[Halld-cal] FCAL monitoring pulse height (was Re: PMT in Athens)

[Matthew Shepherd]

Dear Stratos,

I'm cc'ing the entire calorimetry list since I'm including more info about the pulse sizes.

Unfortunately, the code I need would be on the side of the PMT.  This would require disassembly of the unit, which might not be a good idea.

Let's assume that your PMT is of average gain.

Kei has done some nice work with understanding the correspondence between light output and energy deposition.

I can just summarize quickly:  for the running configurations we expect in the actual experiment, we should aim to adjust the HV so that roughly a peak pulse of 4000 ADC counts or about 2 Volts corresponds to 6 GeV of energy deposited in the block.  That is 2/3 of an ADC count per MeV.

For a typical PMT we observe based on beam test data that running around 1650V produces about 1/2 of an ADC count per MeV while 1700V is about 3/4 of an ADC count per MeV.  I believe you were running your PMT at  1670 V, so it is safe to assume you picked a typical operating voltage expected in the actual detector.

Looking at physics events, our most frequent energy depositions are in the 200 MeV - 2 GeV range.  The system should work well over this range of pulses.

200 MeV * 2 Counts / 3 MeV  * 2 V / 4096 Counts ~ 65 mV

So, for your FEU PMT operating at 1670 V and viewing the Plexiglas through the bar, we'd like to have pulses from the monitoring system that are in the 65 mV - 650 mV range.  As was discussed, the most important thing is long term time stability.  If we could choose an ideal system, we'd want 300 mV pulses in every PMT for each color, but we know that this is not achievable.  The order of importance is (in my opinion):

long term stability
reasonable range of pulses for one color
reasonable range of pulses for more than one color (blue and not blue)
uniform pulses in all detectors

Keep in mind that radiation damage kills the blue transmission so a color that is clearly blue and one that is not blue is desirable.  However, a range of colors provides even more information.

I hope this helps firm up the requirements.

Matt


On Feb 15, 2013, at 7:39 AM, Efstratios Anassontzis <eanason at phys.uoa.gr> wrote:

> Dear Mat
> 
> attached you will find a picture of the barcode on the PMT block that we have in Athens. I couldn't find any reference number but I suppose that you can read the barcode (and it means something).
> 
> Best regards
> 
> Stratos
> <DSC00398.JPG>