by Doug Coulter » Mon Dec 08, 2014 2:46 pm
Well, Philipp, I'd have to do some guessing, which is a domain I'd rather not be in.
I did not change the trimpot or try different time constants on the AC coupling, either of which could account for the differences I see here. I fear that the larger of the two detectors has a flaw of some sort - I did see some junk on its output waveform here. I really doubt any excessive noise was the issue, at least from what I saw on the scope. You did seem to run longer than I did - which averages things better, of course. I was running this over a grounded metal plate here, and in a place that is low-noise to begin with.
Your scheme of waiting for a signal to drop entirely below threshold means variable droop in your S/H if the time varies or the threshold, and that alone would account for (most of) the differences we're seeing here.
I see WAY too much compton scattering in these plots (both of ours) which John Futter tells me is mainly due to how small both of the detector crystals are - they are from PET scanners and were only used for detecting coincidences above some threshold. Indeed, in tests here with those and a "big jug", the big jug shows a far better spectra, with very low compton "noise" between the low energy gamma and high one from Cs-137, quite a few dB down there, if you will. Hard to beat a huge chunk of detector - if a gamma hits the middle, it gets stopped by more of the xtal before it can scatter out. It could be you don't have a "good geometry" experiment there - other nearby things scattering gammas back into the detector, or something between the source (like air) and the detector. It's hard to get right.
Due to one of my scopes getting "built in" to the fusor setup, and another in a place where it's hard to work on this, I bought another and am reviving my own little effort at this. I hope to have some worthwhile results, better or worse, soon. I had some issues with my comparator reversing sense at its common mode input range, since fixed, and a couple of others. And now I see that something in the arduino-uno (the chipkit variety, not atmel) shorting my "hold" periodically, which could be either a very short glitch out of the comparator, or more likely, something odd in chipkit's adaptation of the arduino code and IDE for their own use - I'm currently suspecting the latter, since it does work quite well on a fixed level or low impedance signal generator.
But there I'm just guessing. I will know soon, and report.
I did find with a replacement version of the larger bichron that you can pry off the back fairly easily (it's merely tight-fit and weak silicone). Some of the noise I saw in it's output (the one I sent you) could be explained by a bad resistor or connection in the phototube divider chain - it would be hard to attribute it to the xtal itself, which BTW, is possible to reuse if you take the old phototube off it and supply something else (as I also sent you). It's a pretty major piece of detailed surgery, though.
One I have here I simply cut right behind the crystal on a metal band saw and then popped off the phototube (crazy glue or similar was used, very brittle) and put on another, and then made a light tight interface again. That's kind of brutal. If you could get that tube loose without cutting the main box - and put in another - that would be a much nicer way of doing things. As far as I can tell, there's no reasonably safe way to just pull the crystal out the front. 5 sides are metal encased and 4 are securely glued to the outer envelope. The phototube end has a glass window that is epoxied onto the otherwise metal case to be hermetic (NaI doesn't ever want to see humidity, it degrades very quickly). So hacking off the entire thing was actually the safer way - no risk of breaking the seal, but you definitely ruin the rest of the box doing that.
You might try those BGO xtals, end on, on that dual phototube I sent as well. Though it's not as good as NaI in theory in resolution, it's not bad either - the pulse is faster though - and due to its very high density (and high Z of most of the chemistry), has less compton scattering vs size than NaI does.
Posting as just me, not as the forum owner. Everything I say is "in my opinion" and YMMV -- which should go for everyone without saying.