PMT gain check at Indiana University

Test setup inside dark box LED white paper PMT # JT0298 channel #2 (per HPK numbering) OTHER CHANNELS MASKED WITH BLACK TAPE oscilloscope pulser trigger in C.R.T. Proto Pogo Pin HVB Direct connection of anode to MMCX/cable, with 10 kΩ bleeder resistor to ground. Scope’s voltage integral / Ω is PMT charge. The setup Integrate charge in fixed gate, triggered by pulser that fires LED. No discriminator / no cuts. No external amplifiers. production front board

integration gate 7.5 ns blue trace is PMT charge (since t = 209 ns) purple histogram is charge at end of the gate interval ↑ 80fC Typical results The LED is slow of course (~3ns width), but it is cheap

↑ 80fC Typical single event with signal

A few more details in case you want to know… Scope set to 800 MHz bandwidth limit, 2 mV/div Sampling at 20 GS/s (integral = 150 sample points sum) Interpolation between sample points is turned off. (Worry is, this might cause some unknown error in the integral.) LED pulser rate 500 Hz From look at single-shot acquisitions, even at very low amplitude such that only a few % of events are not pedestal, there is a sizeable fraction of double-pulse events. Either this is afterpulsing in the PMT, or a characteristic of my LED source. In latter case, my results are contaminated with 2-photon events. Probably it doesn’t make a big difference, but this needs to be considered. It is useful to note that math trace does not saturate to screen display limits. As long as ADC (ch1 display) does not saturate, all the data of that event is good. I adjust ch1 gain V/div so that it very rarely saturates.

↑ 80fC A double pulse event – they are not extremely rare…

3150 V Old results for reference – suffers from binning artifacts

3150 V Old results for reference – suffers from binning artifacts

3200 V Improved analysis: Integer waveform data stored from scope Integer sum for charge Binned as integer (here bin=charge/4) As before, then fit pedestal and subtract it, see next slide But, fewer events here (0.5 vs. 7.5M…) pedestal σ = 4.13 fC

3200 V hits (12%, I should lower LED intensity…) mean 65.6 fC (410 k electrons)