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Influence of SiPM Single Photon Timing Resolution on Coincidence Timing Resolution with Fast Scintillator

This project meeting presentation discusses the estimation of coincidence timing resolution (CTR) in TOF PET imaging based on the parameters of the photodetector (analog SiPM) and scintillator. An analytical model is derived for CTR as a function of single photon timing resolution (SPTR), pulse shape, and other parameters. Experimental measurements and analytical calculations are presented.

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Influence of SiPM Single Photon Timing Resolution on Coincidence Timing Resolution with Fast Scintillator

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  1. Influence of SiPM Single Photon Timing Resolution on Coincidence Timing Resolution with Fast Scintillator. md-NUV PET project meeting 1E. POPOVA, 1,2S. VINOGRADOV, 1D. PHILIPPOV, 1P. BUZHAN, 1A. STIFUTKIN, 1National Research Nuclear University «MEPhI» 2Lebedev Physical Institute RAS 18 April 2018 AQUA EPFL Microcity, Neuchatel

  2. For the TOF PET ( and another timing applications) we are interested to Estimate a coincidence time resolution CTR on the basis of known photodetector and scintillator parameters. • Choosing of the best photodetector • Choosing of the best scintillator • Chosing of the best photodetector and scintillator • * Photodetector – analogue SiPM • The goal of our studt – to derive CTR as an analytical function of SiPM and scintillator parameters: • single photon time resolution SPTR, pulse shape (single electron response, SER), PDE - SiPM • trise and tdecay of scintillator, photon numbers - scintillator 2

  3. Before we will talk about analytical model for the CTR, we have to define SPTR itself Uniform light distribution over the sensitive SiPM area, selection of single photoelectron events Draft Timing group, S.Gundaker We assume that SPTRdetector is important parameter that affects CTR, however we often measure SPTR and CTR for the different electronics conditions (different noise and SER) How we can estimate electronics noise and BW influence on SPTR value in order to perform transition between the different setups? Or how we can estimate for the used electronics the SPTRdetector ?

  4. The main idea of experiment: Instead of measuring of CTR with annihilation gammas we study MPTR by using light source with variable pulse shape and intensity Experimental study of SPTR and MPTR and analytical model for MPTR 4

  5. Timing measurements with KETEK SiPM+amplifiers assembly • SiPM + Amplifiers PCB • New timing optimized SiPM Experimental setup: • picosecond laser (405 nm, FWHM ≈ 40 ps) • advanced timing optimized 3x3 mm2 KETEK SiPM chip and specially designed (by S. Ageev) and produced monolithic trans-impedance amplifier(s) (BW 1.5GHz) on PCB assembly • External KETEK evaluation kit amplifier • thermal chamber with light protection T=-30° C • digital oscilloscope LeCroy WaveRunner 620Zi (2GHz, 20GS/s ) • PMT-monitor for calibration light intensity into Npe 5

  6. Digital signal processing LED threshold = 15 mV FWHM ≈ 1 ns • 1_phe pulse shape, Uov = 9.5 V • 1_phe pulse shape, Uov = 4.5 V • DSP: time stamps (Uov = 7.5V) • Linear fit of baseline • Time stamp is the moment of crossing threshold with linear fit of rising edge of pulse • Charge selection: only 1phe pulses (for SPTR data) 6

  7. SPTR measurements Temperature = -30 С° • 3x3 mm2 SiPM, • SPTR = 112 ps • SPTR histogram, Uov = 9.5 V LED 15 mV 7

  8. MPTR measurements • 1) Light source – laser, FWHM = 40 ps, MPTR measurements • 2) Light source – • laser + WLS-fiber, • Tr ≈ 80 ps, Td ≈ 1.8ns, • scintillator-simulated experiment In order to study MPTR dependence on SPTR we carried out two experiments (T = -30°C, Uov = 4.5V, SPTR = 147 ps): 8

  9. Timing resolution - analytical model (S.Vinogradov) Filtered marked point process Analytical model “Amplitude noise” for timing resolution • Number of photoelectrons • Excess noise factor of SiPM (include DCR, XT, AP) • Probability density function of light • Probability density function of SiPM SPTR • Single-electron response function (SER) Constant threshold at the first photon- no CT, no AP, no dark rate ENF≈1 9

  10. Analytical model (laser light) • Gaussian shape of laser pulse and SPTR allows to get CTR dependence on SPTR: • in case if SER is a Heaviside step response it has an analytical form: • in case if SER is a bi-exponential with rise Tr and fall Tf times it has an analytical form: For typical SiPM pulses (Tr = 0.5..1 ns, Tf = 1…100 ns) dependence of CTR on Tr and Tf is rather weak, so it can be approximated as:

  11. MPTR experiment – short laser pulse Uov = 4.5 V, T = -30 °C SPTR Analytical model: Tr = 0.5 ns, Tf = 1 ns Laser trigger electronic jitter? (not include in model) Experimental Fit 11

  12. Experiment MPTR with laser+WLS-fiber • MPTR histograms (Tr ≈ 80ps, Td ≈ 1.9ns): • top – Npe ≈ 0.2 bottom – Npe ≈ 52.3 Experimental Fit • MPTR FWHM, ps (CTR with scintillator simulation) vs Light intensity • Uov = 4.5 V, T = -30 12

  13. Analytical model calculations: MPTR as function of SPTR for scintillator-simulated pulse Npe = 100, Td = 18 ns Npe = 1 Npe = 100, Td = 1.8 ns Npe = 10 Npe = 100 MPTR has regions with different dependence on SPTR Kind of plateau for smaller SPTR value is connected with WLS rise time (80 ps) 13

  14. Summary • PCB assembly of 3x3 mm2 KETEK SiPM with amplifier was measured, SPTR of 112 ps was achieved (9.5V OV, -30 °C). • The multi-photon timing measurements with different pulse shapes were carried out to show how coincidence timing resolution depends on SPTR. • Analytical model of “Amplitude noise” has a good agreement with experiment results for light intensity Npe > 1. • MPTR for short light pulse may allow to extract true SPTRdetector (not affected by noise) – should be checked • Analytical model shows how MPTR depends on SPTR for long scintillator-like pulses, but it should be checked with more experimental data. 14

  15. BACKUP 15

  16. Timing measurements with new PCB – multi-photon TR results • Timing resolution vs Light intensity (in fired pixels), Uov = 4.5 V 16

  17. Timing measurements with new PCB – CTR simulation experiment – results • Simulated coincidence timing resolution vs Light intensity (in fired pixels), Uov = 4.5 V 17

  18. Light intensity calibration: laser & laser+ WLS (CTR simulation) 18

  19. Analytical model: CTR as function ofSPTR and other parameters Modern analytical approaches: • Monte Carlo simulations, • Detection event statistics, • Order statistics of photoelectron detection time, • Cramer-Rao lower bound estimation. 19

  20. 3 1 ´ 10 Experiment Model Experimental fit Time resolution (FWHM), ps 100 10 3 0.1 1 10 100 1 ´ 10 Number of photoelectrons per pulse • MPTR (CTR with scintillator simulation) vs Light intensity Uov = 4.5 V, T = -30 ℃ Experimental Fit Analytical model: Tr = 0.5 ns, Tf = 1 ns 20

  21. Experimental Fit • Uov = 4.5 V, T = -30 ℃ 21

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