I've used my improved hardware setup (a Scionix PMT/Scintillator detector) to further improve other S/W and H/W components of my experimental XRF system. I started using the detector and full-bore Theremino-style filter/amplifier setup, but the results weren't all that much better. I began to suspect some software problems, particularly with regard to the triggering function, so totally re-wrote that portion, switching from a rolling average scheme to a majority-vote approach, where 8 pulses had to exceed the trigger voltage in order to initiate a pulse-detected signal. That by itself resulted in a significant improvement in the quality of the XRF spectra I was getting. At that point I was using some Thorium-doped welding rods as test sources.
The other change is that I reviewed the baseline noise coming out of my relatively simple filter + transistor amplifier design that's totally based on the Theremino design. I had favored it over a low-noise operational amplifier design because the noise voltage coming out of the opamp design was much higher: but that turned out to be due to the fact that it had much higher gain. I missed that crucial difference. Once I reduced the gain in my simulations, the opamp-based design performed substantially better than the discrete transistor design. So I designed a new filter/amp board using an opamp. The results were better, but not as good as I had hoped for. Additional investigation showed that the final RC low-pass filter, which in the Theremino design is just a relatively large capacitor hanging off the emitter of the buffer transistor, was much too "strong" -- the Theremino's emitter-follower's output resistance is pretty low, so even a relatively large capacitor wouldn't exhibit the low-frequency rolloff that my "improved" design did. So I reduced the value of the capacitor in my new circuit -- and suddenly my XRF spectra became a LOT better. At this point I can get a pretty decent XRF spectrum for Cadmium at about 23Kev.
But 23Kev is a long ways away from the ~5.6Kev iron XRF peak. And THAT has remained elusive, because the protective window over my sodium iodide scintillator crystal (in the Scionix detector) appears to crap out somewhere below Cadmium. I've tried detecting lead, at about 10Kev, and see a small peak -- but that's about the end of the road.
So at this point I have concluded a few things. First, the Scionix setup was very valuable in terms of improving my software. If nothing else, it's worth keeping for that alone. The second is that I need to come up with a different low-energy xray detector. One approach could be a cooled silicon PIN photodetector. I've found some made by Osram and Hamamatsu that are fairly large-area, about 3x3mm, with relatively low dark current and relatively low capacitance. Not too expensive, either. Cooling them with a 2-stage thermoelectric cooler (TEC) would reduce the dark current and improve the SNR. So that's one approach, but has its problems when it comes to dealing with condensation. I want to run the PIN diode well below 0C, so will need to enclose the detector with some desiccant -- or evacuate the interior -- to prevent condensation. That's do-able, but raises the bar for other folks who want to reproduce this. Alumina desiccant is good enough and pretty cheap so that probably is the easiest and cost-effective approach.
The other approach is totally different in terms of the detection mechanism. It uses a gas proportional counter, sort of similar to a Geiger counter, but it's operating voltage is lower so the pulses it generates are proportional to the incident photon energy, rather than being avalanche-multiplied to the point where the tube is saturated (this the Geiger mode). It's best suited for lower-energy xrays, which is exactly where I want to be in terms of the analysis work I want to perform (figuring out what kind of steel alloy I've got). If you've got a lathe the detector should be relatively easy to make. But it requires a continuously-flowing gas mixture of argon and carbon dioxide. That' not such a big deal, it is about the same as what's used when MIG welding -- but it would be an additional cost for someone who doesn't already have a MIG welding setup. It also requires a high voltage power supply -- but I already have that, since I needed one for my photomultiplier. I do NOT have a MIG welding setup, so for now I'm concentrating on a silicon PIN diode. But keeping this approach in my back pocket.
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