Dimiter_Popoff wrote:
> On 1/19/2022 17:52, Phil Hobbs wrote:
>> Piotr Wyderski wrote:
>>> Hi,
>>>
>>> the literature says that avalanche gas detectors are capable of being
>>> run in proportional mode where the voltage is below the Geiger
>>> plateau. Would it make sense to operate a sufficiently undervolted
>>> Geiger tube as a proportional detector for the purpose of simple
>>> gamma/beta spectroscopy or is there some inherent property preventing
>>> it? E.g. the gas composition?
>>
>> A proportional counter is a fairly heartbreaking thing to use for
>> spectroscopy--even a sodium iodide scintillator is much much better.
>>
>> OTOH for a hobby project, it would probably work OK.
>>
>>> If it can, then would a 10-way monster like the SBT10A be partly run
>>> in proportional mode and partly in the usual avalanche?
>>
>> ISTM that you'd need to prevent ions from migrating around inside
>> somehow, unless you're okay with a fairly long dead time after each
>> Geiger pulse.
>>
>
> Hi Phil, just related - have you experimented with NaI and some of the
> photodiodes etc. you seem to do a lot of work with? My current
> understanding is that PMT-s are still better by quite some margin
> but then I don't give NaI much thought (although our netMCA is
> HPGe grade we do have customers who use it with NaI though).
Nothing can touch a PMT for dark counts per unit area--they're five or
six orders of magnitude better than an APD or SiPM / MPPC.
A typical 100-um diameter APD has about the same dark count rate as a
_four_inch_ PMT.
Plus you gain another factor of at least 2 by using coupling gel between
the crystal and the PMT face.
You'd think it was right around 2 because you win etendue by the square
of the refractive index, but it's actually better than that--the extra
light at high angles gets totally internally reflected at the
photocathod/vacuum surface, so it gets at least two passes through the
PC, which raises the quantum efficiency considerably.
There are tricks to do even better, e.g. by using prism coupling to make
light rattle round inside the faceplate till it gets absorbed. With a
negative electron affinity (NEA) photocathode, that'll get you almost
100% QE over a wide bandwidth.