Calorimetry
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Physics of Electromagnetic Calorimeters based on Crystal Scintillators Tetiana Hrynova SLAC Graduate Student Seminar Series 21April 2004 Basics • What is Calorimeter? – Device to measure the energy deposited by an incoming particle • How does it work? – Incoming particle looses energy by interactions in the calorimeter material – Calorimeter material proportionally converts energy into detectable signal (scintillation or Čerenkov light, ionization charge) – Photodetector converts light into electrical signal which is proportional to the energy deposited Tetiana Hrynova Physics of EM calorimeters 2 Types Of Calorimeters Air, Ice, Water, Quartz, Lead Glass… Sampling ICECUBE CERENKOV Homogineous P A M E L A Solid State (Si/W) Gas IONIZATION H1 Noble Liquid (Ar, Kr, Xe ) BOREXINO SCINTILLATION ICARUS Scintillator ZEUS Advantages of Crystal EM Calorimeters Good Energy Resolution Good Detection Efficiency Compact Mechanical Structure Uniform Hermetic Coverage Fine Granularity Over a Large Solid Angle Clean Electron and Photon Identification Radiation Damage Readout Speed Cost Tetiana Hrynova Physics of EM calorimeters 4 SM Physics in Precision ECAL Charmonium System Observed Through Inclusive Photons Charmed Meson in Z Decay Crystal Ball Tetiana Hrynova Physics of EM calorimeters From R.Y.5 Zhu New Physics in Precision ECAL J. Gunion, in Snowmass SUSY Breaking with Gravitino CMS Tetiana Hrynova Physics of EM calorimeters Simulations 6 Physics at BaBar Need excellent detection efficiency and resolution from 20 MeV to 4 GeV to study B000, bs, measurement of R ratio using initial state radiation, etc… Tetiana Hrynova Physics of EM calorimeters 7 Outline • Electromagnetic Interactions • Principles of Scintillation Counting • Properties & Applications of Different Scintillators • BaBar EMC – Radiation Dose Monitoring – Total Light Output – Uniformity of the Light Output Along the Length of the crystal Tetiana Hrynova Physics of EM calorimeters 8 Electron Processes Bremsstrahlung dominates at high energies The critical energy is the energy at which the energy lost by ionization is the same as the loss by radiation 800MeV Ec Z 1.2 Radiation length is the mean distance over which (1-1/e)E0 (63.2%) of energy is lost due to bremsstrahlung Ionization dominates at low energies 716.4 g cm 2 A X0 Z ( Z 1) ln( 287 / Z ) Photon Processes The cross-section has a minimum:MeV photons travel long before interacting The mean free path at high energies (e-folding distance N(r)=N0e-r/ for the pair production by a high energy photon) is: =7X0/9 EM Showers: From D. Perkins, Intro to HEP, p368 simple model • Number of particles: N(t)=2t • Average energy of a particle: E(t)=E0/2t • At Shower Maximum E(tmax)=Ec tmax=ln(E0/Ec)/ln2, Nmax=exp(tmaxln2)=E0/Ec • Total Length of Charged Tracks Tch=2/3Ndt= =2/3et ln2dtet(E)ln2/ln2=2/(3ln2)*E0/EcE0/Ec tmaxlnE0; NmaxE0; TchE0 • Need MC for more accurate predictions! Tetiana Hrynova Physics of EM calorimeters 11 EM Showers MC : Longitudinal Profile • Energy Deposition dE (bt ) a 1 e bt E0 dt ( a ) 1 E C j 1 a ln , b 0.5 2 Ec 2 • Maximum of energy deposition E0 a 1 t max ln Cj b Ec where Ce=-0.5 and C=0.5 and • Photon showers are longer b/c of the uncertainty of the position of the first pairproduction Plots from J.Tinslay : 1.0,0.5, 0.1 GeV e- 5 GeV e- on Cu EM Showers MC : Lateral Profile • Exponential Profile • Up to tmax shower radius < X0 • After tmax multiple scattering of electrons causes size to scale with Moliere radius X 0 21.2 MeV Rm Ec • Cylinder of radius 2Rm contains 90% of shower • Soft photons near the end of the shower may travel far depending on cross-sections Tetiana Hrynova Physics of EM calorimeters 13 Scintillation mechanism scintillation Fluorescence(scintillation) – initial excitation and de-excitation by emission of a longer wavelength photon Tetiana Hrynova Physics of EM calorimeters 14 Scintillation Detection • Transparency for Fluorescent Radiation • Crystals wrapped in a reflector to collect as much light as possible • Photo-detector: 16.0-17.5 X0 6.1x6.0cm2 4.7x4.7cm2 – Two 2x1cm2 PID diodes – 85% quantum efficiency – Operated at 50V (depletion voltage 70V) – Shaping time .85ms (2-3 ms optimal) • Amount of the light collected depends on the crystal/photodetector shape Matching Crystal to Photo-detector Photodiode Photodiode requires amplification! x256: 0-50MeV, x32: 50-400MeV, x4: 0.4-3.2GeV, x1: 3.2-13.0GeV Photomultiplier Photomultiplier does not require amplification, but has problems working in magnetic field. Properties of “Perfect” Scintillation Material • • • • • • High Density, High Atomic Number Transparency for Fluorescent Radiation Convenient Emission Wavelength Short Decay Time High Light Yield Uniformity of the Light Output along the crystal length • High Radiation and Mechanical Hardness • Ease to manufacture • Low Cost Tetiana Hrynova Physics of EM calorimeters 17 Properties of Crystal Scintillators Crystal NaI(Tl) CsI(Tl) CsI BaF2 BGO PbWO4 LSO(Ce) GSO(Ce) Density (g/cm3) 3.67 4.51 4.51 4.89 7.13 8.3 7.40 6.71 Radiation Length (cm) 2.59 1.85 1.85 2.06 1.12 0.9 1.14 1.37 4.8 3.5 3.5 3.4 2.3 2.0 2.3 2.37 41.4 37.0 37.0 29.9 21.8 18 21 22 10k 100k 1M 10M 10M 100M 100M Molière Radius (cm) Interaction Length (cm) Radiation Hardness Hygroscopicity Yes Slight Slight No No No No No Luminescence b (nm) (at peak) 410 560 420 310 300 220 480 560 420 420 440 Decay Time b (ns) 230 1300 35 6 630 0.9 300 50 10 40 60 Light Yield b,c (%) 100 45 5.6 2.3 21 2.7 9 0.1 0.6 75 30 d(LY)/dT b (%/ ºC) ~0 0.3 -0.6 -2 ~0 -1.6 -1.9 ? ? Crystal Ball CLEO BaBar BELLE KTeV (L*) (GEM) L3 CMS ALICE BTeV… - - Experiment a. at peak of emission; b. up/low row: slow/fast component; c. measured by PMT of bi-alkali cathode. From R.Y. Zhu Crystals Vendor Map for BaBar It is very difficult to produce crystals in numbers required by HEP experiments (6580 CsI(Tl) for BaBar, 77k PWO for CMS) at one place in the time required! Crismatec Beijing Hilger Kharkov Shanghai Tetiana Hrynova Physics of EM calorimeters 19 Kyropoulos Growth Method Used in Kharkov crystals and with modifications in Crismatec and Hilger crystals From SCINT97 p380 Bridgman-Stockbarger Growth Method Used for Shanghai and Beijing CsI(Tl) crystal production for BaBar. The pictures shown are actually from CMS PWO production in Bogoroditsk, Russia Tetiana Hrynova Physics of EM calorimeters 21 Samples of Crystal Scintillators BGO CeF BaF2 CsI 1.5 X0 Cubic 3 PbWO4 Full Size Samples BaBar CsI(Tl): 16 X0 L3 BGO: 22 X0 CMS PWO(Y): 25 X0 Tetiana Hrynova Physics of EM calorimeters From R.Y. Zhu 22 Scintillation Light of 6 Samples From R.Y. Zhu Calorimeter Resolution E a c b , E E E d f E • Stochastic term(a): – Shower fluctuations, photoelectron statistics, material in front of calorimeter • Constant term(b): detector non-uniformity, calibration uncertainty • c: electronic noise summed over readout channels within a few Moliere radii • Position resolution depends on the effective Moliere radius and the transverse granularity of the calorimeter means summation in quadrature Tetiana Hrynova Physics of EM calorimeters 24 L3 BGO Resolution Contribution “Radiative”+Intrinsi c Temperature Calibration Overall Barrel 0.8% 0.5% 0.5% 1.07% Endcaps 0.6% 0.5% 0.4% 0.88% 12k BGO From R.Y. Zhu CMS PWO Resolution Beam Test 77k PWO Crystal Calorimetry at High Energies Designed Resolution From R.Y. Zhu BaBar CsI(Tl) Resolution Crystal Calorimetry at Low Energies M. Kocian, SLAC, CALOR2002 0.03-3 GeV 3-9 GeV, 12h 0.00613 GeV, 1/2h 6580 CsI(Tl) Good light yield of CsI(Tl) provides excellent energy resolution at B factory energies Energy Resolution • Energy resolution for the detection of photons from 0 in the range from 20 MeV to 4 GeV: E (2.30 0.03 0.3)% (1.35 0.08 0.2)% 4 E (GeV ) E • The stochastic term comes from: fluctuations in photon statistics, electronic noise and beam background • The constant term arises from non-uniformity in light collection ( 0.5%), front and rear shower leakage ( 1%) and uncertainties in calibration (0.25%) • Systematic errors come from fixing of the shapes for energy distributions which are convoluted for the purpose of 0 fit Tetiana Hrynova Physics of EM calorimeters 28 EMC Backgrounds & Rad Damage 10MeV • Mostly photons <10MeV • Leads to formation of color centers which cause absorption bands in the front 10-15cm of the crystal • Caused by impurities • Results in: 100MeV Decrease in the light yield (LY) Non-uniformity of the LY along the crystal length (worsens energy resolution) Tetiana Hrynova Physics of EM calorimeters 29 Absorption, cm-1 Transmitance,% Absorption Bands Exposures: 0, 1, 10, 100, 1000 rad top to bottom Transmitance=1-e-kd, where k is absorption coefficient d is thickness of sample in cm OH- absorption band is located at the maximum of the scintillation emission CsI(Tl) absorption CsI(OH) absorption CsI(Tl) emission EMC Calibrarions • Inter crystal Calibrations • Shower corrections Tetiana Hrynova Physics of EM calorimeters 31 Source Measurement of LY Typical source spectrum: Tetiana Hrynova • Neutron generator activated fluid circulates on demand though an array of thin tubes in front of all crystals • 6.13MeV photons from 19F 16N 16O* 16O • 16N lifetime is 7 sec • Resolution of the light yields is 0.33% • Measured every 1-2 month • Individual crystal calibration! Physics of EM calorimeters 32 Bhabha calibrations M. Kocian, CALOR2002 Crystal Response Uniformity rays Uniformity is influenced by: PMT, PD PD PMT Back (5%) Tetiana Hrynova crystal clarity wrapping surface finish radiation damage Require less then 0.5% contribution to E/E for up to 5 GeV. Front (2%) Physics of EM calorimeters 34 Checking the Uniformity Gain changes obtained using the radioactive source (s) and Bhabha (B) calibrations: possible probe of LY uniformity along crystal length. Statistical errors are: • 0.33 % for the source • 2 % for the Bhabhas No non-uniformity seen. Tetiana Hrynova Physics of EM calorimeters 35 Background Distribution Occupancy obtained using random triggers. LER HER • Single Beam Backgrounds Lost primary beam particles In fwd/bwd direction in horizontal plane • Luminosity Backgrounds Small angle Bhabhas More uniform dose distribution Tetiana Hrynova Physics of EM calorimeters (plot by S.Robertson) 36 Radiation Monitoring in EMC • • • • • Tetiana Hrynova Array of 115 RadFETs Maximum dose seen 1150 Rad Dose has a rate of 4-12 Rad/fb-1 Dose budget is 10 kRad over BaBar life Dose map similar to beam bkg distributions Physics of EM calorimeters Plots by J. Stelzer 37 Leakage Currents (LC) • Leakage current is an average current flowing though the diode • Increases with time because of diode radiation damage • Can be used to calculate the dose accumulated by EMC Plot by I. Eschrich time, s Ecr ( I beamson I nobeams ) Dose dt M cr M cr LYcr Tetiana Hrynova Physics of EM calorimeters 38 Dose accumulated by EMC RadFETs Leakage Currents • Differ by scaling factor, shape similar • EC sees ~30% more dose then measured by RadFETs Tetiana Hrynova Physics of EM calorimeters 39 Crystal LY change by vendor Source of difference – different growth methods results in : • Different distribution of Tl along the length of the crystal • Different distribution of impurities along the length of the crystal Tetiana Hrynova Physics of EM calorimeters 40 Comparison to expected dose rates in CMS BaBar Tetiana Hrynova Dose Rates in Gy(100Rad)/h expected in the regions of CMS ECAL at lumi 1034 cm-2s-1 Physics of EM calorimeters 41 Other Uses for Scintillating Crystals • Medicine – PET/CT scanners, Gamma-cameras • Geophysics – search for minerals • Nuclear Physics / Ecology – radiation control, sample testing • Technology – baggage /cargo scanning Tetiana Hrynova Physics of EM calorimeters 42 Future Calorimeters Si/W for PAMELA MINOS, Plastic Scintillator ATLAS LAr EMC Tetiana Hrynova Physics of EM calorimeters 43 Cosmic Ray Air Shower Detection p, γ，ν interaction Using Atmosphere as a Calorimeter Air Fluorescence Air Fluorescence Detector (Fly’s Eye, HiRes, EUSO/OWL) Air Cherenkov Particle Shower Ground Array (AGASA, Auger..) 1~2km Tetiana FromHrynova Katsushi Physics of EM calorimeters Arisaka, Calor 2002 44 And much, much more… • EMC Calibrations see talk by M.Kocian http://3w.hep.caltech.edu/calor02/abstract/Presentation/cryst al/kocian.pdf • EMC electronics see talk by I. Eschrich http://3w.hep.caltech.edu/calor02/abstract/Presentation/electro nics/eschrich.pdf • PDG2004 • BaBar NIM paper • R. Wigmans, Calorimetry, Oxford, 2000 • J. Birks, The Theory and Practice of Scintillation Counting, Pergamon Press, 1964 • Proceedings of Calorimetry in HEP (even years), SCINT (odd years), Techniques and Concepts of HEP Tetiana Hrynova Physics of EM calorimeters 45 Energy Transfer Losses Scintillation Quantum Efficiency(): =SQ, where is conversion efficiency determined by crystal lattice and not sensitive to radiation Q is determined by type of luminescence center • the light output is ~ QCa where Ca is activator concentration and it might change S is energy transfer to emission center • it is known that CsI(CO3) crystals with concentrations of CO32- two orders of magnitude less than Tl+ ions give competitive light output, thus they capture some part of the charge carriers Longitudinal Shower Profile Z Lead 82 Iron 26 Aluminium 13 • As Z increases, the shower maximum shifts to greater depth • A Z increases, the shower profile decay more slowly beyond shower maximum Tetiana Hrynova Physics of EM calorimeters 47 Light Output of BaBar CsI(Tl) Crystals 241Am Counts spectrum produced by crystal+PMT+ EMC readout system • 241Am source: 60keV • 3.6eV needed to creat e-h pair in Si (16667 e-) • Corresponds to 86.72ADC counts • Using 6.13MeV source determine that 57.8keV/ADC count is deposited (2 diodes averaged!) • This gives 3322 photoelectrons/MeV with PD 0.85ms readout time • Compare to 7300 photoelectrons/MeV with PMT 2.0ms readout time ADC counts Tetiana Hrynova Plot from M. Kocian Physics of EM calorimeters 48 Dose Accumulated by EMC Dose calculated using LC shows: ~45% of EMC dose is accumulated during injection. Tetiana Hrynova Physics of EM calorimeters 49
