[Halld-pid] PMT test results question

[Beni Zihlmann]

Hi Sasha,
upon further discussion with Elton I think I now understand the
problem better.
the main issue is that the Am241 source produces only a few photo
electrons. So the ADC peak you see is a Landau distribution close
to the pedestal. Because you have to trigger on the the same signal
you are looking at the threshold relative to the signal hight is important.
So when a PMT has a much larger gain then a constant threshold will
lead to a different result as compared to a PMT with a low gain because
the threshold will cut into the rising edge of the landau distribution. That
is must likely the reasons for the still rather large variation you see in
the quantum efficiency of the different PMTs.

Generally I think the Am241 source is not a good tool to measure the
photo electron efficiency because of the too small light output. I think
just using an LED with sufficient light output and monitoring the stability
of it will do a much better job. Then you can fit the ADC peak with a 
Gaussian
and the width divided by position will give a much better result.

cheers,
Beni


> Hi Sasha,
>
> yesterday I tested both tubes the ZQ2248 and ZQ2246 using an LED
> light source behind a diffuser. I see exactly the factor of 2 difference
> in signal amplitude and width between the two as expected from the
> difference in gain. The width/spread of the signal amplitude is also
> compatible with this factor of 2. This means that the photon detection
> efficiency or quantum efficiency of the two tubes must be the same
> or very similar.
> I will try to do a more accurate measurement than just looking at
> the scope but already this result tells me that both tubes are fine
> and do not exhibit any vast difference in quantum efficiency at all.
>
> It seems that I overestimate the light output of the AM241 source
> by a huge factor.
>
>
> cheers,
> Beni
>
>
>
>> Hi Beni,
>>
>> I have tried both of your suggestions - going to much lower HV
>> and using light guide. Unfortunately, the only effect achieved was
>> making the signal smaller. The large spread of signals on the scope
>> from Am241 source was still there. At about 1500V, most of the
>> signal already went below 30mV threshold I set for scope triggering.
>> Once every few seconds, I would see a large and varying pulse (supposedly,
>> from cosmic rays) and that's about it. Nothing resembling a stable large
>> pulse from the Am source anywhere from 1750V to 1300V, but only a small
>> signal with large spread.
>>
>> Sasha
>>
>> On Tuesday, October 30, 2012, Beni Zihlmann wrote:
>>> Hi Sasha,
>>>
>>> I would expect that the source should produce thousands
>>> of photo electrons in the PMT. So the signal should be very
>>> very very large.
>>> Can you reduce the HV by a lot? like  running at 1400V or so and see
>>> what that does to the signals in the PMT?
>>> Also you could put a light guide in between the source and the PMT to
>>> make sure that the very low UV part is absorbed before hitting the PMT?
>>>
>>> cheers,
>>> Beni
>>>
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