1. CLAS 12 Status of the test bench Calculation of the time resolution
Meeting – Frascati, Nov , 2008
B. Genolini, M. Guidal, M. Imre, A. Maroni, T. Nguyen Trung, S. Niccolai, J. Pouthas, C. Théneau

2. 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

3. 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

4. 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.)

5. 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

6. 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))

7. 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

8. 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

9. 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)

10. 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

11. 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

12. 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

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