CLAS 12 Status of the test bench Calculation of the time resolution Meeting – Frascati, Nov. 26-27, 2008 B. Genolini, M. Guidal, M. Imre, A. Maroni, T. Nguyen Trung, S. Niccolai, J. Pouthas, C. Théneau http://ipnweb.in2p3.fr

Current status Received: Existing scintillators for the coincidence Reference readout PMT (XP20D0) HV_set PS module BC408 + wrapping Output signal Additional scintillator (thicker) for enhanced trigger resolution Wide band amp. (≈ 2 GHz) 60 cm Received: The scintillator (BC408) – new and shorter (60 cm) than the previous All components to make the SiPM readout the SensL SiPM: noisy / need improved test bench Hamamatsu: the 3×3 mm2 SMD, 1×1 mm2 devices Extruded scintillator + WLS fibers Preliminary measurements on the old test with Hamamatsu SiPM Test Bench ready for testing Further work Stuff the PCBs (including the PS + preamp) Measurements with SiPMs Test bench with the extruded scintillator

SiPM signals (1×1 mm2) Hamamatsu “MPPC” Single pe Single pe + afterpulses Double pe Acquisition: 1.5 GHz BW, 10 GSPS 100 ns 10 µs Hamamatsu “MPPC” Acquired on the “old” scintillator Rise time: about 1 ns

SiPM signals (3×3 mm2) Hamamatsu MPPC Full scale 200 ns 200 ns Zoom 20 ns 20 ns Hamamatsu MPPC Larger rise time due to the increased detector capacitance Acquired on the “old” scintillator (several p.e.)

Zoom on the SiPM circuit case The new test bench Reference readout PMT (XP20D0) Coincidence scintillators Mobile support Trigger scintillator Test readout: PMT or SiPM Trigger: the time reference is taken from the thickest scintillator, validated by the coincidence of the two others Mobile support to scan the scintillator Test readout: PMT as the reference, or SiPM (in a box, for shielding) Zoom on the SiPM circuit case

Measurements with PMTs mercredi 15 mai 2019 Time resolution (including the trigger resolution): 180 ps RMS (fit= 150 ps) – 440 ps FWHM 200 ps Mobile average over 100 events total duration: 2.5 days Blue: Poisson distribution for the same photon average Preliminary results (low thresholds) Intrinsic time resolution: < 94 ps (72 ps) (“old” test bench: 110 ps (78 ps))

Simulations Questions: Method What is the best method to measure the time resolution? What is the influence of the noise on the resolution? (especially with a SiPM) Method Favor analytical method Computation only when analytical formulae were non available No Monte Carlo

Definitions and hypotheses F (t) integration Poisson distribution of photoelectrons (p.e.) based on the average time distribution Cumulative function (F) serves to calculate the probability of p.e. count on a time interval: Ref: . F. Post and L. I. Schiff, Statistical Limitations on the Resolving Time of a Scintillation Counter, Phys. Rev. 80 (1950) 1113

Example: exponential model Add the photon emission and the PMT noise The model takes into account the noise rate and the average number of emitted photoelectrons (R) Analytical expression for the PDF of the first pulse (and also for the Q-th one)

Resolution of the first pulse Noise = 2×107 cps, R = 5×10-2 p.e. Calculation with the analytical expression of the 1st pulse PDF + simplified expression of the tts. Important: resolution calculated for the first pulse after a fixed trigger Landau distribution: expected to add a correction factor In the case of a PMT, with a fast decay: resolution dominated by the PMT transit time spread (tts) Averaging of signals due to the overlap of close signals In the case of a fast detector, without overlap of close signals (especially if measurement on several channels), the intrinsic time resolution can be observed

First pulse method and noise Noise = 2×107 cps, R = 5×10-2 p.e. At high level of noise, need for a large value of R. Coincidence / high threshold needed

Resolution of the coincidence Method No simple analytical expression found, but possible to evaluate the PDF of the first coincidence of Q p.e. Threshold to be determined according to the noise and the number of emitted p.e. Application to CLAS12 (orders of magnitude) Total noise: 10 SiPMs × 1 Mcps ≈ 107 cps Expected noise p.e. over 10 ns = 0.1 Long calculations / rough estimate for the resolution = a few ns Proposed method: make a coarse trigger defined as a given number of photons in coincidence (“high” threshold) Make the trigger as the first pulse in the coincidence window (“low” threshold) Coinc. window trigger Coarse trigger

Next steps Measure and model the SiPM afterpulses (additional noise / dead time) Modify the analytical formula for a Landau distribution