WARP. C. Galbiati, C. DMSAG Aug

Transcription

1 WARP C. Galbiati, C. DMSAG Aug

2 WARP Collaboration INFN and Università degli Studi di Pavia P. Benetti, E. Calligarich, M. Cambiaghi, L. Grandi, C. Montanari, A. Rappoldi, G.L. Raselli, M. Roncadelli, M. Rossella, C. Rubbia, C. Vignoli INFN and Università degli Studi di Napo F. Carbonara, A. Cocco, G. Fiorillo, G. Mangano INFN Laboratori Nazionali del Gran Sasso R. Acciarri, F. Cavanna, F. Di Pompeo, N. Ferrari, A. Ianni,O. Palamara, L. Pandola Princeton University F. Calaprice, C. Galbiati IFJ PAN Krakow A.M. Szelc INFN and Università degli Studi di Padova B. Baibussinov, S. Centro, M.B. Ceolin, G. Meng, F. Pietropaolo, S. Ventura

3 Outline of the Talk Introduction to WARP Technology Goal and strategy of the experiment Neutron backgrounds Summary of Main Results from 3.2 kg prototype Gamma backgrounds 140 kg Detector Auxiliary Measurements Needed Isotopically Depleted Argon Schedule Funds

4 Basic Parameters Suitable materials for detection of ionizing tracks: Dense, homogeneous, target and also detector Do not attach electrons High electron mobility (except neon in some conditions) Commercially easy to obtain and to purify (in particular, liquid Argon) Inert, not flammable Cos t Element Density (ρ/cm 3 ) Energy loss de/dx (MeV/cm) Radiation length X 0 (cm) Collision length λ (cm) Boiling 1 bar (K) Electron mobility (cm 2 /Vs) Neon high&low Argon Krypton Xenon

5 Noble Liquids One competitive advantage in WARP: Collaboration with Rubbia s group, longest and deepest experience in development of detectors with Cryogenic Noble Liquids As well known, noble elements (He,Ne, Ar, Kr, Xe) have the unique physical property of a negligibly small electron attachment probability (magic elements). In ultra pure liquids, thermalised electrons have an attachment probability f.i. : Ar + e Ar - far less of 1 in collisions Thermal electron mobilities in these liquids are relatively fast ( 1 cm/µs at few kv/cm) with the exception of He, Ne in which a bubble of 15 Å is naturally formed around a free electron from which He, Ne is excluded Ion mobilities are very low ( 5 cm/s at 1 kv/cm) Krypton is today strongly polluted by 85 Kr from nuclear industry ( 1 Bq/m 3 of ordinary air) due to nuclear reprocessing, (La Hague, Sellafield) Argon is polluted by radioactive 42 Ar due to 2-n capture of atmospheric nuclear blasts and by 39 Ar due to cosmic rays ( 1 Bq/kg of Ar) Isotopic enrichment of Argon or Xe-Kr separation may be required for some specific low counting applications Argon, Xenon and Neon offer practical cases for cryogenic applications

6 Purity Because of their low temperatures, noble liquids can be easily purified, since most of the contaminants freeze out spontaneously Standard polar molecules filters (oxi-sorbs) ca be used to separate out many electronegative contaminants to a very high degree and with very simple and cheap methods The residual contamination in the case of LAr may be currently as low as 10-2 ppb Oxygen equivalent for industrial applications, corresponding to a free electron lifetime of about 1/100 sec and a mean free path of many meters at V/cm Lower but acceptable purities have been achieved in the case of Xenon

7 Electron drift properties in liquid Argon R.M.S. Diffusion Speed Drift time Electron Lifetime [ms] Detection Limit 10 ms Drift Path [m] 1 ms 5 m 5 m

8 Extraction of electrons from liquid to gas Of particular interest is the possibility of extracting ionisation electrons through the interface liquid-gas, since electrons can be easily multiplied in a gaseous medium. In order to do so, one has to overcome the potential barrier binding electrons to the liquid, introducing a local, accelerating electric field. It is found that: At relatively low fields, electrons require a long time to be freed (slow component, typically a fraction of ms) above a given threshold, the extraction is prompt. At fields of the order of a few kv/cm the prompt extraction is practically complete, both for Xe and Ar. E-fields for Xe are about 2.5 times larger than for Ar. Extraction Efficiency

9 Noble Gas Luminescence (I) I s /I t = 0.3 (e) I s /I t = 1.3 (α) I s /I t = 3.0 (ff) τ singlet = 7 ns Ionization to Scintillation ratio S2/S1 Pulse Shape Discrimination τ triplet = 1.8 µs

10 Noble Gas Luminescence (II) The main luminescence from high density noble gas scintillators excited by charged particles is in the vacuum ultraviolet region and has the same spectrum as the so-called second continuum spectrum in gas discharge. This second continuum spectrum is attributed to the transitions from the two lowest molecular states ( 1 Σ and 3 Σ ) to the ground state These molecular states have two main origins. The first one (indicated with [1] in the previous Figure) is the direct excitation of excited states by primary charged particles and secondary electrons. This excitation process is followed for sufficiently high pressures by the quick formation of molecular states by a three-body collision process: R * * + R + R Æ R S u or 3 + [ S u ] + R The second origin (indicated with (2) in the Figure) is the formation of two molecular states through a recombination process between thermalized electrons and molecular ions R2 *. The primary particles and secondary electrons produce atomic ions R * and electrons. The atomic ions are quickly converted to molecular ions R2 * for sufficient densities by the process [2]: R + + R + R Æ R R The secondary electrons lose kinetic energy promptly through the excitation of excited states and/or production of electron-ion pairs and then through the elastic collisions, and they are finally thermalized These thermalized electrons recombine with molecular ions forming highly excited atoms via dissociative recombination [3]: R e Æ R * * + R

11 Noble Gas Luminescence (III) The highly excited atoms relax via the reactions [3]: and [5]: R ** + R + R Æ R 2 ** + R and finally form excited molecules via reaction of diagram [1]: R ** 2 + ( R) Æ R * + R + R In the study of the time dependence of the luminescence in high-pressure noble gas scintillators excited by charged particles, it is therefore necessary to consider two distinct origins of the luminescence For the first one, called excitation luminescence, the decay depends on the lifetimes of two excited molecular states, if the reaction rate for the process [1] is higher than the decay rates of molecular states. It has been clearly shown that excitation luminescence has two components, a fast one and a slow one, which corresponds to 1 Σ and 3 Σ transitions, respectively The second origin, called the recombination luminescence, has a finite rise time and the time dependence of the luminescence should represent the kinetic characteristics of the recombination process and of the lifetime for molecular states, on condition that the reaction rates for the processes [1], [2], [4], and [5] are larger than the reaction rate for the process [3] and tile decay rates of the excited molecular states ( )

12 WARP: the motivation TARGET: Atomic number 40 No loss of coherence at intermediate energies Complete retention of gold plated events ( kev) WIMP CANDIDATES IDENTIFICATION: Highest discrimination between nuclear recoils and beta/gamma-like background 39 Ar, 0.8 Bq/kg need 3 x 10 8 rejection against betas (for 140 kg detector) WARP Collaboration, Benetti et al., astro-ph/

13 WARP: the Target Form factor very different from Xe, Ge targets Lower A results in lower rate per unit mass at 10 kev threshold For Mχ>100 GeV, Gold Plated events (>60 kev) still abundant! Could possibly run with a significantly higher threshold than other experiments and be very competitive

14 Multiplication Electrons Primary Surviving in Gas Secondary Recombination Scintillation Scintillation Drift Photons Photons Towards (S1) (S2) Anode Two-Phase Argon Drift Chamber Energy Deposit

15 The WARP Technology Highest discrimination of minimum ionizing events, in favor of potential WIMP recoils, with two simultaneous and independent criteria: Pulse shape discrimination of primary scintillation (S1) based on the very large difference in decay times between fast ( 7 ns) and slow (1.6 µs) components of the emitted UV light Minimum ionizing: slow/fast ~ 3/1 Nuclear recoils: slow/fast ~ 1/3 Both prompt scintillation (S1) and drift time-delayed ionization (S2) are simultaneously detected with a pulse ratio strongly dependent from recombination of ionizing tracks. Precise determination of events location in 3D: 5 mm x-y, 1 mm z Additional Rejection for multiple neutron recoils and γ background

16 (A) First Two Discrimination Methods Electron S1 S2 (B) Drift time Argon recoil S1 S2 S1 S1 Events are characterized by: the ratio S2/S1 between the primary (S1) and secondary (S2) the rising time of the S1 signal Minimum ionizing particles: high S2/S1 ratio (~100) and by slow S1 signal Alfa particles and Ar recoils: low (<5) S2/S1 ratio and fast S1 signal

17 γ-like The simultaneous application of both identification techniques allows efficient discrimination of nuclear recoils from γ-like events Ar-recoils Ar-recoil indicative red box (energy dependent): - 8<S2/S1< <F<0.87 Neutron-induced ion recoils(am- Be calibration)

18 γ-like Ar-recoil indicative red box (energy dependent): - 8<S2/S1<22 Both methods are necessary S2/S1 ratio method alone would mix-up Ar-recoils with low S2/S1 γ-like population tail S2/S1 ratio The Need for both identification methods

19 Ar-recoils kev kev The effect of the S2/S1 ratio cuts (energy dependent) is shown for neutron-calibration data. It strongly depletes the γ-like population (F<0.6) It leaves the Ar-recoil population unaffected (F>0.6)

20 Third Discrimination Method Identification of Multiple Recoils Single Hit Double Hit drift Multiple recoils can be identified and measured, if separated along the drift coordinate by a few mm (few μs drift)

21 3.2 kg prototype reproducing on a smaller scale the design 2.3 of 140 kg liters detector. First tests started in 1998 in the framework of the ICARUS R&D program. First installation at LNGS dating More than two years of operation in underground location, in several configurations (with and without gamma and neutron shields). Thought of as a technological demonstrator, was indeed able to perform a WIMP search campaign for over three months of continuous data taking. Results reported in many conferences. 3.2-kg prototype

22 Nuclear Recoil Photoelectron Yield The Am-Be neutron-induced recoil spectrum is compared with MC predictions. Checks are needed to verify the consistency with results claimed by other collaborations for the ratio of nuclear to electron yields. Our estimate is being subjected to further verifications. YAr=(1.65±0.25) phe/kev Additional measurements with neutron sources underground, and a new measurement with monoenergetic neutron beam at an accelerator will be needed to complete the job.

23 kev Selected events in the n-induced single recoils window: None

24 The Need for both identification methods The importance of using both discrimination techniques is even more evident if the data from the run with an exposure of 96.5 kg x day are considered. The effect of both cuts is summarized in the plot for all the events falling in the range kevion. The main background to Ar-recoils surviving the S1 shape cut is constituted by spurious physical events that mimic the S1 typical for recoils. Measured S1 Shape Rejection Power F>0.68: kevion = 3.7 x kevion = 1 x 10-6

25 Results of WIMP search 90% C.L upper limit no recoil-like events are observed above 42 kevion in a total fiducial exposure of 96.5 kg x day (2.8 x 10 7 trigger). The evaluated 90% C.L. upper limit for spin-independent interaction, in the standard WIMP scenario, is plotted. Energy resolution due to statistical fluctuations and to a non uniform light collection has been taken into account. The dominant systematic effect is due uncertainties on scintillation yield. An error of 15% on YAr corresponds to a variation of MW=100 GeV /c 2 and of MW=50 GeV/c 2.

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27 Radon Daughters 5 mm resolution x-y 1 mm resolution z Not a problem, an opportunity for monitoring the detector

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29 WARP 140-kg Detector The WARP 140-kg detector to be installed and commissioned at LNGS- 140 kg active target, will possibly allow to reach into cm2- cover most critical part of SUSY parameter space Active Veto 100 liters Chamber Complete neutron shield! 4π active neutron veto (9 tons Liquid Argon, 300 PMTs) 3D Event localization and definition of fiducial volume for surface background rejection Detector designed for positive confirmation of a possible WIMP discoveryactive control on nucliderecoil background, owing to unique feature (LAr active veto) Cryostat designed to allocate a possible 1400 kg detector Passive neutron and gamma shield

30 Background Sources Neutrons External neutrons efficiently absorbed by 70 cm of polyethylene Internal neutrons by spontaneous fission, (α,n) interactions, cosmic ray muons (especially in lead). Main sources: stainless steel dewar, PMTs, cosmic ray muons Neutrons rejected with very high efficiency (larger than 99.99%) by Veto Detector (10 cm mean free path length) Beta and γ-rays passing selection cuts for nuclear recoils Dominated by 39 Ar (1 Bq/kg) External sources (rock, detector materials, etc.) much smaller Required rejection power for non-depleted Ar 3x10 8

31 Residual Neutron Background Source Residual, non-vetoed Recoil Events in Inner Detector ( kev) [events/year] Dewar (12 tons) 0.22 Veto PMTs (300 units) 0.70 Internal PMTs (40 units) 1.03 Steel in chamber (20 kg) 0.05 Steel in shielding (8 tons) < 0.15 External neutrons 0.02 Cosmic rays ~ 1 Total 3.3 After cuts [multiplicity of hits in Internal Detector, coincidence with gammas] <1 Spectrum from all sources dominated by nuclear form factor Neutron background becomes negligible above 50 kev 4-5 yrs of neutron-free data taking above 50 kev

32 Expected Sensitivity

33 140 kgs Detector: Schedule

34 140 kgs Detector: Tasks (I) Task People Cryogenics and Purification Detector Mechanics and Cabling PMTs Preparation and Test C. Vignoli, C. Montanari, E. Calligarich M. Cambiaghi, E. Calligarich G. Fiorillo, F. Carbonara High Voltage for PMTs High Voltage for Inner Detector Feedthroughs Electronics for the Inner Detector G.L. Raselli F. Pietropaolo F. Pietropaolo, M. Nicoletto S. Centro, F. Pietropaolo, P. Cennini Electronics for the Outer Detector DAQ for the Inner Detector DAQ for the Outer Detector Slow Control C. Galbiati, P. Cennini S. Ventura C. Galbiati M. Rossella External Shield F. Cavanna Detector Assembly M. Cambiaghi Logistics B. Baibussinov, C. Vignoli, C. Montanari

35 140 kgs Detector: Tasks (II) Task People Calibration and Monitoring Software Tools for the Analysis Montecarlo Simulations Computing and Data Storage Isotopically Purified Atmospheric Argon Argon from Geological Reservoirs A. Szelc, A. Ianni, L. Grandi O. Palamara, A. Cocco, L. Grandi A. Cocco, L. Pandola, A. Ianni A. Cocco, S. Ventura P. Benetti C. Galbiati, F. Calaprice

36 Argon depleted from 39 Ar (I) Need 3 x 10 8 rejection of beta/gamma background Rejection measured from pulse shape discrimination ranges from 4 x 10 5 to 1 x 10 6 Open question: non gaussian tails? Rejection from ratio of ionization to scintillation ~ 10 3 Open question: are the two discrimination really independent? Depending on the answers to these questions, it may be necessary or just beneficial the procurement of Argon depleted from 39 Ar (1/100 of 39 Ar/ 40 Ar in air)

37 Argon depleted from 39 Ar (II) Centrifugation Samples of isotopically purified argon produced and certified by third parties 5 kg to fill the 3.2 kg prototype being procured, end 2006 delivery Small sample of argon from geological reservoirs acquired Methods for certification and quality control are under evaluation Survey of possible sources in the fall Aiming at delivery of 5 kg spring 2006

38 Auxiliary Measurements to address Open Questions High Statistics with beta sources to address: Gaussian Tails for Pulse Shape Discrimination Limited discrimination from S2/S1 Independence of the two main discrimination methods (Pulse Shape Discrimination, S2/S1) Light Yield Investigation Measurements with neutron LNGS (Fall 06) Measurement with monochromatic neutrons at accelerator facility (Spring 07)

39 Funding Needs People small number of students, postdocs with respect to competing efforts three promising Graduate Students joined Princeton group: David Krohn, John Appel, Ben Loer Serious effort to keep three students Need 1 postdoc LNGS Electronics to pursue upgrade of Outer Detector (see next slides) Future Needs: Plant for large scale production of Geo Ar

40 Upgrade of Veto Detector Additional Physics with a 9 ton optical-only detector Neutrino fluence in SN collaps, cross section: SN physics with Veto Detector: kpc can give about 40 events above 10 kev Neutral currents: sensitive to all flavors, time of flight measurement of mass Oscillations in DM rate in Veto Detector (in progress) Need upgrade of PMTs (300 to 400, INFN) and upgrade of electronics for Veto Detector

41 The End