I'm a little behind in the discussion since H-M was down for a bit. Anyway, to comment on some of the recent discussion points....
1. A/D resolution and speed. My comment here is based on reading through the Theremino gamma spectrometer information. They are using the 16 bit A/D on a USB sound card, so it's limited to a maximum sampling rate of 192KSPS. That's not nearly fast enough to capture the pulses coming out of a PMT, so they are stretching the pulses using a simple 2-pole low pass filter, then amplifying by 100 to get the pulse amplitude back high enough to get decent numbers out of the A/D. This is a "nice" approach because it permits the use of _very_ inexpensive A/D's; and the drivers are generally included with the computer OS (the Teensy approach would use the provided audio library to accomplish the same thing). The LP filters also improve the SNR which helps improve the resolution of the multi channel analyzer.
The cheap A/D kit from PJRC can be found
here
The Theremino group also has some information regarding the energy resolution (in FWHM) vs A/D bits. You really do want at least 14 bits. I believe more bits is better due to the method they use to find the pulse peak in the presence of noise and the inevitable misalignment of sample time relative to the peak -- it is highly unlikely you will actually sample the peak, so some form of curve fit or interpolation is needed. The energy resolution is completely dependent on how accurately the peak voltage can be measured. You'd then conclude that one of the fancier audio A/D's that can get to 24 bits would be better yet: but in fact those A/D's really don't give you true 24 bit resolution. From what I've read on the web about this, the lower 4 bits are basically noise. So 16 bits is the sweet spot in terms of "real" system performance.
2. Regarding shielding. Ebay has 1/8" thick lead sheet for not a lot of money. I was figuring on cutting strips and rolling them into tubes, with the ends overlapping to avoid leakage. Another approach would be to nest a small tube inside a larger one and fill the gap with lead shot. You'd need a thick enough layer to make sure no stray x-rays got through there, but that also could be a way to form odd-shaped shielding pieces -- make a mold, fill it with lead shot and pour in some casting resin.
3. PMT's ARE sensitive to magnetic fields. SiPM's are not.....although the Hall effect still is in play, apparently it doesn't affect solid state detectors to any significant degree.
On a slightly different note, I found some WAV files some folks had made, recording pulses coming from a scintillator/PMT combo. They also can be found on the Theremino web site. I have some ideas on improving the data processing over & above what the Theremino folks are doing, and I can play with those by using the WAV files -- no hardware needed! In addition, there's no reason why we couldn't do the same and share the WAV files to evaluate/compare our results. The only downside is that even a few minutes worth of recording produces large WAV files. It probably would be best to put them on something like Google Drive or the like -- I suspect H-M wouldn't permit the attachment of files that are tens of MB in size....
