[Halld-pid] ST First Tests

[Fernando J Barbosa]

Hi Mark, Werner,

I have been optimizing and testing the ST readout for the 50 um and 100 
um SiPMs. There are 4 SiPMs connected together (current sum) to one 
readout channel on each of two SiPM PCBs (50 um and 100 um). The sensor 
capacitance is the same in both cases so one optimization is all that is 
required for fast risetimes for the group of 4 SiPMs.

The prototype is fully functional but I determined that the micro-coax 
ribbon cable assembly (connects the readout to the breakout PCB) is not 
good enough due to cross-talk (primarily due to the connectors on these 
cable assemblies - a single micro-coax cable assembly provides the power 
to the electronics, 3 bias supplies, 6 outputs [3 channels w/ 3 for ADCs 
and 3 for TDCs] and has a thermocouple two-wire output for readout). The 
readout is temperature compensated via thermistors. The workaround has 
two coax cables (ADC and TDC), in addition to the micro-coax, which 
bypass the micro-coax signal connections. So, the final design will 
change but the current prototype will be useful for tests with ST 
detector prototype.

I have attached two pics: ST50 is for the 50 um and ST100 is for the 100 
um. These were taken with a laser and a diffuser stimulating all 4 SiPMs 
in the group. Some comments:

1. 50 um - Fast pulses with width of 37 ns (~10%-10%)
2. 100 um - Fast pulses with width of 125 ns (~10%-10%), step/kink ~ 5 
ns after peak.

The leading edge slew rate is about the same for both but the 100 um has 
a slower recovery w/a kink (the quench resistor on each pixel would need 
to be ~3x smaller for similar recovery but there are other factors at 
play, i.e. Geiger current) . I will investigate this a little further 
but these are definitely characteristics of these SiPMs and I will write 
a GlueX note soon.

In the meantime, let me know if there is interest in pursuing the 100 um 
further. Even though the rates are expected to be in the KHz range, the 
long recovery time will likely lead to pileup.

Best regards,
Fernando
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