[Halld-pid] PMT test results question

[Beni Zihlmann]

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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