[Solid_ec] Npe estimation from beam tests, suggestions on HV, and possible meeting on Monday?

[Xiaochao Zheng]

Dear all:

As we discussed during the meeting, we need to estimate whether the peak we see in the Preshower and the Shower are MIP or electrons, or something else. I did a quick estimate of how the integrated FADC channel relates to the number of photoelectrons, Npe. Assuming Ye's calculation simply add all FADC readings within a pulse together, then:

(integrated FADC) = (8E-6)*Gain*Npe

where Gain is the PMT gain.

Using this equation, the Shower peak we see in SDU1 and SDU2, with the PMT gain clearly known (measured at SDU to be 5E6 at the current HV setting), corresponds to Npe=122. This likely is the MIP and now shower particles, based on the vertical cosmic test that the MIP Npe is about (0.5-1.0)pe per shashlyk layer (and we have about 200 layers). However, there is a question about how SDU2 compares with SDU1, since SDU2 is expected to produce 80% higher light yield than SDU1 due to using newer scintillators. this means if they use the same PMT gain then the SDU2 spectrum should be 80% higher than SDU1. We don't see this.

For THU shashlyk, there are two peaks observed at 2000 and 5000. We don't know the PMT gain so can't reliably estimate the Npe. And the SDU measurement on the PMT gain clearly differs from the manufacturer chart for the same type of PMTs, so we can't trust the chart to estimate its gain from the HV.  The THU shashlyk does use a much higher HV though, but it's likely that this PMT simply needs much higher HV to produce a similar gain. (Rather than, for example, the peak at 2000 corresponds to the SDU 1 & 2's 5000, because the THU 5000 peak is much stronger and if it's also present in SDU 1 and 2, it would be at channel 12500 and should be seen.)

For the Preshower, we also don't have any measured value for the PMT gain. OUr best estimate is on THU preshower, where the sPE peak seems to be isolated from the pedestals. Using the equation above, and if assuming the HV follows the photonics gain chart #B, I got Npe=0.6 for the SPE peak location and npe=25 for the main peak. So likely the HV for this PMT is not too far from the chart and the actual Npe is 40. This is not too far from the Npe=78 that we tested at UVa. (Also, note that we see a similar reduction in the FASPD's light yield, from ~(9 ~ 10) at UVa to ~7 in the beam test.)

For Preshower of SDU1 and SDU2, we don't see the sPE peak and we don't know the gain. So we don't really know what the NPe is.

Based on the above information, I would suggest:

1) For all 3 preshowers, increase the HV in small steps until the SPE peak is clearly isolated from the pedestals. This way we will know exactly what Npe is. And we can even deduce the PMT gain.

2) For the shashlyk, it's more complicated. If the main peak now at 5000 is the MIP (corresponds to a pulse of height 1000~2000 and (12-20)ns wide), should we lower the HV and attempt to see shower particles?  The main difficulty is, if at a 76 deg angle, the electron rate is 1E4 lower than other particles, then we will never be able to see the electron signal even with the aid from FASPD. It will simply be buried in the pion background. We will need a Cherenkov to see it.  Also, if the electron response is factor 4 higher than pions, we should see them showing up as saturated signals (since FADC range is 4091). But we don't see them right now.

On the other hand, maybe we should lower the HV and give it a try anyway?


Perhaps we can have another meeting on Monday to discuss this? We could set a time after the main SoLID meeting.

Xiaochao

----- Original Message -----
From: "Xiaochao Zheng" <xiaochao at jlab.org>
To: "solid ec" <solid_ec at jlab.org>, "Solid daq" <solid_daq at jlab.org>
Sent: Thursday, December 8, 2016 6:57:18 AM
Subject: [Solid_ec] SoLID EC/DAQ meeting 9:00am Thursday 12/8/2016 CC F227

Dear All:

We will have our SoLID EC/DAQ meeting at 9:00am US EST on Thursday 12/8/2016. The location at JLab is CC F227.  

The call-in instruction is shown below. Meeting minutes can be found at
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Thanks,

Xiaochao

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