The BaBar CsI(Tl) Electromagnetic Calorimeter

Transcription

1 The BaBar CsI(Tl) Electromagnetic Calorimeter Jane Tinslay Brunel University 3rd December 1999

2 Contents Introduction Physics of Electromagnetic Calorimetry Particle interactions with matter Electromagnetic showers Scintillation and radiation damage processes Energy resolution The BaBar Calorimeter Design and construction Reconstruction Calibration Current performance Conclusions

3 Introduction A device which measures the total energy deposited by a primary particle is called a calorimeter. The primary particles energy is degraded by interactions with the calorimeter material. A scintillating crystal calorimeter converts this energy into scintillation light. The scintillation light is converted to an electrical signal. Electrical signal proportional to primary energy. Other experiments using crystal calorimeters- Crystal Ball-NaI(Tl),KTev- CsI, L3-BGO, CLEO, BELLE, BaBar-CsI(Tl),CMS -PbWO 4.

4 Why does BaBar need a calorimeter? Generic B decays contain an average of 5.5 photons and 5.5 charged tracks. ~50% of photons < 200 MeV. Need excellent energy, position resolution in order to reconstruct π 0 s. Need excellent photon detection efficiencies down to low energies (20 MeV). Charged, neutral PID. Crystal calorimeter most suited - best energy resolution

5 Particle Interactions With Matter Electron/Positron Processes At low energies, ionisation dominates. At high energies, bremsstrahlung dominates. Radiation length, 716.4gcm 2 A X 0 = ZZ ( + 1) ln( 287 ( Z) ) Mean distance over which all but 1/e of energy is lost due to bremsstrahlung. Bremsstrahlung γ e - e - Nucleus de --- dx E --- X 0

6 Critical energy, E c = energy at which the energy loss by ionisation is the same as the loss by radiation. OR E c = 800MeV Z E c = energy at which the ionisation loss per radiation length is equal to the electron energy. E c = 610MeV Z + 1.4

7 Photon Processes Pair Production Compton Scattering γ eē+ γ γ e - Nucleus Nucleus Photoelectric effect γ e - Nucleus

8 Mean free path at high energies λ = 9 --X 7 0 CsI In the MeV range, cross section reaches a minimum so low energy photons may travel a long way before interacting. I = ( µt) I 0 e µ = 1 -- λ

9 Electromagnetic showers When a photon/electron/positron enters a crystal an electromagnetic shower is initiated. A cascade of secondary photons, electrons and positrons produced by the particle interactions described previously. Average shower properties described by simple model. Define the scale variables: t = x X 0 e - γ e - e - γ e - e - e + e + γ γ y = E E c γ e - e + e - X

10 After t generations... Number of particles present Nt () = 2 t Average energy of a shower particle Et () = E t Shower Maximum t max = ln( E 0 E c ) ln2 Total track lengths of all charged particles L = t 2 max -- Nt ()t d 3 0 E y E c

11 An over simplified model - more accurate predictions from Monte Carlo techniques. Longitudinal profile de ( bt) a 1 e bt = E dt Γ( a) t max = ( a 1) b = ln y + C i Where C e =-0.5, C γ =+0.5, b =0.5 Photon induced showers are longer because of the uncertainty in the position of the first pair production.

12 Lateral Profile Exponential profile. Up to t max shower, contained in a cylinder with radius < 1X 0. Soft photons found near the end of shower - may travel far depending on cross sections. After t max, multiple scattering of electrons causes lateral size to scale with the Moliere radius - determines optimal transverse crystal dimensions. X Mev R m = E c Roughly 95% of a shower is contained laterally in a cylinder with a radius of 2R m

13 Average energy deposited/0.18x 0 (Mev) Gev γ 0.5 Gev γ 0.1 Gev γ Depth into crystal from front face (X 0 ) Average energy deposited/0.18x 0 (Mev) Gev e Gev γ Depth into crystal from front face (X 0 )

14 CsI(Tl) Scintillation and Radiation Damage Mechanisms Scintillation Mechanism CsI(Tl) is an inorganic scintillator. Scintillation mechanism depends on energy states - determined by the structure of the crystal lattice. Well defined valence and conduction bands. Impurities, called activators (Thalium) added to modify energy levels - increase the probablility and λ of scintillation light. Conduction band Electron Exciton band Exciton Modified energy levels due to impurity Hole Valence band

15 Fluorescent light with a relatively short decay time (see previous diagram). Phosphorescent light (afterglow) with a long decay time - Impurity is in a metastable state. Extra energy (eg, thermal energy) needed to excite it to a level where it can return to ground state. Issues: Transparent to own radiation Decay time Radiation hardness Light Yield Wavelength of scintillation light Radiation length Ease of manufacture Cost

16 Radiation Damage All crystals suffer from radiation damage -change in crystal response. Not thought to be a damage to the scintillation mechanism. The formation of colour centres in the crystal produces absorption bands.

17 Colour centres are formed when an impurity atom is displaced from its lattice position by ionising radiation, into which an electron can drop, causing absorption bands. Results in an overall loss in light output. If the photon attenuation length in a given crystal isn t long enough, radiation damage will produce a non-uniform light output. Every crystal is different - impurities introduced during manufacture. Crystal non-uniformity introduces a constant term to the energy resolution. Non-uniformity isn t always bad - the compensation for leakage effect.

18 Energy Resolution A beam of mono-energetic photons incident fired into a calorimeter will have a spread in measured energy. σ Energy resolution : E E Contributions: Fluctuations in scintillation photons stats Fluctuations in photo-electron stats Noise-coherent+incoherent Leakage fluctuations Calibration Front material (efficiency) backgrounds non-uniform light collection σ E Energy σ A = E E B 2 + C 2 E

19 A = Shower+readout fluctuations B = Noise C = Calibration, leakage, non-uniformity, dead material BaBar σ = E 4 E Achieved through high crystal light yield, reflective crystal wrappings, two photodiodes, very low electronic noise. Angular resolution Determined by the transverse crystal size and average distance to the interaction point. ( σ θ ) 2 = 3mr mr 2 E

20 The BaBar Calorimeter Design and Construction

21 EMC located asymetrically about interaction point. Non-projective crystal geometry by15-45 mr in θ - minimise lost photons cos( ϑ) cos( ϑ) (lab) (CM) cm cm e IP cm e + Barrel: cos( ϑ) (CM) 5760 crystals. 48 θ rows, each with 120 crystals in φ. 280 modules of 3*7 crystals. (except for last row - 6 crystals in θ) Weighs 23.5 metric tonnes.

22 Endcap: Conic Section. 820 crystals in 20 modules. Each module contains 41 crystals. Tilted at 22.7 o wrt the vertical. 8 θ ring-number of crystals in φ varies from Angle coverage in CM: cos( ϑ) Inner ring of lead-shielding. Weighs 3.2 metric tonnes

23 Crystals: Trapezoidal in shape. Range in length from 17.5 X 0 in the EndCap, to 16 X 0 in the back barrel. Typical front face dimensions 4.7*4.7 cm 2 Typical back face dimensions 6.0*6.0 cm 2 Tolerances: 250 µm transversely, 1mm longitudinally. Crystal wrappings: two 150 µm layers of Tyvek (front and side faces), one layer of 30 µm aluminium foil. Read out by two silicon PIN diodes, connected to preamplifiers. Implements two independent electronics chains - reliability + signal/noise. With digital filtering incoherent noise = 150 KeV per crystal (negligible). Noise dominated by beam backgrounds.

24 Crystals are tuned for uniform light collection - roughening/polishing surface.

25 Properties CsI(Tl) CsI PbWO4 Density (g cm-3) Radiation length (cm) Moliere radius (cm) Decay time (ns) Peak Emission λ (nm) Light yield (Photons/ Mev*10 3 ) Hygroscopic slightly slightly no

26 Features Radioactive source system Thin tubes on the front face of the calorimeter carry a fluorocarbon which has been excited by neutron irradiation - decays resulting in 6.13 Mev photons. Used as a calibration and monitoring tool. Light pulser system Xenon flash lamps feed light onto the rear face of all crystals. Used as a monitoring and diagnostic tool - electronics linearity. RadFETs 56 in barrel, 60 in End Cap - small devices which measure dose.

27 Modules made from 300 µm CFC, and supported from the rear by an aluminium strongback. Mounted in an aluminium support cylinder-supported off coil. Cooling and cables located at the back of the modules.

28

29 Reconstruction Digis->Clusters->Bumps->Cands Digis: Crystals with >1 MeV (TDR=0.5 MeV). Cluster: Collection of adjacent digis. Summed energy > 20 MeV. Bump: Split each cluster up into one or more bumps-look for local maxima. Cand: Energy+position corrected bump or cluster, selected under a particular particle hypothesis. PID Charged: e -, µ, π shower shapes, E/P Neutral: γ, merged π 0, K 0 L - shower shapes

30 Calibration Scintillation light -> final particle energy depends on three calibrations: Electronics calibration: Linearises the electronics response using a charge injection system. Inter-Crystal calibration: Determines the response of an individual crystal. Calibrates to the deposited energy. Time dependent. Cluster calibration: Applied to candidate particles made out of clusters of crystals. Sets the final energy scale. Corrects for lost energy due to clustering, leakage. Time independent. Monitoring - light pulser + calibration methods track changes - eg, radiation damage. Inter-Crystal Calibration Different response depending on incident energies which should (if the crystals are reasonably uniform) be small. Fixes calibration points along a calibration curve.

31 Techniques: Bhabhas High energy point (3->9 GeV) Well known topology. High rate (200 hits/crystal = ~8 hours data taking at nominal luminosity) Source Low energy point (6.13 MeV) Fast (doesn t depend on luminosity!). Has seen changes in crystal response. Radiative Bhabhas - intermediate energies. constant Energy

32 Cluster Calibration Theta, particle type dependent. MC derived correction. Pi0Calibrator - shifts pi0 mass peak to correct position - data. Polynomial derived correction. Should be relatively stable over time.

33 Current Status Problems at startup: Coherent noise. Electronics non-linearity (diagnosed by light pulser). = High digi cut = worse resolution. Major improvements with coherent noise-torroids on power supplies, oscillating capacitor removed. Digi cut now at 1 MeV (was at 5 MeV), TDR aim is 0.5 MeV. Pi0 cluster calibrator derived from data implimented. Beam backgrounds not as bad as expected - accumulated dose of ~50 rads in hottest crystals (improved vacuum, collimators). Budget is 10 krad over 10 years 10% difference in source and bhabha constants.

34 Entries/Bin EMC Bhabha Clusters Constant = 1674 Mean = BaBar Entries MC E/E_expected (fwd brl) Constant = Mean = Sigma = e-05 BABAR Sigma = Forward Barrel E measured / E expected deposited E measured / E expected

35 π 0 Mass E γγ > 500 MeV Entries BaBar π 0 -mass = MeV π 0 -width = 7.7 MeV m γγ (GeV) (MC gives 4-6 MeV width)

36 Conclusions CsI(Tl) Crystal calorimeter should deliver the necessary the photon/electron energy resolution required. Detector is functioning - big improvements with detector performance - still things to understand...

37 Bibliography SLUO Lecture Series - Lectures 13, 14 (Jim Brau), 15 (Helmut Marsiske) PDG - Passage of particles through matter Techniques and Concepts of High Energy Physics VI -p.325, lectures by Richard Wigmans. BaBar Physics Book, TDR Introduction to Experimental Particle Physics - Richard Fernow Radiation Damage: Papers by Ren-yuan Zhu, H. Chowdry.