surface of the detector electronics, so the aluminum of the junction boxes provided both mechanical support and a discharge path. Likewise, the potted PMT bases were wrapped in foil tape with conductive adhesive (McMaster- Carr PN 76925A). For further details of the electronics and PMT base construction see [42] and [44]. Following the high voltage failure of CREAM III described further 34

51 in Section 3.2.5, protective cladding was added to all exposed Reynolds cables to prevent accidental, potentially invisible damage TCD Electronics The TCD electronics power and read out the scintillator-pmt system within the instrument requirements of low mass and power consumption, large dynamic range, and fast response time. The CREAM instrument high voltage (+31 VDC) is transformed into the required 12V power and 0 5V control voltages for the PMTs via Vicor industrial grade DC-DC converters and ripple attenuator modules. Table 3.1 lists the converters used for each voltage. Peak detectors and time over threshold electronics process the PMT dynode signals. The TCD electronics also include local triggers and a master TCD trigger as well as digital readout electronics. Vicor Model Attenuator Output Voltage Used by VI-JW0-IY VI-RAM-I1 +5V Analog circuits VI-JW0-IZ VI-RAM-I1 +5V Digital circuits VI-JW1-IY VI-RAM-I1 +12V Ultravolts (PMT power) VI-JW0-IZ VI-RAM-I2 5V PMT control voltage VI-JWY-IZ none +3.3V TDC circuit power VI-JWY-IZ none 2V TDC circuit power Table 3.1: Power converter module map. As described at the start of this chapter (3.1.2), detecting the incident cosmic ray in the face of albedo from the calorimeter strongly drives the TCD s design. The peak detectors and timing measurement particularly reflect this constraint. Classically, one integrates scintillator light to determine the energy deposited, which is proportional 35

52 to the incident ionizing particle s charge squared. Since the scintillator has 3 ns total response time [46] and the scintillation light takes 4ns to travel to the PMT from the center of the scintillator paddle, integrating through the tail of the pulse would include particles backsplashed from the Calorimeter, substantially degrading the final charge resolution. Instead, we use the peak height and timing information to reconstruct the energy deposited without substantial albedo contamination. Peak Detectors The peak detectors are attached to PMT dynodes 7, 9, 10, and 12. This provides the requisite dynamic range by sampling the signal at various gains. The deepest taps (e. g. 10 and 12) provide the signal amplification necessary to detect the lowest charge particles (down to Z = 1) but become saturated for the highest charges. The shallowest taps (e. g. 7 and 9), with the least amplification, allow us to detect the heaviest charges, through Iron, for a total dynamic range of 28 2 = 784. The scintillator saturates some for the highest charges, so the actual gain necessary is slightly less. Also see Table 3.2. For each dynode tap, the circuit shown in Figure 3.4 detects and retains a peak for 50 µs [42]. The capacitor C1 is charged by the combination of operational amplifier U1-A (Maxim Max4225) and diode pair D1, which allow current flow only when the input (dynode) voltage is increasing with respect to the voltage across the capacitor. The voltage follower (U1-B) which buffers C1 has enough differential offset current to fully charge the capacitor without an input signal, so it is used in conjunction with matched junction gate field-effect transistors (Q1-A and -B) which prevent this backcharging. Analog Devices AD7854L Analog-to-Digital Converter (ADC) digitizes the final signal. Should the signal not be requested by the rest of the system (an internal 36

53 Gain relative PMT Tap Digitizer to anode Approximate Detected Charge(s) Anode TDC 0, 1 1 H, He : 1 Z 5 Dynode 12 ADC H, He : 1 Z 5 Dynode 11 TDC 2, Boron and heavier: Z 5 Dynode 10 ADC Boron - Neon : 5 Z 10 Dynode 9 ADC Carbon - Calcium: 6 Z 20 Dynode 7 ADC Oxygen - Iron : 8 Z 28 Table 3.2: Map of PMT taps to electronic readouts and associated detected charges. For CREAM II only, thresholds were adjusted such that Helium rather than Boron lay at the lower edge of Dynodes 10 and 11. TCD trigger), a reset signal is sent through an Analog Devices ADG417 switch, which quickly discharges the capacitor to 10mV. Timing We also use the time associated with the signal crossing programmable thresholds as a measure of the energy deposited in the scintillator. Since higher charge particles deposit more energy, their rise time should be faster. This has been demonstrated for protons versus helium in [47]. Two thresholds are placed on each of the Anode and dynode 11, and the time from threshold crossing to a common stop is measured with an internal precision of 50 ps. The paired threshold crossings are approximately proportional to the rise time, and thus provide a means of measuring the charge. Figure 3.5 diagrams the pulse traces and thresholds. Thresholds 0 and 2 are also used in forming the TCD trigger, as discussed further in Section Figure 3.6 shows the important sections of the timing circuitry. The emittercoupled logic (ECL) continuously draws current to maintain a constant voltage across 37

54 Figure 3.4: Circuit used to detect and hold peaks from the PMT dynode taps. [42] the differential amplifier (U1), preventing the transistors from saturating and allowing the rapid response time necessary for triggering the detector and avoiding substantial albedo contamination. The rapid response time supersedes the low power consumption requirement in this case. Upon exceeding the threshold, the binary VALID bit is set. The capacitor C1 stops charging on receipt of the COMMON STOP, issued by the local trigger, or when the charge on it exceeds the programmable DC LEVEL, providing a self-reset. Either a self-reset or an external RESET signal causes the rapid discharge of C1 and clears the circuit in preparation for the next threshold crossing. If on the other hand, the local trigger provides a CONVERT signal after the COMMON STOP, the capacitor C1 is read out at TIME and digitized by an ADC (Analog Devices AD7854L). Table 3.1 connects the circuit voltages indicated in the figure to their internal TDC sources. 38

55 Figure 3.5: Pulse from PMT Anode (left) and dynode 11 (right) with threshold crossings shown. Thresholds indicated by TDC 0 and TDC 2 in red are incorporated into the local trigger formation. [42] TCD Triggers The TCD provides CREAM with two triggers: ZLo and ZHi. A three-fold coincidence of threshold crossings for any PMT on each of the instrument s four sides, in addition to other criteria (see below), forms both triggers. Thus, the trigger is first initiated locally when the signal exceeds any TDC threshold in any PMT. The local triggers are combined in the TCD master trigger, which determines if at least one PMT on three of the four sides of the instrument has crossed threshold TDC 0 (anode) for ZLo or TDC 2 (dynode 11) for ZHi. Given a three-fold coincidence, the master trigger first sends a RAW COINC signal to the local trigger electronics and then, if the whole instrument has also triggered, sends them a CONFIRM and an event number generated by the Science Flight Computer (SFC). If the CONFIRM is not received within 1 µs, the detector electronics clear and wait for the next event. 39

56 D R Q Q C E C E C E C E B D I S C V A L I D R E S E T R e s e t a n d T r ig g e r S y s t e m s I N H R U 1 Q Q A D C O M M O N S T O P R 2 G N D R V U 2 C L K E V G N D R 4 1 K P M B T H 1 0 B Q 2 B Q 1 R V P M B T H 1 0 P M B T H 1 0 B Q 7 V R E F C 1 R p F K P M B T H 1 0 Q 3 R 7 R 8 R V 5 V G N D 1 K 2. 7 K R 9 A D U R G N D T I M E L E V E L U 6 M A X Figure 3.6: Major components of the timing circuit. [42] A confirmed trigger allows the local triggers to issue a COMMON STOP to the timing circuit and the CONVERT signal necessary to translate all analog signals into digital data. The readout electronics include a valid event number in the data sent to the SFC, which allows verification of the event coincidence. They then return ALLDONE, and the trigger and other electronics clear and await the next event. An INHIBIT signal, initiated either by the TCD master trigger or to it by the main instrument, prevents the TCD from triggering at all for full instrument debugging. Any type of completed trigger causes all timing and peak detector measurements to be digitized and recorded. Figure 3.7 diagrams the TCD trigger logic. As seen in the figure, the ZLo trigger also requires at least one of the S3 PMTs to exceed their TDC 0 threshold. This contains the geometry within the instrument and during analysis can ensure events are down-going. For further discussion of S3, see 40

57 Figure 3.7: The TCD trigger logic. Each set of four PMTs reside on one side of the detector and are instrumented with ADCs attached to peak detectors and TDCs clocking time since exceeding a threshold. Signals surpassing threshold 0 can form a low charge (ZLo) trigger, while those over threshold 2 contribute to the the high charge (ZHi) trigger. [42] 41

58 Section 3.7. Likewise, the ZHi trigger requires a minimum signal registered by the sum of the CD s PMTs (Section 3.4). The CD requirement also prevents triggering on the numerous non-relativistic particles funneled by the Earth s magnetic field to the poles, and thus overly abundant in an Antarctic circumpolar flight. Further, while an ideal event will create a four-fold signal in the TCD as the light travels to both ends of both traversed scintillator paddles, the three-fold requirement as implemented prevents damaged PMTs from lowering the trigger rate without triggering on substantially more noise. In this way, the TCD trigger drives the value of the TDC thresholds 0 and 2. As shown in Table 3.2, ZLo triggers on nuclei from Hydrogen to heavier elements. Since Hydrogen is 10 times more abundant than Helium and over 100 times more so than that of the next most abundant cosmic ray nucleus, it dominates the ZLo trigger. Helium has a proportional contribution to ZLo triggered events. ZHi trigger is thus set to allow separate triggering for nuclei with charge greater than Z = 4: Boron and heavier. This is a natural separation as the intervening elements (Lithium and Beryllium) have exceedingly low flux. For CREAM I, it also corresponded to relativistic charges with enough energy to be observed by the TRD. For CREAM II, lacking a TRD, the science focus moved to calorimeter events, and the TCD trigger thresholds changed accordingly. The TDC 2 and CD thresholds were lowered in concert to allow triggering on events with Z 2, including Helium in the ZHi trigger. While these settings performed as expected, for CREAM III and beyond it was decided to increase the proportion of triggered charges heavier than Helium for their scientific merit, so the TCD trigger settings were returned to their CREAM I characteristics. 42

59 Electronic Infrastructure and Data Flow The peak and timing electronics are grouped onto their own circuit boards, with one assigned to each PMT. Two peak and timing boards for adjacent PMTs are grouped into a single electronics box, shown in Figure 3.2, and combined with a power board, a local trigger circuit board, and the board containing the microcontroller and supporting parts for reading out and transferring the data from the TCD to the SFC. Thus, there are two electronics boxes per TCD side and one for S3, for a total of nine boxes. Figure 3.8 shows a schematic and photographic representation of a single TCD electronics box. Figure 3.8: TCD electronics box: schematic and photographic representations. Each box controls two PMTs, with each one s anode and dynode taps running to a single set of peak and timing (TDC) circuit boards, 0 or 1. Ethernet runs over one of the larger, square connectors on the microcontroller board; the other connectors thereon provide backup I/O options. [42] The boards are electronically coupled and powered via a 50-pin system bus while the 12V supply enters the power board over a 10-pin Molex Mini-Fit Jr.connector. A 16-pin bus separately connects only the peak and timing boards to the local trigger. 43

60 10-pin ribbon cables connect the local triggers together by side of the TCD and then connect the sides to the TCD master trigger. The master trigger distributes commands back to the local triggers via three 14-pin ribbon cables, one to each half of the TCDs electronics boxes and the last to S3. Data from each of the electronics boxes flows to the SFC over an Ethernet system. The four microcontrollers in the boxes on each half of the TCD are cabled to two Parvus PRV port Ethernet switches, and these are connected to a third switch which relays the TCD and S3 data to the SFC. The low power ( 1.5W), high temperature-rated (85 C) switches operated sufficiently well during bench tests at low temperatures ( 40 C), and thus fit the balloon payload requirements ([48], [44]). The microcontrollers, Arcturus Networks Coldfire uc5272, come with two Ethernet connections and two serial ports as well as paired USB ports, providing connection redundancy useful for debugging the system and controlling the TCD independent of the remaining instrument during ground testing. To satisfy the flight constraints, we used the extended temperature range ( 40 to 85 C) model with 4 MB NOR flash memory, 8 MB SDRAM running an included uclinux operating system running at 48 MHz [49]. The uclinux OS runs two servers, one for incoming configuration data and commands (cserver) and one for outgoing event data (eserver), which communicate with the SFC over the previously described TCP/IP Ethernet network. After a CONFIRM signal from the TCD master trigger triggers a device driver to read all the data from the electronics boards into a system buffer, the eserver transmits the data to the SFC. cserver receives simple commands from the SFC allowing the various detector parameters (PMT high voltage, thresholds, and timing circuit levels) to be set and 44

61 responds to periodic configuration state requests from the SFC during flight. This allows monitoring of the TCD configuration without incurring excessive dead time. cserver also transmits the system reset, clearing the electronics with an ALLDONE Initial Verification The PMT-scintillator systems were thermal and vacuum tested at the Columbia Scientific Ballooning Facility in August of 2003, prior to the first flight. During the initial pump-down, the Tedlar wrapping covering the paddles began to puff up as the pressure decreased. To eliminate the probable tearing, the test was stopped and a vent cut into the Tedlar. A coil of drip-irrigation tubing was inserted into the vent, and the vent was then sealed around the end of the coiled tube to prevent ambient light from entering the Tedlar-wrapped paddle while still providing an exit for the air. As no change in trigger rate was observed following this procedure, the lighttightness was verified. The test then continued with nominal results for several hours at a temperature of 20 C and pressure of 4torr [44]. The function of the TCD was further verified with a beam test at CERN on 31 Oct A 158 GeV/n Indium beam fragmented on a thin target, generating nuclei from Hydrogen to Gallium with A/Z = 2. These were incident upon a pair of orthogonally oriented TCD scintillator paddles. Prototype peak detectors read out two of each PMT s dynode taps, with dynode 12 capturing charges 1 Z 3 and dynode 9 measuring Z 5. Figure 3.9 shows the results of combining the channels after applying a 3 rd order polynomial to correct each channel s non-linearity. The gap labeled No Data arises from a lack of overlap between dynodes 12 and 9 which was rectified, as expected, by also utilizing dynodes 10 and 7 during flight. This prototype 45

62 system obtained a good charge resolution of 0.2e for Oxygen and 0.4e for Iron [42]. The beam test was not repeated following CREAM I since the detector remains nearly identical and the scintillator response is also characterizable from flight data. Figure 3.9: A prototype TCD scintillator and electronics system placed in a beam of Indium fragments measured Oxygen with a resolution of 0.2e and Iron with 0.4e [42] TCD Improvements In September of 2004 the assembled CREAM I instrument was subjected to a final thermal-vacuum test at Goddard Space Flight Center. Over the course of the four day test, nine of the eighteen PMTs failed, eight on the TCD and one on S3. Testing revealed that the original Emco C40 HV power supplies, chosen for their 46

63 compactness which allowed integration directly into a PMT base, had failed in vacuum by overdrawing current from the main power supply, suggesting internal arcing [44]. Thus, the supplies were replaced with the Ultravolts in junction boxes described in Section The new systems, with all paddles and power supplies connected as for flight, were vacuum tested for over a day each in October 2004 at Wallops Flight Facility prior to the launch of CREAM I. This power system proved successful for CREAM I and II, with a few upgrades to prevent single-point failures and to increase mechanical robustness following CREAM III. The CREAM III high voltage to the CD and S3 3.7 failed during turn on at float altitude. The two power sources only shared a control voltage which, at a low 0 5V, makes it exceedingly unlikely to have caused the observed failures as the PMT high voltage system runs at 12V. Prior to the flight of CREAM III, a shipping accident caused forces in excess of 5 g to damage the integrated instrument. As all systems were thoroughly inspected and retested to the greatest extent possible after the incident and prior to launch, and reinspected after recovery with no damage discovered, the most likely culprit is damage to the Reynolds cables connecting the PMTs to their high voltage sources. Such cable damage may only be visible under a microscope, and thus not readily apparent with simple visual examination, and may have been caused during the shipping incident or by accidental mishandling after TCD integration into the full instrument. Thus for all following flights, all Reynolds cables are protected with cladding. Further, we increased the testing rigor, operating the PMTs and Ultravolt power supplies in vacuum for a minimum of one week. Another upgrade to the TCD design allowed vacuum testing of complete PMT and junction box groupings connected as 47

64 for flight prior to integration; the CREAM I junction boxes system for mounting onto the instrument frame only allowed in situ vacuum testing under a bell jar for 30 minutes following potting. This grouped testing was applied to CREAM IV and V. 3.3 The Transition Radiation Detector The Transition Radiation Detector (TRD) consists of eight layers of Ethafoam radiator embedded with sixteen layers of thin-walled, Xenon-Methane gas filled proportional tubes. A high energy cosmic ray incident on this system leaves a track of energy deposition proportional to its charge and energy, providing a non-destructive measure of the particle s Lorentz factor γ. This system allows the opportunity for measurement in two energy regimes: via ionization caused by a lower energy cosmic ray nucleus in the gas tubes and via the transition radiation caused by a higher energy nucleus passing through the radiator Detector Physics As a relativistic, charged particle traverses the boundary between materials with differing dielectric constants, its electric field changes, generating transition radiation. These photons tend to be soft X-rays of 1 100keV for particles of high Lorentz factor in typical TRD media. They are emitted in the forward direction with an angle of 1/γ, in practice coincident with the incident particle path for high Lorentz factors. For the CREAM TRD, Dow Ethafoam 220, with typical cell spacing of 870 µm and cell walls of 35 µm provides a radiator material with many boundaries: 11 interfaces/cm [50]. Layers of stretched foil often have a slightly better transition radiation response 48

65 than foam but require additional structural support [51], while the Ethafoam is originally designed as supportive, protective, low mass (ρ = g/cm 3 ) packaging material [52]. Embedded in each foam layer to record the transition radiation photons are two layers each of 16 thin-walled, 2 cm diameter, aluminized Mylar proportional tubes filled with a 95%/5% Xenon/Methane gas mixture in (see Figure 3.1). Each layer of foam is rotated so the proportional tubes lie orthogonal to adjacent layers to allow stereoscopic tracking. Four foam layers form a single TRD module; CREAM I flew with two TRD modules separated by a Cherenkov Detector (see Section 3.4) for a total TRD active area of cm 2 and a total depth of 2 35 cm. The multiple layers of foam and proportional tubes not only allow 3-dimensional tracking, but also provide many independent measurements of the transition radiation, statistically reducing the impact of individual fluctuations in energy deposition on the final energy measurement [53]. At lower Lorentz factors, ionization of the gas mixture by the incident cosmic ray nucleus dominates the interaction mechanism. In this case, the passing cosmic ray strips electrons from the gas. The liberated charge cascades to the sense wire, held at 1.5kV relative to the grounded tube, and is proportional to the energy lost by the traversing particle. In Xenon, the increase in ionization energy loss with increasing Lorentz factor, known as the relativistic rise, begins at γ 4 and saturates at γ 1000, complementing the onset of detectable transition radiation [51]. Further, the Xenon/Methane gas mixture used in the CREAM TRD captures the transition radiation X-rays below 35 kev with high efficiency [50], yielding a detector with 49

66 sensitivity to Lorentz factors from 4 to The large dynamic range is digitized by a dual-gain, 12-bit ( 4,000channel) amplifier system. Using the detector with high charge particles (Z 3) improves the final energy resolution. The transition radiation and ionization yields increase as Z 2, giving at minimum an order of magnitude signal amplification for a given Lorentz factor. As the yield increases, the average energy loss for heavier nuclei becomes comparable to or exceeds the maximum energy transferred in a single interaction for the transition radiation and ionization processes. This implies that multiple interactions must occur, and thus, by the central limit theorem, the deposited energy distribution becomes nearly Gaussian for energetic, heavy cosmic rays, rather than the broadened Landau distribution seen for singly-charged particles in a multi-layer detector. See [51] for further discussion. Thus, the CREAM I TRD data is recorded only for ZHi triggers (see Section 3.2.3) and has an excellent energy resolution of 15% for Carbon and 7% for Iron nuclei at γ = 3000 [40]. Since the majority of the calorimeter backsplash has charge Z = 1, there is little effect on the TRD measurement. While a backsplash particle can deposit enough energy to be detected in the TRD, this is almost always in an isolated position and can thus be positionally rejected. This is borne out in the analysis (Section 5.2.1). Further, the TRD has excellent tracking performance, with reconstruction in the TRD to better than 2mm [54] Flight Constraints The TRD conforms to several constraints derived from flight conditions, notably mass limitation, long duration, low power consumption, and restricted data rate. 50

67 The proportional tubes can operate in unpressurized environments, as observed during flight and during the preceding instrument thermal-vacuum testing of September 2004, eliminating the need for a massive pressure vessel. Further, the electromagnetic nature of transition radiation does not require as large amounts of target mass as compared to techniques such as calorimetry for an equivalent geometric factor, permitting a total TRD flight mass of 200kg [40]. The flight duration requires the proportional tube end caps to be specifically designed to reduce gas leakage to less than that through the tube walls, preventing complete gas loss before the end of the anticipated 100-day Ultra-Long Duration Balloon flight. During the 42 day CREAM I flight, the gas manifolds were periodically connected together to balance the gas pressure resulting from differing loss rates in the various tubes. This proved sufficient for maintaining the gas pressure, and the reservoir gas was not tapped, suggesting, along with the previously performed lab tests, that a future, 100-day TRD operation would be successful. The TRD used high efficiency Vicor VI-200 series converters to provide the +5 and ±12 Volts needed along with custom electronics to limit the total power consumption to 37W. Two Ultravolt 2a2a-p20 (0 2kV, 20W) modules potted in aluminum boxes the same as the TCDs (Section 3.2.2) supplied high voltage of 1.4 to 1.5kV to the two TRD modules. Over 30 days of vacuum testing prior to flight indicated stability during the flight. Since both the total data storage and the data downlink rate are limited, the recorded data was limited to signals greater than the pedestal mean plus a constant (in ADC counts, 3 for the flight). The noise pedestals were determined every 5 minutes during flight and typically varied during that interval by < 1 count. Satellite downlink 51

68 provided a sample of 10% of the data for in-flight tuning and verification. Thus, the data were reduced from 512 tubes with two 8-bit high- and low-gain channels to a level within flight constraints while maintaining sufficient data quality with low power consumption. Further details may be found in [40] Calibration Calibration of the TRD is crucial for determining the incident cosmic ray s energy and allowing the excellent energy resolution necessary for accurate deconvolution of the cosmic ray s steeply falling energy spectrum. In the summer of 2001, a prototype TRD, consisting of a single layer of Ethafoam and proportional tubes, was placed in beams of protons, pions, and electrons at CERN. Two small multi-wire chambers provided auxiliary tracking to 100µm, permitting sufficient event geometry reconstruction. Together, this provided calibration for Lorentz factors ranging from γ 150 to over and showed good agreement with the GEANT4 detector simulation [50], [40]. Figure 3.10 shows the simulated response for the CREAM I prototype TRD with the beam test results overlaid. The relativistic rise is evident for Lorentz factors from about 4 to 10 3, at which point the transition radiation begins to dominate. The blue lines graphically portray the relationship between TRD signal resolution and Lorentz factor resolution. The well-understood dependence of transition radiation on charge (strictly as Z 2 because it is a purely electromagnetic process) allows the extension of this calibration to nuclei with Z > 1. For further details see [51]. 52

69 Figure 3.10: The TRD response simulated with GEANT4 (small, red dots) and from exposure to singly-charged particles with a range of Lorentz factors (larger, shaped symbols) [54]. 3.4 The Cherenkov Detector Detector Material and the Cherenkov Response A thin acrylic radiator coupled to eight PMTs divides the TRD in half and measures Cerenkov radiation for energetic incident particles. This ensures that the abundant low energy cosmic rays funneled to the poles by the Earth s magnetic field do not swamp the ZHi trigger (see Section 3.2.3) as only charged particles with v > c/n, the speed of light in the medium, will emit Cherenkov radiation. The CREAM CD has a refractive index of n 1.5 and thus a threshold of β The number of 53

70 emitted photons per unit length and wavelength for a particle above threshold then goes as: ( ) d 2 N dλdx = 2πZ2 α 1 1 λ 2 β 2 n 2 (λ) (3.3) where α is the fine structure constant ( 1 ) ([43]) and n varies little for the CREAM 137 CD over the relevant wavelenghts. Figure 3.11 plots the signal response to the incident particle s energy for various multiply-charged incident particles. Since the Cherenkov signal is also sensitive to the square of the incident charge, it can be used with the TCD to simultaneously determine the incident particle s charge and velocity (see Section 4.3). Figure 3.11: The Cherenkov signal from a representative detector for multiply-charged particles (Z = 25 (Mn), 26 (Fe), 27 (Co), and 29 (Cu)) versus energy per nucleon. Note the low energy threshold and asymptotic maximum at high energy (β). 54

71 The CREAM CD is specifically designed for high efficiency under balloon flight conditions. Doping of the 110 cm 110 cm 1 cm plastic radiator with a wavelengthshifting dye ensures nearly uniform Cherenkov photon reradiation within the medium. Springs hold wavelength-shifting bars against the four edges of the CD; the small air gap left by this attachment mechanism helps trap the photons in the bar, permitting improved signal detection. Eight 5in Photonis XP1910 PMTs at each end of the bars convert the photons to electric current. Custom circuitry subsequently digitizes the data via 12-bit, peak sensing ADCs coupled to the PMTs tenth dynode outputs and a fast summing amplifier for the PMTs anode signals. Fast discrimination on the summing amplifier provides the necessary TCD ZHi trigger input. For CREAM I, the CD electronics were located with the TRD electronics; for the flights following CREAM I, the CD readout systems were integrated into the TCD s with a new Pulse Shaper Board, allowing the PMTs anode signals to be digitized by a pair of TCD peak detector boards (see Section 3.2.3) and rapidly summed for the ZHi trigger input, as described in detail in the following section (3.4.2). Control of the minimum threshold voltage on the summing amplifier and the high voltage for the PMTs was also transferred to the TCD electronics, while the power itself remained otherwise isolated. This is supplied by the same the same power configuration as for the TRD, described in Section The CD consumes 10W and weighs only 100lbs, well within flight constraints CD Electronics From CREAM II onward, the CD is read out through the TCD/main-instrument interface. The initial Pulse Shaper Board (PSB) design accepted the 8 PMT outputs 55

72 associated with the ends of the wavelength-shifting bars attached to the 4 sides of the CD plastic. Passing the short, inverted PMT pulse through a series of operational amplifiers (op amps) (AD8039) resulted in an approximately 1.5 V, upright, 100 ns pulse with an 0.5V tail. The circuit fully recovers after 1µs, ensuring low dead time. The electronics provide a sum of the 8 PMT channels to both an external (SMA) connector and to the discriminator onboard. In flight, the external output is terminated with a 50 Ω resistor. The discriminator uses a MAX9203 comparator to determine if the PMTs sum is greater than an externally set threshold. When this occurs, a 74HC221D multivibrator modulates the signal to a square pulse of 2V for just under 100 ns, approximately 85 ns after the initial input pulse. The discriminator output is used in conjunction with a 3-fold coincidence from the TCD threshold 2 TDCs to form a ZHi trigger (see Section 3.2.3). The CD threshold is typically set such that the instrument triggers on nuclei of charge Z 3 with speeds β in excess of 0.7c. The CREAM III instrument experienced forces in excess of 5g during a shipping accident which, among other things, caused damage to the CD electronics. The PSB, as tested following the incident, showed excessive heating of the op amps associated with the initial PMT pulse shaping. Replacing the op amps on the 8 PMT channels returned the board to working status. The board was fully tested, both in the lab prior to installation on the instrument and with muons on the ground in the full instrument configuration. When turning on the CD system at float altitude following the launch of the CREAM III instrument, one of the two high voltages powering the CD could not be 56

73 ramped up to its normal operating voltage. Though the second of the two still appeared operational and thus powering half the CD s PMTs, no ZHi triggers occurred. Lab testing of the recovered PSB showed the same op amp failure as after the shipping incident. Further investigation into the shipping incident events revealed that one of the Calorimeter targets which had slipped from its moorings had landed on a CD High Voltage (HV) Reynolds cable. Any exposure of unprotected HV system components to near vacuum conditions, such as those experienced at float near the top of the atmosphere ( 125,000 ft), can cause arcing. These voltage fluctuations most likely damaged the op amps on the PSB in flight. To prevent this from happening on future flights, we redesigned the PSB with protection diodes to regulate the voltage input into all the board s circuitry. The CREAM IV board includes dual Schottky barrier diodes (ZC2812E) at all the PMT inputs as well as at the threshold input. The Schottky diodes, using electrons as the charge carrier (rather than holes), quickly clamp the input if it exceeds a safe ±5V. Zener voltage regulator diodes (1N5341B-D041) perform a similar function on the ±5V power supplies. These new PSBs have been tested in the lab and on the ground with muons, and performed without failure during the CREAM IV flight ( ). 3.5 The Cherenkov Camera For the third flight of CREAM, a new charge detector was added: the Cherenkov Camera (CherCam). Rather than simply measuring the total Cherenkov light output, the CherCam images the cone of light produced when a relativistic, charged particle passes through material with a speed faster than light in the medium (see for a discussion of the threshold). The cone opening angle is well defined by θ C = 1 βn(λ), 57

74 where again the material is chosen such that the index of refraction n does not significantly vary over the relevant wavelengths λ for a refractive medium of thickness L >> λ. The CherCam derives from a similar AMS detector [55] Detector Materials Figure 3.12: Schematic view of the Cherenkov Camera, including the aerogel radiator and PMT imager plane. Note the light-absorbing layer at the top which removes light emitted from albedo particles traversing the radiator from the detector [55]. Figure 3.12 depicts a schematic view of the CherCam, including the Aluminium honeycomb support structure and light absorbing layer followed by aerogel radiator, a minimal expansion gap of 12cm, and the PMT array imaging plane. The light-absorbing layer prevents backsplashed albedo particle from impacting the measurement by creating stray photons in the aerogel radiator. Two stacked tiles of 58

75 cm 2 Matsushita-Panasonic SP50 aerogel arranged in squares of 4 into a 5 5 grid comprise the 110 cm 110 cm 20.8 mm radiator plane (see Figure 3.13a). (a) CherCam aerogel radiator (b) CherCam PMT imaging plane Figure 3.13: Aerogel radiator arranged for installation into the CherCam (3.13a) and PMTs arranged for imaging the cone of Cherenkov photons prior installation into the CherCam [55]. As variations in radiator depth and refraction index cause distortions in the Cherenkov cone which affect the final charge measurement, the thickness spread must be < 0.2mm and the optical index dispersion δn/n < 10 3 for a final charge resolution of σ Z = 0.25 [56]. Mapping the tile thickness showed a typical variation of 0.5mm. To compensate for this, an algorithm optimized the stacking tiles into pairs for an overall deviation from horizontal of less than 0.2 mm. Deviation of a laser beam across the tile combined with its mean density computed from the thickness map and weight gave tile-to-tile fluctuations less than 10 3 and internal fluctuations less than that [56], establishing n = Beneath the expansion gap lies the imaging plane, comprised of square blocks of 16 PMTs each, for a total of in Photonis XP3112 PMTs, housed in a 59

76 15mm thick layer of black epoxy ertalyte material. Figure 3.13b shows the arrangement of round PMTs, giving an active detection surface of 50% of the total surface area. While light guides could minimze the inactive detector area, modeling indicated that reconstruction was more efficient without the complex refractions inherent in the light guides [56]. The PMT anode signals are read out by 16-bit ASICs (Application Specific Integrated Circuit) identical to those developed for AMS [56], [55], which record and amplify the signal peak while automatically correcting for the DC offset [57]. 100 purpose-built high voltage modules power the CherCam PMTs while a separate unit powers the front-end electronics Validation Like the other detectors, CherCam underwent thermal-vacuum and beam testing. A long duration (23hrs) test at 5mbar with thermal cycling from 10 to +10 C and thermal cycling from 10 to +35 C for over 30hrs at atmospheric pressure showed normal operation with only nominal pedestal variation, < 5ADC counts, well below the 60 ADC counts generated for a single photoelectron. Further powering on at 10 C simulated Antarctic testing conditions and was a success. The CherCam had two beam tests at CERN, the first with a prototype 64-PMT module with partial flight electronics in Oct 2006 and the second with the full flight electronics in Oct The Z = 1 test particles, with incident momenta between 100 and 300 GeV/c, tested the detector s lowest sensitivity limit. Since the 50% active detecting area implies up to a factor of 2 loss in number of photons for a given trajectory, a 4-plane silicon strip beam tracker (not flown) provided tracks for 60% 60

77 of the beam events. The loss is corrected for by counting the signal only from PMTs within the tracker-determined Cherenkov cone, as seen in Figure In flight, the Calorimeter and, to a lesser extent, the SCD and TCD provide the necessary tracking. The beam test demonstrated a preliminary CherCam charge resolution of σ Z = 0.2e [55]. Further, the main features of the CherCam simulation matched those of the beam test, with study ongoing to explain the remaining deviations and thereby better characterize the detector response. Figure 3.14: The simulated and beam test signals for the CherCam after counting only the signal from the trajectory-determined Cherenkov cone [55]. 61

78 3.6 The Silicon Charge Detector The Silicon Charge Detector (SCD) uses an array of Silicon PIN diodes to measure the charge on an incident cosmic ray. A high energy cosmic ray passing through the reverse-biased sensor creates electron-hole pairs which form a current proportional to the energy deposition in the detector. For relativistic cosmic rays, the energy deposition is uniform along the length traversed and proportional to Z 2. With 2912 PIN diode pixels and tracking from the TRD and Calorimeter, the cosmic ray signal is isolated to a few pixels, allowing the elimination of the effects of the many albedo particles incident on the SCD. Charge mis-identification for a low-z (< 3) cosmic ray with energy near ev is expected to be less than 2 3%. At lower energies, less albedo, and therefore less confusion, is created, while at higher charges the increased energy deposition in the Silicon permits differentiation from the mainly singly-charged albedo particles. In this way, the fine segmentation of the SCD enables charge identification in the face of substantial background. It also improves noise subtraction by isolating it to a small area Detector Material The sensors were fabricated from 5in diameter, lightly doped n -type Silicon wafers 380 ± 15 µm thick. The thickness and specific material were chosen for maximum depletion of the substrate without incurring danger of voltage breakdown in near vacuum conditions. The SCD sensors ran at a reverse-bias of 100V DC across the p-n junction. Figure 3.15 diagrams a single SCD PIN diode traversed by a high energy cosmic ray. 62

79 Figure 3.15: A relativistic cosmic ray loses energy as it traverses the Silicon substrate of the SCD PIN diode, creating electron-hole pairs. The high reverse-bias voltage accelerates the current flow to the p-n junction, where the signal is read out [58]. Following fabrication, the sensors are cut into a 4 4 array of pixels with cm 2 active area. The sensors pixels are mounted together on a ladder which provides mechanical support for both the pixels and their analog electronics. The pixels are tilted slightly in both lateral directions to eliminate non-instrumented area as in Figure 3.16a, resulting in a total active area of cm 2 for 7 sensors per ladder and a 2 13 ladder array. The detector is a slim 2.16cm, preventing further restriction of the instrument s geometric factor. Each pixel s signal travels along a Kapton tape Flexible Printed Circuit Board wire-bonded to it, to the read out electronics. These include charge sensitive and shaping amplifiers as well as track and hold and calibration circuits. The last reduces the analog circuit noise by injecting known charges into the electronics channels and measuring the output every 2hrs during flight. 16-bit MAX1133 ADC chips digitize the analog signal with range sufficient to resolve Z = 1 to 28 (Nickel). 63

80 (a) Overlapping SCD pixels. (b) Fully assembled SCD. Figure 3.16: The slight tilt of the SCD pixels prevents dead space in the SCD active area (3.16a) [59]. The assembled CREAM I SCD, showing the fabricated pixels and the digitization electronics to the sides (3.16b) [40]. The SCD receives power from one of three Vicor VI-J00 series DC/DC converters with VI-RAM active output filter modules housed in the Calorimeter power box. This converts the instrument power from 28 ± 5V to ±6.5V and 12V. The SCD electronics use the lower voltages, consuming about 50W, while EMCO Q01-12 DCto-DC converters upgrade the 12V supply to the necessary 100V sensor bias voltage. Figure 3.16b shows the assembled SCD, which has a mass of merely 14kg Inherent Noise Semiconductors have inherent noise from the tunneling of charge carriers across the insulator, known as the leakage current, which limits the smallest pulse detectable. Heavy doping, as for the SCD s p-n junction, increases this, so all sensors were required to have leakage current < 8 na/cm 2 at full depletion of 100V. To further measure signal to noise, sensors were exposed to normally incident βs (e ) from a 64

81 90Sr source at full depletion voltage. Less than 20% of the fabricated sensors failed these tests, and were replaced with working sensors [59]. Since leakage current increases with increasing temperature, the electronics channels were read out every 5mins during the fight, allowing constant monitoring of the SCD pedestal values. During the first flight, SCD pedestals varied by 20 ADC counts/ C for flight temperatures (after launch) between 25 and 33 C with daily variation of only 4 C [59]. This also allowed sparsification of the recorded data by retaining only pixel channels with signal above the pedestal threshold (see also [40]) Validation In September 2004, the SCD participated in the full instrument thermal and vacuum test. With temperatures ranging from 10 to +40 C at 4Torr, convection did not readily cool the SCD, resulting in increased detector noise from increased leakage current, particularly above 33 C. Thermal straps of Copper installed thereafter prevented this from affecting the flight data, where only 1.9% of the pixels were dead or too noisy to use [59]. A beam test at CERN in November 2003 exposed the SCD to fragments from an Indium beam incident up a thin target at 158 GeV/nucleon (see also Section 3.2.4). Each pixel was individually exposed, and the detector showed clear charge separation for all nuclei (Z = 2 to > 28) with charge resolution better than 0.2e for Helium and 0.3e for Iron [40], [58] CREAM II and beyond: the Dual-layer SCD For the CREAM II flight, a new SCD based on the first but with two layers of active silicon was created. This improved the charge resolution by permitting 65

82 improvements in the sensor fabrication process, associated electronics, and noise reduction systems. The same Silicon sensor was used, but pixel size was increased to 2.1 cm 2, allowing the total number of pixels in a layer to be reduced to 2496 without changing the active area per layer. The dual layers of SCD, oriented orthogonally to each other as seen in Figure 3.17a, cover 84% (0.52 m 2 ) of the total active area. Wiring improvements decreased the probability of signal interference. The dual-layer SCD s increased power consumption, to 136 W, necessitated improved thermal abatement in the form of Aluminium heat plates under the analog boards, Copper bars in two corners, and a reflective Aluminium wedge on the anti- Sun side of the instrument to help radiate away the excess heat. These increased the mass to 56 kg, still low for any of the CREAM sub-detectors, while maintaining the SCD operating temperatures below 38 C, as for the previous flight. (a) Photograph of the assembled, dual-layer SCD. (b) SCD signal from the top and bottom layers. Figure 3.17: The assembled SCD with the top layer rotated 90deg from the bottom (3.17a) and the consistent response of the two layers to Calorimeter-tracked cosmic rays during the second CREAM flight (3.17b) [60]. 66

83 Leveraging experience gained in from the first SCD fabrication, after nearly doubling the number of pixels only 1.7% were dead or too noisy to use. The dual-layer SCD was refurbished after the flight of CREAM II and again placed in a CERN beam, this time of singly-charged particles, in A charge resolution of 0.11 e was found for both layers, demonstrating clear improvement over the original SCD using the same Calorimeter tracking. Requiring consistency in charge measurement between the two layers increased the charge resolution by an additional almost 10%. Figure 3.17b shows the consistency between top and bottom SCD layer measurement for CREAM II flight data. Events lying off the equivalency diagonal are due to albedo, hit pixel misidentification, and charges passing through one layer but not the other. Further details on the dual-layer SCD may be found in [60] and [61]. A dual-layer SCD has flown on all subsequent CREAM flights. 3.7 Hodoscopes and S3 CREAM I included three sections of scintillating fibers, placed above each of the two Carbon target segments and the calorimeter. The uppermost two sections, of layers of mm 2 scintillating fibers, form a hodoscope system, providing a longer lever arm for tracking, particularly from the calorimeter. The fine spatial segmentation limits contamination from albedo while the scintillating nature of the fibers provides supplemental charge sensitivity. The topmost hodoscope section consists of four layers of fibers, each oriented orthogonally to the previous, and denoted S0/S1, seen in Figure The active area of cm 2 matches the geometry of the top of the upper Carbon target which tapers trapezoidally to cm 2. This is 67

84 matched by the second paired layer of orthogonal fibers grouped to form S2 and the top of the second Carbon target. Figure 3.18: The S0/1 hodoscope, with clear fibers collected into HPD readout geometry visible on the edges. [40]. The hodoscopes fibers (Saint Gobain BCF-12MC) are polished on both ends, aluminized on one, and read out by hybrid photo-diodes (Delft Electronics Products BV s PP0380BB HPD) on the other. Aluminization both increases the signal intensity and enhances the fiber response s uniformity to particles incident anywhere along its length [40]. Four of the HPD s pixels are illuminated by blue Light Emitting Diodes (LEDs) which aid in pixel-fiber alignment during construction and calibration during flight; S2, having proportionally fewer HPDs uses only three pixels per HPD for the 68

85 LEDs. Following CREAM I, tracking from the calorimeter was deemed sufficient and power and weight were saved by eliminating the hodoscopes. Just above the calorimeter, a single layer of the same scintillating fibers as used in the hodoscopes comprises S3. Designed to participate in the ZLo TCD trigger (Section 3.2.3) rather than for tracking, S3 has an active area of cm 2 read out by PMTs and electronics identical to the TCD s (see Sections and 3.2.3). Being sensitive to both the incident cosmic ray and calorimeter albedo, the timing measurements from S3 and the TCD allow us to construct a particle s time of flight as well as discriminate against backsplash. The single S3 PMT high voltage power supply was doubled for CREAM IV to prevent another single point failure, as occurred for CREAM III; with some adjusting of thresholds, the system can be nearly as effective in triggering off of one functioning PMT. 3.8 The Calorimeter A high energy cosmic ray passing through dense material has an increasing probability of interaction, creating a hadronic shower of particles. Decay of neutral pions creates a narrow electromagnetic shower core which can be measured by a calorimeter to determine the energy deposition and thereby, the most probable incident cosmic ray energy. Weight limitations primarily prevent the balloon flight of a hermetic calorimeter, but do allow a sampling calorimeter, which has flown on all CREAM flights. 69

86 3.8.1 Detector Materials Layers of Tungsten plates and scintillating fibers sample 0.3% of a minimum ionizing particle s energy, and the Calorimeter is deep enough to contain the electromagnetic core of showers initiated by nuclei with energies at least ev. This is aided by densified graphite targets (ρ = 1.92 g/cm 2 ) which preceed the Calorimeter and provide locations for nuclear interactions. About half the incident protons and up to 60% of Iron nuclei will interact in the Carbon targets, creating Calorimetermeasurable showers [40]. The trapezoidal shape of the two, 9.5 cm-thick targets maximizes the geometric acceptance while minizing the associated target weight. Figure 3.1 shows the shaped targets and Calorimeter with respect to the rest of the instrument. Beneath the targets and with a vertical depth of 20 X 0 radiation lengths in less than 10 cm lies the scintillating fiber and Tungsten Calorimeter. Interleaved with the fiber layers, g/cm 2, 3.5mm thick Tungsten layers provide further regions for nuclear interaction and the conversion of shower products into detectable particles. The same scintillating fiber and HPD setup is used in the Calorimeter as for the Hodoscopes (see Section 3.7). The 0.5mm diameter fibers, on the order of the Moliére radius ( 0.9cm), are grouped into ribbons of 19 1 cm 50.3 cm fibers each, with 50 ribbons in one layer. Scintillating ribbon layers alternate orthogonal orientations to provide stereoscopic tracking. UVT acrylic light guides pipe the light from the flat ribbons to bundles of mm diameter light-tight fibers coupled to the HPDs. Figure 3.19 shows the fiber bundles extending from the side of the Calorimeter to the HPDs (box on left). Neutral density filters and optical division of the fibers into sub-bundles of 37, 5, and 70

87 Figure 3.19: The assembled Calorimeter. Fiber bundles carry the scintillator ribbon light to the HPDs, seen on the left side of the picture. [62]. 1 fiber provide low, medium, and high range read outs, respectively. The limited linearity of the digitization ASICs drives this requirement, rather than the HPDs, which are linear to an order of magnitude beyond the expected range. Each HPD reads out all ranges on one side of one layer (as for the Hodoscopes, only one end of each ribbon is read out, and that end alternates between sides for neighboring ribbons), utilizing 55 of the 73 available pixels. 3 more are designated for LED alignment and in-flight calibration while 6 are digitized with no optical input to aid pedestal determination. The Calorimeter power module contains three Vicor VI-J00 series DC/DC converters with VI-RAM active output filter modules which power the Calorimeter, the 71

88 Hodoscopes, and the SCD. The Calorimeter receives ±3.5V and 12V for the electronics and HPDs while the Hodoscopes use ±6V and 12V. All high voltage circuits and sections of the HPDs are potted to avoid surface charge accumulation, which can lead to coronal discharge and part failure Calibration and Validation Pedestals collected every 5 mins during flight allow monitoring of the pedestal drift, determined to be 12 ADC counts/ C on average. To fit the limited downlink bandwidth, recorded data must be a settable amount above the pedestal. This limit enabled transmission of all high energy data during the flight. Periodic injection of known amounts of charge into the Calorimeter electronics allows subsequent normalization of their relative gains. Scanning the Calorimeter ribbons with a 150GeV electron beam at CERN in 2003 permitted normalization of gain differences caused by both electronics and ribbon photon and HPD efficiencies. This required rotating the detector preceded by combinations of Tungsten and Lead absorbers to illuminate all Calorimeter layers and accounting for the energy dependence of the longitudinal shower profile with well-fit Monte Carlo simulations. Further details may be found in [63]. By applying the inter-ribbon response corrections adjusted linearly for flight HV gain differences and subtracting the pedestal, corrected on an individual event basis for coherent noise, the CREAM I Calorimeter response uniformity improved from 4.8% to 3.8% [64]. Exposing the Calorimeter to beams of nuclei ensured a reasonable Calorimeter response at lower energies. Monte Carlo simulations enable extrapolation to cosmic ray energies beyond ground-based accelerators reach. Protons, the most populous of 72

89 cosmic rays, had a recorded energy deposition of 0.15% at 350GeV with resolution of 46%, and a resolution of 68% at 150GeV. The Calorimeter was also placed in the Indium fragment beam and showed energy deposition consistent with protons for all nuclei and similar energy resolutions, e.g. 46% for 1.9 TeV (total energy), SCD-selected Carbon showers initiated in the graphite target. A similar selection of 8.2TeV Iron had an energy resolution of 21% and agreed with the Monte Carlopredicted energy deposition. The decrease results from increases in shower fluctuation with energy. Figure 3.20 shows the Calorimeter response linearity to the increasingly energetic beam fragments, ranging from mass number 1 (protons) to 58 (Nickel). The scale on the right shows the correspondingly linear incident particle energy while the data is completely consistent with Monte Carlo simulations for Helium, Oxygen, Lead, and Iron (A = 4, 16, 30, and 52), adding confidence to its use. Once the ribbons are calibrated, their low-, medium-, and high-gain ranges are stitched together with flight data, the next-highest gain measurement continuing the linear response just before the previous one saturates Beyond CREAM I CREAM II flew with a nearly identical copy of the CREAM I calorimeter. Difficult Antarctic payload recovery conditions warranted mechanical modularization of the layers, which proved their usefulness in the following flights. Further information may be found in [62]. For CREAM III, the CREAM I Calorimeter was refurbished with improvements made to the optics, electronics, and mechanical systems. The addition of trays and rods for transporting the Calorimeter modules further eased 73

90 Figure 3.20: The deposited energy measured by the Calorimeter (left) shows a relatively linear response to nuclei with increasing energy from Indium beam fragments. The scale on the right corresponds to the also linear incident particle energy, ranging from 1 to nearly 10TeV. Blue crosses mark results of Monte Carlo simulations for Helium, Oxygen, Lead, and Iron (A = 4, 16, 30, and 52), well predicting the data. [65]. recovery. Switching to multi-clad scintillator fibers increased the scintillation signal by up to 60% by adding a second boundary with even lower refractive index, increasing internal reflection [66]. Two electronics adjustments decreased noise levels for CREAM III and following flights: a 16-bit ADC replaced the 12-bit ADC and the ASIC reference voltage was used only to maintain a stable pedestal over varying temperatures. The data record size, being already set up as 16-bit, did not grow. Thermal testing showed a decrease 74

91 in pedestal variation by almost an order of magnitude, to 1.4 ADC counts/ C. The new circuit s pedestal noise also decreased by about an order of magnitude. Combined with an increase in signal intensity, this allowed lower thresholds, increasing the Calorimeter s sensitivity to lower energy showers without increasing the noise in an event or the number of noise-only events. CERN electron beam testing again showed a linear response to increasing energy and also evidenced improvements in energy resolution and uniformity. Further discussion may be found in [67]. These improvements, applied to Calorimeters flown on CREAM III and beyond, improve the spectra measured for all nuclei both in range and resolution Calorimeter Trigger The Calorimeter triggers on high energy shower events, defined as signal above threshold in N consecutive layers, where both N and the threshold are set in flight. N was typically 6, and the threshold ranged from 60MeV for CREAM I to 10 MeV for CREAM II, and lower for the subsequent lower-noise flights (see preceding subsection). The threshold is applied to the low-gain channel of every ribbon in a layer. In case of malfunction, layers can be set to appear always on. The number of events (and their record size) is low enough to allow downlinking of all high energy data, ensuring that it is retained even in the event of instrument recovery failure. 3.9 Master Trigger The master trigger accepts the subdetector triggers initiated by the TCD or Calorimeter and initiates data collection from all subdetectors. A comparison of the event number sent to all the subdetectors and included in their data verifies the collation of all the data. To maintain trigger rates appropriate to both science goals 75

92 and downlink and disk space limits, prescalers allow retention of only a settable fraction of each trigger type, except for the calibration trigger, which triggers only on command. In general, the Calorimeter trigger rates did not need prescaling, while of order 15% of the TCD ZHi and ZLo triggers remained CREAM on Ice The instrument is integrated and tested before being shipped to Antarctica, where we verify all science and support systems a final time in tandem. Launch occurs after the establishment of the Antarctic polar vortex, under clear, low wind conditions, as seen in Figure We check initial flight performance and tune the instrument once it has reached float altitude ( 120,000 ft) and before the payload disappears over the horizon. This line of sight (LOS) period allows us to use faster, higher bandwidth radio communication to transmit instructions to and data from the instrument. At the end of LOS, communications travel via satellite to the US-based command and monitoring centers. The following sections describe the testing and tuning procedures for the CREAM III TCD AND CD, for which the author had primary responsibility on the Ice. The other detectors were verified in a similar fashion using muons and pedestal runs prior to launch, and their relevant thresholds and other parameters adjusted for optimum performance following the TCD and CVD s during LOS. In the final section, we report on the recovery efforts, in which the author also directly participated Integrated TCD and CD Ground Test The TCD and CD are tested following integration with the main instrument using muons from cosmic ray air showers. We use the following method both prior to 76

93 Figure 3.21: The CREAM III instrument suspended from the Boss launch vehicle s crane, with the science team in the foreground and the balloon during inflation in the background. shipping and following arrival in Antarctica. After the TCD is powered and the high voltages are turned on or ramped up for the TCD and CD, respectively, we trigger on thousands of muon events. We then check that all CD ADCs have data and that the lower threshold TCD ADCs and TDCs have events. The cosmic ray muons do not have enough energy to create signal in the highest threshold TCD dynodes, so we do not expect signal beyond pedestal in them. The comprehensive performance test (CPT) expands on this limited performance test (LPT) by verifying that the ZHi trigger rate responds appropriately to changes in CD threshold and HV levels. A 77

94 higher than standard CD threshold and lower CD high voltages both correspond to lower trigger rates, while a lower threshold increases the trigger rate. Lower CD high voltages also decrease the spread in CD data by requiring higher energy muons to trigger the system. Tables A.1 and A.2 contain step-by-step instructions for both the limited and comprehensive performance tests for CREAM III. Color coding delineates the portions prescribed for each subdetector. The following figures show the results of the LPT performed just prior to launch 19 Dec With only 3,000 events, we can verify the aliveness of all the TCD s PMTs lower ADCs, seen in Figures 3.22 and 3.23 for ADCs 0 and 1, respectively. ADC0 peaks are all centered, as designed, around 1,000 channels. We do not reproduce here the higher ADCs, associated with shallower dynodes, because, as expected groundlevel muons do not have enough energy to produce a substantial signal in them. This can be seen in Figure 3.24, which shows the progression of deeper to shallower dynodes for S3. We finally verify that the CD PMTs all have signals similar to their sum, reproduced here in Figure Initial TCD and CD Tuning The initial CREAM III settings were chosen to minimize the time spent tuning the instrument in the air (during LOS and overall) and thus utilized experience gained from the previous two flights. Tables 3.3, 3.4, 3.5, and 3.6 show the ground and initial values set for each of the TCD, S3, and CD thresholds and high voltages. The TCD thresholds 0 and 2 were chosen to correspond to the desired minimum charge for their respective trigger (ZLo and ZHi). Thus threshold 0 was set to 0.2 MIP, corresponding to Hydrogen, while Boron corresponds to roughly 7MIP for threshold 2 and the ZHi 78

95 Figure 3.22: Response of all TCD PMTs ADC0 to ground-level muons demonstrates detector aliveness. All peaks are centered at 1,000 channels, as designed for response uniformity. trigger. Thresholds 1 and 3, being not included in a trigger, were set at 1 and 3.5MIP respectively to fall between the low and high Z triggers. 79

96 Figure 3.23: Response of all TCD PMTs ADC1 to ground-level muons demonstrates a minimal level of detector aliveness. Once the instrument reaches minimum float altitude, at about 35km, high bandwidth line of sight commanding is utilized to turn the instrument on and perform 80

97 Figure 3.24: Response of all the two S3 PMTs ADCs (upper and lower rows) to ground-level muons demonstrates detector aliveness. The highest ADCs in the rightmost plots correspond to the shallowest dynodes, which the muons are generally not energetic enough to create a signal in. initial tuning. Since LOS is limited to the time before the instrument passes beyond the horizon, initial TCD and CD tuning procedures use only about 10 times 81

98 Figure 3.25: We verify aliveness and response by ensuring each of the CD s PMTs ADC signals match the sum, shown here. TCD Threshold TCD Trigger PMTs (TDC) Channel MIP (minimum element) ZLo (H) not included ZHi (Li/Be/B) not included Table 3.3: Initial flight threshold settings for the TCD. 82

99 S3 PMT Threshold MIP Trigger (TCD) (TCD Channel) (minimum element) ZLo (TCD internal) not included Table 3.4: Initial flight threshold settings for S3. High Voltage PMT TCD S3 pairing: HV (TCD 0, 1 2, 3 4, 5 6, 7 8, 9 10, 11 12, 13 14, 15 16, 17 Channel): Table 3.5: Initial flight High Voltage settings for the TCD and S3, based on adjusting the muon peak to 1,000TCD channels. 1 TCD Channel 1mV. the data of the ground-based L and CPTs. Trigger rates are first checked to remain below the 20Hz system readout maximum. If the ZHi trigger exceeds 20Hz, the CD threshold is raised to remove more of the Helium tail; Helium is about 26 times more abundant than Oxygen. If the ZLo trigger rate exceeds the maximum, the S3 threshold is similarly raised to remove more of the low energy and noise events. With a properly triggering instrument, the thresholds are then adjusted to ensure all relevant elements are visible in the appropriate peak detector scale for the given 83

100 CD TCD Channel Volt MIP Threshold mv 1 HV HV Table 3.6: Initial flight settings for the CD high voltages and threshold. triggers. For the ZHi trigger, the TCD threshold participating in the trigger (2) is raised until there is no Helium peak in the second shallowest dynode (ADC 1) for all TCD PMTs. Similarly, if the minimum between Carbon and Helium is not on scale, the threshold is lowered. Given enough data, Boron replaces Carbon in the tuning procedure. If the TCD threshold is lowered below its noise-triggering value (350 channels), we lower the CD threshold, return the TCD threshold to its starting value, and begin the TCD tuning procedure again. Figure 3.26 shows the result of a reasonable tuning for the ZHi trigger in the CREAM I TCD PMTs ADC1. The equivalent plot for CREAM III is unavailable as no scientifically relevant trigger was ever found following the HV failures (3.2.5). The ZLo trigger collects Hydrogen and Helium, which should be on scale for the deepest TCD dynodes. As for the ZHi trigger, we adjust the participating thresholds, TCD threshold 0 and S3 threshold 0, until both Hydrogen and Helium peaks are on scale, while maintaining them above the noise floor ( 75channels). We must also ensure that the heaviest desired elements are on scale. For this we turn to Iron, the most abundant of them in the CREAM range, which should appear most strongly in the shallowest dynodes of the TCD s PMTs (3), and also appears 84

101 Figure 3.26: The ADC1 values for the CREAM I TCD PMTs show reasonable tuning for the ZHi trigger, with the Carbon, Oxygen, and Iron peaks visible particularly in PMTs 0, 2, and 13. PMT 11 had an odd electrical failure mode also involving a dead anode, and so should be ignored. ([44] has further details on the failure). in the deeper dynodes albeit outside their linear response range, as seen in Figure If it is present but saturated in the TCD, we can lower the TCD high voltage 85

102 slightly, being careful to keep the lighter elements (e.g. Boron and Carbon) on scale in the deeper dynodes, and retuning if necessary. If it is not present at all, we lower the CD high voltage to allow triggering on the more-energetic elements, since the CD response is proportional to the incident charge. It can be advantageous to maintain the two CD high voltages at slightly different values in order to have greater dynamic range in the ZHi trigger. Tuning for the heaviest elements may be done during the line of sight commanding period immediately following launch, or it may be shifted to later in the flight, allowing other subdetectors to tune while the data flow rate is maximal. As the actual duration of line of sight depends on local weather conditions and the Timing Charge and Cherenkov Veto Detectors needs must be balanced against the other subdetectors, the decision to finish tuning is made in situ. For CREAM III, having spent several hours exploring various options for reviving the TCD and CD, including slowly ramping up the HVs and holding them at lower levels and altering the CD and S3 thresholds, we turned to tuning the other subdetectors during the LOS. With a working TCD, we accumulate 100,000events in about 0.6days and may commence fine tuning. A plot of the TCD dynodes versus the CD s will show individual charge regions corresponding to an identical charge and energy measurement made by both detectors for a given event. Figure 3.27 is an example of this from the CREAM I data for only 4,300events; with more events, we can tune more carefully. The TCD versus CD signal plot can also reveal regions where one detector may be measuring charges which should be off scale. For instance, if the CD has Helium events in a plot versus the TCD ADC 1, we raise the CD threshold to eliminate 86

103 those events, while bearing in mind that some tail from the Helium distribution is inevitable. Likewise, if Boron is not on scale on the CD or TCD axes, the corresponding threshold should be lowered. At this point it should be clear if the increased dynamic range in the ZHi trigger gained from having two different CD high voltage values is necessary. Tables A.3 and A.4 contain the CREAM III flight documentation for the tuning procedure. Figure 3.27: The sum of the TCD s ADC0 signals versus the summed CD s PMTs ADC signals from events within the center of the detector clearly show large numbers of Carbon and Oxygen incident cosmic rays (lower and upper large red spots, respectively). Below each of the Carbon and Oxygen nuclei lie the less populous but still visible Boron and Nitrogen nuclei which we can use for fine tuning. These events were selected by having signals in the four PMTs defining a pair of crossed paddles, as seen in Figure

104 Following LOS, we returned to attempting to obtain useful data from the TCD and CD on CREAM III. We tried gradually ramping up the HVs, having waited sufficient time for a slow current to, in effect, heal any burned out electronics as occasionally happens, but the HV did not remain steady, instead falling slowly to 0 from an already low value. We then set the S3 and CD triggers to always On, in hopes that there would still be some relevant data from a TCD-only TCD trigger (3.2.3). Unfortunately doing so only allowed the low energy cosmic rays particularly abundant at the polar regions due to funneling in the Earth s magnetic field to swamp the desired high energy ones. So despite our further attempts at reviving the TCD, CD, and S3 system, we were unable to obtain any scientifically relevant data during the CREAM III flight. Section enumerates the steps we took to diagnose and prevent the failure, most likely due to a microscopic disruption in the HV wires insulation following a shipping accident, from occurring in future flights. All of the detectors performed to specifications for the CREAM IV flight and have shown no signs of catastrophic failure for CREAM V, presently preparing to fly from Antarctica Flight The data are continuously monitored throughout the flight, and adjustments made as necessary to maintain high quality while the payload circumnavigates the Antarctic continent. Figure 3.28 shows the path CREAM I took following its launch from Williams Field just outside the US base of McMurdo Station on 16 Dec 2004 through its three revolutions to its landing on 27 Jan Figure 3.29 plots the CREAM I altitude throughout its record-breaking 42days flight. 88

105 Figure 3.28: CREAM I circumnavigated Antarctica three times, with the first circuit s path traced in red from McMurdo Station, then second in green, and the third in blue. CREAM I landed on the far side of the Transantarctic Mountains from the US base. We use the high statistics thus obtained to good effect in our analysis (Chapter 5). CREAM II launched exactly a year later, and flew 28days, until 13 Jan CREAM III also flew for 28days, from 19 Dec 2007 until 16 Jan 2008, and CREAM IV launched a year after CREAM III, flying until 1 Jan 2009 for a total of 19.5days. 89

106 Figure 3.29: The altitude of CREAM I, shown in days following launch, held steady around 37km for the majority of the flight. The changing angle of the sun causes diurnal temperature variations which slightly change the density of the surrounding air, and thus the balloon s altitude. Aside from their shorter durations, the following flights had flight paths and altitude histories similar to CREAM I in the relevant aspects Recovery Approximately one month after it establishment, the polar vortex begins to subside. This, combined with resources further limited by the consequences of dwindling daylight hours, sets the time for termination of the flight. Since CREAM flies, and 90

107 is scheduled to fly, nearly consecutively for a total of ten flights, recovery of the instrument as intact as possible is critical for permitting sufficiently rapid construction of the following flight s detectors using refurbished and reverified portions of the old detector(s). The payload is terminated near a manned base, preferably as close to the main US base of McMurdo as possible, and is typically accessed with a Twin Otter airplane. This has been the the case for all CREAM flights thus far. As the payload is too large and heavy to fit whole into the aircraft, science crew members, accompanied by a safety person, travel to the landing site and disassemble the instrument. During the first recovery flight, the parachute and balloon pieces are typically stripped from the payload under balloon personnel supervision and returned to the base. As possible, the exterior components of the payloads, such as the antennae and Command and Data Module, are also removed at this point and transported back to the base. Such was the case for the CREAM III, in which the author participated. During the final recovery flight, the instrument was fully disassembled into its subdetectors for the CD, CherCam, and SCD, which are light and compact enough to move as a unit. The TCD, removed first along with the exterior electronics boxes, separates most easily into its component pieces, and, as described earlier (3.2.1), benefits from low-temperature brittle glue. Separating the payload between the CherCam and SCD allowed us to work in teams, removing both simultaneously. Rapid disassembly can be a major benefit under often bitter Antarctic conditions, as it proved for the CREAM III recovery. The Calorimeter and its targets, being particularly heavy, have thus benefited from hardware specially designed to easily and securely remove layers from from the detector (3.8.3). These worked well during the CREAM III recovery, 91

108 though another noteworthy lesson from the recovery is that excessive amounts of tape can majorly hinder safe, rapid recovery. We continue to improve our recoverability by sharing such information and finding improved solutions. Figures 3.31 and 3.30 shows images of the CREAM III payload after landing and during the final recovery stage. Figure 3.30: CREAM III landed on the Ross Ice shelf, about 3hrs south of McMurdo in a Twin Otter. As seen in this image recorded shortly after arrival at the landing site, everything was relatively intact. The payload tipped away from the solar panels, as had also occurred for previous flights. Photograph courtesy of S. Nutter. 92

109 Figure 3.31: Most of the way through the second day s recovery, we had disassembled everything except the Calorimeter, seen as the black rectangle between the people. The TCD paddles with PMTs attached are the black U-shaped objects lying in the lower left of the image while the CD, upper pallet structure, SCD, and CherCam lie in a diagonal from the bottom right corner towards the Twin Otter airplane. As is apparent in the image, conditions were cloudy, windy, and thus rather cold. Photograph courtesy of S. Nutter. 93

110 Chapter 4 Analyses of CREAM Cosmic Ray Data Once we have recovered our data, we perform the analyses described in Sections 4.1 and 4.2 to turn the first two flights raw signals into calibrated charge and energy measurements. For CREAM I, we use the TRD as our primary energy detector and the TCD and SCD in combination for our charge measurement. Signals from the CD extend our lowest energy range, and we have performed the first, balloon flight cross-calibration of a TRD and Calorimeter with excellent results. CREAM II utilizes the Calorimeter and SCD to determine the charge and energy for both Calorimeter and TCD-triggered events. Section 4.3 describes a technique for simultaneously extracting charge and energy from the TCD and CD which in the final analysis revealed important information about the inclusion of δ-rays when measuring energy with the TRD and CD. In the final section, 4.4, we use our complementary charge detectors aboard CREAM I to examine the number of cosmic rays which undergo spallation within the detector. As few measurements of cross-sections exist at these energies for nuclei such as Nitrogen and Oxygen, and models are therefore less well tested, this analysis provides a unique glimpse of the nuclear physics occurring within our instrument. The author worked in particular on these two analyses. 94

111 All data used in the analyses were gathered during stable instrument operation periods, excluding periods such as high voltages adjustment, generally during tuning at the beginning and shut down at the end of a flight; when the TRD gas was redistributed; and of intense solar flaring. The last occurred only once with substantial enough magnitude to disrupt data taking during the flights of CREAM I and II, on 20 Jan 2006, and was one of the most intense flares ever observed [68], [69]. 4.1 Charge Determination Accurate initial tracking permits us to select the associated TCD region or SCD segment most likely to contain the energy deposited by the incident particle without confusion from Calorimeter albedo. It also allows us to correct for non-uniformities in the detectors responses and for the distribution of pathlengths traversed by particles of differing incident angles. With gain-matching, a relativistic energy requirement, and attenuation and other non-linearities also accounted for, we obtain charge resolution for Z 4 of an excellent 0.2e. The following subsections describe these steps in greater detail Tracking: CREAM I TRD The initial TRD track is determined with a fast, iterative, least-squares minimization procedure applied to the corrected TRD tube signals. After removing the pedestals described in Section 3.3.2, the high- and low-gain ranges were combined and the tubes signals corrected for gain variations due to temperature and pressure fluctuations, the former including diurnal variations and the latter due to gas loss and subsequent rebalancing. The gains were matched to 3% across the entire flight, and further details may be found in [54] and [40]. 95

112 The least-squares minimization procedure uses all corrected TRD tube signals to calculate a track in the first iteration. In the next, only signals from tubes within 12cm of the first track are included in the track calculation. A third iteration, decreasing the envelope of accepted signals to 6 cm ([44]), provides tracks with a precision of 5mm in both the xz- and yz-planes as measured by the full-width, half-maximum of the error found in a Monte Carlo simulation using the identical tracking method. This Monte Carlo was further confirmed by using the TRD tracks to identify the edges of the TCD paddles, which appear with lower TCD signal than events which pass through the complete thickness of a TCD paddle. [70] and [53] contain further details. Each event s track is improved by further constricting the envelope to 2 cm and by requiring any signal used in the fit to have a magnitude within 2σ of the average of the previous fit signal after correcting for pathlength differences. The latter is a requirement of the uniformity of the energy loss in each tube and the precision with which it can be determined. It also helps eliminate albedo as signals therefrom are generally lower in magnitude and tend to be isolated, as indicated in Section 3.3). Taking the xz and yz projections as independent and comparing them yields an RMS difference of a reasonable 8% for through-going Oxygen nuclei [70]. Finally, we included a Minuit algorithm incorporating the 3-dimensional TRD structure and using a χ 2 minimization to produce the track with signals most consistent with the expected, pathlength dependent signal. These track or event geometry parameters are used in the spatial non-uniformity or mapping corrections applied to the TCD and CD. 96

113 With the charge determined, it is incorporated into a final tracking procedure which uses a maximum-likelihood fit in place of the previous χ 2 minimization. This permits the inclusion of the differing energy loss distributions for differing charges and impact parameters. Such flexibility allows us to account for, e.g., the variation in energy loss between paths through mostly gas and those through mostly glass TRD tube wall, seen by particles which merely graze a tube. With tubes separated by about 3 cm, including such effects as edge clippers substantially improves the tracking resolution. The resulting 1mm RMS tracking also improves our eventual energy determination. ([54] contains further details on the tracking procedure.) Charge Identification: TCD Several steps are required to obtain a measure of the incident particle charge from the TCD, including pedestal subtraction, linearization and stitching, and attenuation and relative gain corrections, both in time and in among the PMT s dynodes. Since the TDC (timing) and ADC0 (peak detector) measurements are primarily for Hydrogen and Helium and the analyses present here focus on Boron and above, we refer the reader to, e.g., [71], and only delineate the process used to obtain charge from the highest three peak detectors (ADCs 1-3) (Section 3.2.3). After subtracting the pedestal of 425 ADC channels, each ADC s dynodes are corrected for non-linearities using a quadratic function phenomenologically fit to the data. The dynodes on each PMT are then shifted and scaled so that they may be equitably conjoined. This stitching selects the dynode with the most-linear, linearized response for each event, switching to the next highest dynode as the signal increases. 97

114 TRD tracking gives us the position-based quadratic normalization used to flatten the TCD response map. The final gain corrections come from scaling each PMT so that Oxygen events always give the same magnitude signal and by a phenomenological quadratic fit to the temporal signal variation. The initial Oxygen selection also required a minimum signal in the Cherenkov Detector to ensure the particle was relativistic. We again use the TRD tracking to find the four PMTs reading out the ends of each of the two paddles in the x and y scintillator layers. These PMTs signals, divided by their mapped attenuation and gain corrections, comprise the final TCD charge. The signals in each direction are averaged if they both exist. Otherwise only the extant PMT s signal forms that layer s charge variable. Particles incident on the detector from varying angles have differing path lengths through the detector material. This leads to an increase in signal for particles arriving at shallower angles. We correct for this by dividing the signal by cos(θ), where θ is measured from the detector plane and determined with the TRD tracking. To first order, the signal from each layer is proportional to Z 2. Remaining discrepancies are accounted for by matching the measured charge peaks (S) to their known Z through the phenomenologically determined function: ( ) S Z =. (4.1) cos(θ) An event s resulting x and y layer scintillator signals are averaged to form the final TCD charge determination. Further discussion may be found in [53] and [70]. Figure 4.1 shows the excellent charge resolution we obtain, even clearly separating the lower flux secondaries such as Florine, Calcium, and Aluminum from their somewhat more abundant primary counterparts (Neon, Magnesium, and Silicon). 98

115 Including a cut on the Cherenkov detector signal ensures that the incident cosmic ray is relativistic, providing further uniformity of response and thus better charge resolution. For Oxygen, we measure an RMS resolution of 2% [53]. Figure 4.1: Charge as determined by the TCD after linearization, attenuation correction, gain-matching, and path length correction for relativistic incident cosmic rays. The lines represent Gaussian fits to the peaks, denoted with red triangles. [53] Tracking: CREAM I, II Calorimeter An independent analysis uses a linear χ 2 fit to determine a Calorimeter track. The raw calorimeter data first has its pedestals, varying with temperature and described 99

116 in Section 3.8.2, subtracted. As measured in flight, noisy or inefficient ribbons are masked off and coherent noise removed before correcting for nonuniform light output and HPD gain variation based on our beam tests (Section 3.8.2). For CREAM II, only 0.7% of all calorimeter channels were masked [72]. The readout gain ranges are also normalized using flight data such that the medium range channel s signal is equivalent to the low range signal just before the read out electronics saturate and lose their linearity. As the optical division of fibers into the low and medium ranges is 37 and 5, respectively, to first order the normalization constant is 37/5 = 7.4, with corrections based on matching the actual flight data signal ranges. The high range signal, corresponding to the highest energy events, is generally not needed in the following analyses as the flux of these events is quite low. Candidate Calorimeter track points are then selected by finding in each layer the maximum signal above a threshold equivalent to an energy deposition of 10 MeV. The signal-weighted average of the location of the maximum ribbon and its two neighbors localizes the candidate point. For a set of candidate points, a χ 2 minimization procedure then computes the best track, with quality ensured by requiring at least three candidate points for any event s track and a final χ 2 < 10. We then use the SCD to improve the track. The track is projected back to the SCD with an estimated resolution of 1cm; events with tracks lying outside the SCD geometry are excluded. Conservatively examining a circle of 3cm radius centered on the projected hit location and covering about 13 pixels, we find the SCD pixel with maximum signal. We use this signal to calculate the incident charge. By including its SCD pixel location (or locations, one each for the top and bottom SCDs for all flights following CREAM I) with the candidate track points and refitting, we measure the 100

117 track at the top SCD with a resolution of 7mm RMS [72]. Over 95% of events with energy over 3 TeV have well-reconstructed Calorimeter tracks using this method, even if they had insufficient energy deposition in the Calorimeter to trigger it but instead were recorded via the TCD trigger as described in Section 3.9 [72] Charge Identification: SCD Prior to determining the charge, the SCD signal is first pedestal-subtracted using values obtained every 5 minutes during flight, with variations correlated with temperature changes, as described in Section Again noisy and inefficient signals were masked off, resulting in the loss of only 1.7% of the CREAM II SCD channels [72]. The remaining pixels are corrected for variations in gain. As mentioned in the previous section (4.1.3), the Calorimeter track can be used to select the SCD pixels with highest signal within a conservative track resolution circle. Likewise, if using the TRD track, the highest signal SCD pixel must be within 2cm of the TRD-projected hit location, again about 3 times the track resolution. This type of requirement successfully eliminates the majority of albedo contamination as the track is generally insensitive to albedo and the cosmic ray signal exceeds the albedo s in the finely segmented SCD. Using either the Calorimeter or TRD tracking, the SCD signal S is corrected for path length by multiplying by cos(θ), where θ is the angle between the vertical instrument axis and the incident particle s track. Since the SCD signal (S) should be proportional to the charge squared (Section 3.6), the signal is normalized such that S cos(θ) = Z (4.2) n 101

118 where Z, determined from the observed charge populations locations and relative intensities, also known to first order from previous experiments, allows us to determine the normalization constant n. For CREAM II and beyond, the top and bottom SCD signals are averaged as well as path length corrected and normalized. Further, we improve our results by require a level of consistency between the measured top and bottom SCD signals in the later flights. Given the limited statistics, both at higher charges and energies described below, we do not attempt to increase our charge resolution by limiting our data set with restrictions on θ beyond those already imposed by the instrument geometry. Figure 4.2 shows the SCD charge reconstructed with Calorimeter-determined tracks in the manner thus described and with a top to bottom consistency requirement of 20%. A multi-gaussian fit to the elements from Boron to Silicon yields charge resolutions of 0.2 to 0.23e, as listed in Table 4.1. Statitistics limit the resolution for higher charges, particularly above Sulpher, so for Iron, the most-populous of the heaviest elements, we estimate the resolution from the peak width, seen in Figure 4.3, at 0.5e. More details may be found at [72]. Z: C N O Ne Mg Si Fe σ (e): Table 4.1: SCD Charge Resolution using Calorimeter tracking for CREAM II data. [72] 102

119 Figure 4.2: CREAM II SCD charge reconstructed using calorimeter tracking and requiring top and bottom SCD signals to be with 20% of each other. The lines show the results of a multi-gaussian fit to Boron through Silicon and yeild the charge resolutions listed in Table 4.1. [72] 4.2 Energy Calibration The Cherenkov Detector As for the TCD, we use the TRD tracking to remove spatial variation across the the Cherenkov Detector. We first correct each of the eight PMTs at the ends of the wavelength-shifting bars (see Section 3.4) for its variation with time. As the Cherenkov Detector is not segmented in any way, each PMT views the entire detector 103

120 Figure 4.3: CREAM II SCD charge for Sulphur to Nickel reconstructed using calorimeter tracking and requiring top and bottom SCD signals to be with 20% of each other. Low statistics prevent a multi-gaussian fit. Instead, the charge resolutions for Iron as listed in Table 4.1 is estimated from the peak width. [72] area. The variations in signals detected in a given region on the detector therefore differ for each PMT, so we apply a separate mapping correction to each PMT s signal based on the event s TRD track location. Each map is derived from a single charge species and the response flattened by normalizing to the mean signal from all PMTs. This accounts for intra-pmt gain variation. 104

121 Figure 4.4 shows the results of flattening the detector s signal for a selection of Oxygen nuclei. Prior to corrections, the response uniformity was 20%; as flattened, the response is uniform to an excellent 2% across 95% of the detector area [53]. Figure 4.4: The Cherenkov Detector s response to a selection of Oxygen nuclei after flattening corrections. The response is uniform to an excellent 2% across 95% of the detector area. [53] The signals from all PMTs are summed and path length corrected. As the PMTs are generally linear through the range of signals generated by the majority of the incident cosmic ray s charge and energy (maximized for relativistic particles as β 105

122 approaches 1), we need only apply a minor linearization, of the form: S Linear = 7.992S (4.3) The resulting signal should now be proportional to Z 2 and the incident particle velocity as in Equation 4.6. Section 4.3 explores this in greater depth while the following section describes our final method of utilizing the Cherenkov signal The TRD We corrected and combined the TRD signals as described in Section to obtain the excellent tracking reported therein. We then include each event s mean signal in a χ 2 minimization procedure to determine the incident cosmic ray energy with the best precision over the broadest range of energies. To do this, we use a Minuit fitter to minimize the difference between the measured signals from the TRD, TCD, and CD and that expected from the instrument Monte Carlo, weighted by the signal resolution as: ( ) 2 ( ) 2 ( ) 2 χ 2 STRD S TRD,MC STCD S TCD,MC SCD S CD,MC = + +. (4.4) σ TRD σ TCD By computing the three Monte Carlo signals over a range of energies, we are able to step towards the energy which best fits all available data. An initial estimate of the energy is made by determining the lowest χ 2 for the range of energies log 10 (E/n) = 0.5 : 5 in steps of 0.05 with energy E measured in GeV. We then do a parabolic interpolation of the minimum χ 2 and energy with the χ 2 values for the two surrounding energies: log 10 (E/n) ± The critical point of this parabola becomes the starting energy for the fitting algorithm. The TCD and TRD resolutions are relatively constant with energy, at 0.06 and 0.08 respectively, and improve gradually with increasing energy. Figure σ CD

123 graphically shows the TRD signal resolution in the relativistic rise region and its mapping to resolution in Lorentz factor, equating to σ γ The CD signal resolution is calculated with the Monte Carlo and rapidly worsens, as expected for β 1. In this manner, the resolution weighting limits the major CD contribution to the lowest energy bins, 2GeV/n, where it is critical for breaking the degeneracy in TRD signal seen in Figure At the highest energies, x-ray transition radiation begins to impact the signal resolution by causing increased fluctuations around the mean. Low statistics prevent extension of our measurement at these energies, so we have not at this time extended the analysis to include the transition radiation; neither is it modeled in the Monte Carlo. Instead, we limit our highest energy bin to 4 TeV/n to maintain reasonable statistical errors. Further details on the Monte Carlo and energy determination may be found in [54] and [44]. Deconvolution We gather events into energy bins with width δe = , about 1.3 times that of the energy resolution. This finite resolution means there will be some spill over from one bin into its neighbors. For a flat spectrum, this effect would wash out, as an equal number would enter a bin as leave it. However, for a spectrum with slope, and particularly for the steeply falling cosmic ray spectrum, a greater number of low energy cosmic rays will shift into higher energy bins, resulting in an overall shift in the spectrum, as seen in Figure 4.5. Deconvolution allows us to remove this shift. Since our energy resolution does vary some with energy, we extend our TCD, TRD, CD, and SCD Monte Carlo simulation to generate events which are passed through identical analysis procedures (and code) as the data. We then create a transfer matrix 107

124 Figure 4.5: Example of the effect of finite energy resolution on a cosmic ray-like spectrum. Deconvolution shifts the spectrum to its original location. [73] T ij which maps the number of events a measured in an energy bin (m i ) to the (Monte Carlo) true number of events (n j ) as: n j = I T ij m i j = 1,..., J (4.5) i=1 where I and J are the maximum measured and true energy bins, respectively. As noted previously, the Monte Carlo simulation accurately reproduces all beam test measurements as well as flight data average signals and their fluctuations. 108

125 The deconvolution depends on the simulated spectral index, which must match the unknown measured spectral index. To ensure this, we iterate through several simulation indices and rapidly converge on a spectral index which accurately reproduces the measured spectral index. Small residual discrepancies did not significantly affect the final results, as observed in, e.g. [74] and [72]. We then apply this transfer matrix to our data to most accurately measure the energy spectrum The Calorimeter As described in Section 3.8, the Calorimeter samples the electromagnetic core of the hadronic shower initiated in the graphite targets or Tungsten plates above it. The raw data is pedestal-subtracted and gain-matched, and the read out ranges combined prior to developing the most probable track, as related in Section Using this track, the signals were also path-length corrected and then summed for a total calorimetric energy deposition measurement. Given the particle s charge (e.g. from the SCD, Section 4.1.4), we can map the deposited energy to the most probable incident energy via Figure As noted in Section 3.8, we use a Monte Carlo simulation to extend the CERN beam calibration up to hundreds of TeV, where the map finally begins to deviate from linear behavior, coincident with our lack of statistics. Deconvolution The FLUKA Monte Carlo was further augmented to allow a deconvolution of the energy spectrum in a manner similar to that for the TRD-based measurement (Section 4.2.2). Again isotropically incident cosmic ray nuclei with power-law spectra were generated and passed through the Calorimeter analysis procedures to obtain 109

126 the transfer matrix. As noted in previous sections, the Calorimeter Monte Carlo has been finely tuned to reproduce beam test measurements and flight data. Figure 4.6, showing energy deposition as measured in flight and simulation for Carbon nuclei, exemplifies the excellent agreement with flight data. Figure 4.6: Energy deposited in the Calorimeter for a selection of Carbon nuclei as measured with flight (points) and Monte Carlo simulation (histogram) data, demonstrating the excellent agreement of the two. [72] We apply the transfer matrix thus determined to our data. We checked that the results were robust to the assumed Monte Carlo spectral index by scanning several 110

127 values within ±0.2 of the assumed value and found no substantial variation in the final spectra for all measured nuclei. Further details may be found in [72] Energy Cross-Calibration Being the first instrument ever to simultaneously fly both a TRD and a Calorimeter, we have the unique opportunity to cross-calibrate these techniques. Each analysis was performed in a manner similar to that described above, with the only major deviation being the use of the TRD track to select Calorimeter signals for summation. Figure 4.7 shows the TRD energy deposition ( de ) for the event-correlated energy dx determined with the CD, at low energies, or Calorimeter, at high energies for a selection of Oxygen nuclei. Carbon nuclei were also included for the Calorimeter events and are shown as separate points following TRD energy normalization by Z 2. All points lie along the line derived from the TRD Monte Carlo, and hence we confirm the TRD simulation at these energies [75]. Likewise, the Calorimeter energy scaling, determined using its separate calibration beam data and Monte Carlo, is consistent with the TRD s, further confirming the scale and extension of the deposited energy to incident energy map to highest energies [75]. Thus, we find excellent agreement between our detectors and their Monte Carlos. 4.3 Simultaneous Charge and Velocity Determination Recalling the responses of the TCD scintillator (Section 3.2) and CD acrylic radiator (Section 3.4.1), both proportional to the square of the incident particle s charge Z and varying with its velocity β, suggests that we might be able to disentangle the two. Plotting the linear, attenuation- and gain-corrected TCD peak detector (ADC) signals versus the normalized Cherenkov signal clearly shows in Figure 4.8 the cosmic 111

128 Figure 4.7: The energy deposition in the TRD, de, versus the corresponding energy, dx denoted log(βγ), measured in the Cherenkov Detector (green circles) or Calorimeter (red circles) for a selection of Oxygen nuclei. A selection of Carbon nuclei (blue squares), normalized for the TRD Z 2 dependence, is also plotted against its Calorimeter energy. The dotted line represents the TRD Monte Carlo expectation. Excellent agreement is seen among all three detectors, and the absolute energy scales from the Calorimeter and TRD Monte Carlos agree, as represented by the upper energy axis. [75] ray elements by population (as determined in earlier cosmic ray experiments, e. g. [4]) with a spread across the varying velocities First Analysis We first posit the functional dependence of the CD signal on the incident particle s charge and velocities as: ( Cer = k C Z ) n 2 β 2 (4.6) 112

129 Figure 4.8: TCD linearly stitched, attenuation- and gain-corrected scintillator signal for the X and Y layers (right) and their pathlength corrected combination (left) plotted versus the normalized Cherenkov signal. where n is the Cherenkov detector s index of refraction (1.49) as in Equation 3.3 and the constant k C absorbs those from Equation 3.3 and allows us to normalize the detector signal to Z 2 for β = 1. Rather than using the full Bethe-Bloch equation (Equation 3.1), we first try the approximation: Z 2 Scin k S (4.7) β 2 where the constant k S again allows normalization of the corrected scintillator signal to Z 2 for β = 1. Thus, from Figure 4.8 with a scintillator signal of 0.6 and 113

130 Cherenkov signal of 95 for Carbon, we expect k S 0.6/ and k C ( ) /n 1.53 at β = ZvBflat Z β 0 Figure 4.9: Charge for a Carbon subset determined using the approximate Bethe- Bloch equation (4.7) versus the velocity (β) for normalization constants k S = and k C = Figure 4.9 shows the result of plotting the analytical solutions of these two equations for charge and velocity: k S β 2 = Cer + 1 (4.8) Scin k C n 2 and Z 2 = Cer k C + S k S 1 n 2 (4.9) 114

131 for a subset of Carbon nuclei with k S = and k C = While the average charge of 6 is correct and the range of β reasonable, the slope does not demonstrate the expected charge and velocity independence. This remained true when varying the constants over the ranges k S = : 10 and k C = 0.1 : 10 and when tested on scintillator and Cherenkov signals corrected using a different method (see [44]). Thus, we turned to implementing the full Bethe-Bloch equation Second Analysis Since the simplification of the Bethe-Bloch equation (Equation 4.7) did not yield the expected independent charge and velocity determination, we examined implementing the full Bethe-Bloch equation as Equation 3.1: ( Scin = k S K Z2 Z mat 1 β 2 A mat 2 ln 2m ec 2 β 2 γ 2 T max β 2 δ ) I 2 2 (4.10) with the constant k S again allowing normalization to Z 2 for β = 1. This form no longer permits a simple analytical solution. However, since the Cherenkov and scintillator have a similar charge dependence, their ratio is independent of Z. Figure 4.10 shows the theoretical curve for CREAM s particular materials. The calculation was performed for a signal ratio range of : , corresponding to β of : , ranging from the Cherenkov threshold to near the maximum speed possible for a particle with mass. Thus, using Figure 4.10 as a look-up table, we first determine the particle s velocity. We interpolate β using a weighted average for events with signal lying between the ratio values, calculated with precision. The charge then comes naturally from inverting the Cherenkov signal Equation 4.6. Figure 4.11 shows the result of this method, with the constants k S and k C set to 1 for simplicity. While the charge bands 115

132 β Scin/Cer Figure 4.10: Velocity β for the theoretical, charge-independent scintillator to Cherenkov signal ratio, given by Equations 4.6 and 4.10 are narrow and well-defined, they are not flat, indicating a remaining dependence of charge on velocity. This was tested for constants ranging from k S = 0.01 : 135 and k C = 0.01 : 230, with none evidencing the requisite charge and velocity independence. Several factors which may cause the charge bands non-zero slopes, most notably involving details of the cosmic ray-detector interaction, are explored in the next section Final Analysis The observed charge and velocity dependence may be explained by examining in more detail the cosmic ray-instrument interactions and the detectors responses 116

133 sunz:sunbeta Entries Mean x Mean y RMS x RMS y Figure 4.11: Charge and velocity determined using the exact form of the Bethe-Bloch equation (4.10) for all CREAM I ZHi triggers with constants k S and k C set to 1 for simplicity. While the charge populations are narrow and well-defined, they do not demonstrate the expected independence of charge and velocity. thereto. Scintillator saturation cannot be perfectly accounted for, as modest changes in the slope of higher charges in Figure 4.8 show. This is also seen in a separate analysis ([44]). Likewise, imperfect attenuation and gain corrections can manifest as charge and velocity dependence. Further, δ-rays or energetic knock-on electrons produced in ionization interactions in the upper TRD module deposit energy in the CD which is not included in Equation 4.6. The stochastic nature of the δ-ray production process increases the 117

134 width of the energy deposition distribution, thereby reducing the detector s resolution. At the same time it increases the highest energy at which the detector saturates by providing extra energy deposition for high energy events. The number of energetic electrons increases with incident particle energy up to about 10 GeV/n (γ 10), where the signal again saturates. This is clearly seen in Figure 4.12, the Cherenkov signal normalized to Z 2 determined from a Monte Carlo simulation of Oxygen nuclei including δ-ray production versus the incident particle s Lorentz factor. The black points indicate the Gaussian mean of the δ-ray inclusive signal. The red line clearly below the black points shows the signal expected from 4.6 without the energetic electrons. Also indicative of the difficulty in using Equation 4.6 to describe the CD response function, no analytical function was found to fit the δ-ray inclusive signal. Instead, five, piece-wise continuous polynomials with continuous first derivatives were stitched together to describe the response. The saturated region (γ > ) uses a flat line while the lower energy region is quartered and fit with 3 rd -order polynomials. Further information and details may be found in [44] and on the same effect seen in the TRACER experiment in [76]. The final analysis determines a given event s energy by minimizing the total difference between the measured TCD, CD, and TRD signals and that expected from their response maps, including the continuous polynomial fit for the CD, weighted by the detectors resolutions. The CD dominates this reconstruction for incident cosmic ray energies only up to 2GeV/n [44]. 118

135 Figure 4.12: Cherenkov response normalized to Z 2 from Monte Carlo simulation of Oxygen nuclei interacting in the instrument, including δ-rays produced in the top TRD layer and depositing energy in the CD. The solid, red line shows the Cherenkov response without including δ-rays from Equation 4.6. [44] 4.4 Spallation in the CREAM detector Using the two complimentary charge detectors on the CREAM I instrument, we can determine the non-negligible fraction of cosmic rays that undergo a chargechanging interaction in the 7g/cm 2 between the TCD and SCD. This provides valuable information for correctly calculating the observed cosmic ray flux as well as potential opportunities for deepening our understanding of nuclear physics. As noted in Sections 3.2, 3.6, and 3.3, the TCD and SCD are mainly sensitive to the charge of the through-going cosmic ray while the TRD provides a measure of the 119