A study on stochastic term of calorimetric energy resolution Ilhan TAPAN and Fatma KOCAK Uludag University Physics Department Bursa-Turkey Title: Excess Noise Factor of Neutron Irradiated Silicon Avalanche Photodiodes The XIV International Conference on Calorimetry in High Energy Physics, Beijing, China, May 10-14, 2010.

- Simulation and Results Outline - Introduction - PbWO4 Crystals-Avalanche Photodiode Combination - Calorimetric Energy Resolution and Stochastic Term - Simulation and Results - Conclusion Motivation: CMS & APDs

Introduction The energy measurement with scintillation crystal is based on the energy released of the incident particles in the crystal material. An electromagnetic shower is produced by a high-energy electron, positron or photon enters the crystal. The light generated in the shower development is detected by photodetectors. The ultimate limit for the energy resolution is determined by fluctuations in the development of showers. Motivation: CMS & APDs

Introduction CALORIMETERS SUPERCONDUCTING COIL IRON YOKE TRACKER MUON ECAL HCAL Scintillating PbWO4 crystals Plastic scintillator/brass sandwich IRON YOKE TRACKER Silicon Microstrips Pixels Total weight : 12,500 t Overall diameter : 15 m Overall length : 21.6 m Magnetic field : 4 Tesla MUON ENDCAPS MUON BARREL Drift Tube Resistive Plate Cathode Strip Chambers ( ) CSC Chambers ( ) DT Chambers ( ) RPC Resistive Plate Chambers ( ) RPC

Introduction An homogenous scintillating crystal detector Made out of 75,000 crystals Motivation: CMS & APDs Subdivided into a barrel and two endcap Barrel section contains 61,000 crystals two APDs per crystal

PbWO4 Crystals-Avalanche Photodiode Combination Disadvantages  Temperature dependence Low light yield Advantages  Dense and Radiation hard Short radiation length Fast Properties of PbWO4 Density 8.28 g/cm3 Radiation length 0.89 cm Interaction length 19.5 cm Moliére radius 2.2 cm Emission peak 420 nm Light yield 120 photons /MeV Radiation hardness 107 rad Motivation: CMS & APDs Scintillation light spectrum of PbWO4 (CMS ECAL TDR)

PbWO4 Crystals-Avalanche Photodiode Combination Avalanche Photodiode (APD) Advantages  compact and robust very high QE internal gain very good time resolution insensitive to B Motivation: CMS & APDs Disadvantages  small sensitive areas and noisy gain fluctuations dependence on high radiation

Si-APD Parameters (Hamamatsu S-8148) PbWO4 Crystals-Avalanche Photodiode Combination Hamamatsu silicon S-8148 APD Produced by epitaxial growth on low resistivite n+-type silicon substrate, followed by ion implantation. Si-APD Parameters (Hamamatsu S-8148) Active area 5x5 mm2 Quantum efficiency at 420nm 72% Operating voltage 380 Volt Gain [M] 50 Capacitance [C] 80 pF Excess noise [F] ~ 2 (1/M) (dM/dV) at M=50 3,3% (1/M) (dM/dT) at M=50 -2,2% Motivation: CMS & APDs

Calorimetric Energy Resolution and Stochastic Term Energy resolution in the ECAL E is the energy of the incident particle Term Contribution to Aim for CMS ECAL Barrel a Stochastic term Photoelectron statistic Shower fluctuations ~ 2.8% GeV1/2 b Constant term Calibration Non-uniformities ~ 0.55% c Noise term Electronic noise Dark current 155 MeV at low luminosity 210 MeV at high luminosity Introduction: ECAL Energy Resolution

Calorimetric Energy Resolution and Stochastic Term APD contributes to all the terms ECAL energy resolution a sensitive area, quantum efficiency, excess noise b gain sensitivity to operating voltage and temperature, aging and radiation damage c low capacitance, serial resistance and dark current By neglecting the intrinsic resolution, the APD photo-electron statistics contribution to stochastic term is given by Introduction: ECAL Energy Resolution Npe is the number of primary photoelectrons Npe = Nph .QE Nph ; photons from crystal, QE ; quantum efficiency F is the avalanche gain fluctuation or excess noise factor

Calorimetric Energy Resolution and Stochastic Term The relative fluctuation of the APD signal in the proportional mode Npe is the number of primary photoelectrons : S.D. of the number of primary photoelectrons; M : Avalanche gain σM : S.D. of the avalanche gain APD photo-electron statistics contribution to stochastic term

Calorimetric Energy Resolution and Stochastic Term The total stochastic term of the energy resolution for crystal- APD combination is composed of a contribution from shower containment (lateral leakage contribution) and a contribution from APD signal fluctuation (photo-electron statistics). 1- Event to event fluctuations in the lateral shower containment ......(alateral), 2- Photo-electron statistics contribution from APD (ape)

Simulation- alateral The lateral shower shape determines the distribution of the energy deposition in a cluster of crystals around the impact point. The contribution to the stochastic term coming from fluctuations in the lateral shower containment (lateral leakage) of PbWO4 crystals has been simulated by GEANT4 for 0.2-100 GeV electrons. Crystal is same size used in CMS ECAL, a truncated-pyramidal shape; a length of 23 cm (25.8X0) front side 2.2x2.2 cm2 rear side 2.6x2.6 cm2

Simulation- alateral Energy deposition in single crystal Electrons at different energies were injected in the central of the crystal. 78% of the energy of the incident electron was deposited

Simulation- alateral Energy deposition in 9 crystal blocs of a 3x3 matrix central crystal of the 3x3 matrix Events deposition in the central crystal to that in all nine crystals is 85%. Events Energy (MeV) Toplam enerji E (MeV) Energy deposits in 3x3 PbWO4 crystals for 1 GeV electrons injected into the center of the central crystal Energies deposited in the nine crystals

Simulation- alateral Comparison of Geant4 and EGS4 200 MeV 400 MeV 600 MeV 800 MeV 1 GeV Event Numbers Energy (MeV) Energy spectra obtained by summing up all energy deposits in the nine crystals for the incident electrons of 0.2, 0.4, 0.6, 0.8, and 1.0 GeV, respectively. Energy resolution sE/E of the 3x3 crystal matrix Shimizu, H., et al. 2000. NIM A: 447,p.467

Simulation- alateral Energy deposition in 5x5 matrix Deposited energy as a function of incident electron energy 78% of the energy of the incident electron deposited in the central crystal. The total deposited energy in the 9 crystals 93% in the 25 crystals 96%. Deposited energy fraction as a function of incident electron energy

Simulation- alateral alateral Intrinsic energy resolution (alateral) for the 1x1, 3x3 and 5x5 crystals matrices as a function of incident electron energy

Simulation- ape PbWO4 – APD Simulation The light generated by 0.2-100 GeV electrons in the PbWO4 crystal has been obtained using with the GEANT4 simulation code. APDs electron The Single Particle Monte Carlo technique has been used to calculate APD output signals and their fluctuations

Simulation- ape Cherenkov and scintillation lights in the electromagnetic shower As the scintillation light is emitted in the wavelength region of 320 nm to 600 nm peaking at around 420 nm, the Cherenkov light is emitted with a characteristic spectrum. Cherenkov, scintillation and total photons spectrums at the end of the crystal

Simulation- ape PbWO4 Spectrums for 1 GeV electron Number of photons Cherenkov Spectrum Spectrum Scintillation Spectrum Cherenkov Spectrum Scintillation Spectrum Spectrum Number of photons Number of generated photons in the crystal Number of photons at the end of the crystal Wavelength (nm)

Simulation- ape Photon absorption by APD Wavelength (nm) Number of photons Explanations & Work: Simulation Mechanism Quantum efficiency variation with wavelength for the S8148 APD structure Total photons spectrums at absorbed in the APD

Simulation- ape A Single Particle Monte Carlo code by tracking the generated charge carriers through the APD motion of particles is spatially restricted in the device model each charge carrier is assumed to be independent of the others diffusion, drift and impact ionisation processes the charge released by an incident photon or by an impact ionisation can modify externally applied field Explanations & Work: Simulation Mechanism

Simulation- ape Avalache gain, number of electrons left from the avalanche region per number of primary photoelectrons entered. As the photon absorbtion depth is a function of wavelength, the charge generation in the avalanche region decreases the avalanche gain and increases the excess noise factor. Explanations & Work: Simulation Mechanism APD Gain and excess noise as a function of wavelength

Simulation- ape The behaviour of the mean signal and its fluctuation results from the combined effect of the wavelength dependent absorption coefficient of the incident photons and of the depth dependent avalanche gain in the depletion. Explanations & Work: Simulation Mechanism APD Signal variation versus wavelength Relative fluctuation in the APD signal versus wavelength

Simulation- ape APD Signal fluctuation or photo-electron statistics contribution on stochastic term has been calculated for Cherenkov, scintillation and total photons from PbWO4 . Relative fluctuation in the APD signal (ape) as a function of incident electron energy

Simulation- stochastic term a Stochastic term (a) variation as a function of incident electron energy for 3x3 crystal matrice

Simulation results- stochastic term a For 1 GeV incident electrons ape (photo-electron contribution) : 2.07 % alateral For 3x3 crystal matrice: 1.90 % For 5x5 crystal matrice: 1.42 % The total the stochastic term contribution to the energy resolution for 3x3 matrice The CMS test beam results : 2.8 % % CMS Collaboration, 2008. JINST 3 S08004, The CMS experiment at the LHC, p.90

Conclusion Simulation shows that; PbWO4 crystal- Hamamatsu S8148 APD are good combination for energy measurements. The Hamamatsu S8148 APD had been optimised for the photons with around the peak wavelength of the PbWO4 emission spectrum. PbWO4 crystal has low light yield, In the case of the entire emission spectrum of PbWO4 crystal; one part of the photons, between in the wavelength region of 320-450 nm cause an increase of the F for the S8148 APD structure

Conclusion TAPAN, İ., M.A. AFRAILOV, F. KOCAK. 2006. NIMA, Volume 567 (1): 268-271.

Conclusion ~120 photons/MeV ~66,000 photons/MeV PbWO4 ~120 photons/MeV ~66,000 photons/MeV The high photon values and the low signal fluctuations make crystal-APD combination an excellent choice for energy measurements.

Additional slides Contribution 26% At the end of the crystal Scintillation – 1 GeV C+S – 1 GeV Cherenkov – 1 GeV Contribution 26% Frequency At the end of the crystal İn the APD Number of photons

Additional slides Npe=number of output photons*0.074*QE The number of leaving photons as a function of incident electron energy Npe=number of output photons*0.074*QE

Additional slides

Additional slides Lateral shower development comparisons The ratio of the energy contained into a single crystal (E1) over the energy contained into a 3×3 and a 5×5 crystal matrix center around the hit crystal (respectively E9 and E25). MERIDIANI, P. Optimization of the discovery potantial of the Higgs Boson in the decay channel with the CMS detector. PhD Thesis. Universita Degli Studi Di Roma “La Sapienza”.