What I've done is take a large 2n3055 BJT NPN power transistor, and decap it (it is a large metal-can type). Then I carefully removed any coating from the exposed silicon (it typically has a dab of silicone potting compound on it).
Then, I had a weak alpha source at ~5MeV lying around the lab from previous work. This was inserted into the can with the beam facing downward towards the exposed silicon, and the can reattached and made lightproof.
Then I threw together the circuit shown here using the modified transistor (the base is left floating). What I expected to happen was that at TP1 (relative to GND), with my scope AC-coupled, I should see small voltage spikes followed by a decay. This is caused by alpha particles impacting the silicon and knocking loose enough electrons to permit some current flow.
However, I just see... more or less nothing, maybe some electrical noise from fluorescent lamps in the room next door. Certainly not the spike+decay curve I've seen with other detectors.
Did I make a wrong assumption somewhere? It's been a while since I worked with discrete transistors much, and I feel like I am missing something silly.
Or is this more or less right, and I should maybe question whether my alpha source is still good? Or whether the signal strength is in a voltage domain I can even clearly see without amplification? Or maybe I should suspect that a thin passivating glass layer is added to big BJTs these days, enough to block the alpha?
The source is past expiry, but not by that much. I'm mostly interested in characterizing and documenting the detector as an academic exercise.
Maybe -- easy to check, at least. I'll just shine a light on it :)
The coating came off pretty easily though. The bonding wires pass visual and mechanical inspection, and do not short on the case or other parts of the transistor.
That doesn't actually mean it's OK, there are cases where transistors and other descrete components are "semi-burnt" (tests check out, yet it doesn't work or doesn't work as it should). The ”not so reliable" test would be to use a multimeter and see the voltage drop between B-E and B-C. The definitive test would be to make an actual amplifier circut, use the transistor in it and see if it works and if it distorts the sound (there are also cases where the PN substrates are somewhat depleted or damaged, either through use or a manufacturing error, so it works, but distorts the signal).
Do the light test, see if that passes, then do the multimeter test, see if that passes as well. If they both check out, 99% chance the transistor is OK. That 1% can be eliminated with the test circuit amp test.
Wow, OK. It failed pretty hard. Fail on the light test, and failed to switch with the base saturated. Also measures a resistance close to zero between all pins.
I'm actually quite surprised! The potting compound 'surgery' went very smoothly, like peeling off a sticker. Well, these things happen when abusing semiconductors I guess. I've got spares, so no big deal. If it fails again, I'll go find an alternative BJT that does not have potting compound.
Thanks for the tip!
Glad I could help 👍.
OK, I repeated the experiment with a new transistor (which tested OK after modification).
Sadly, the results are the same. Oh well!
Hm... don't know why that happens, never made a particle detector, but I have modded TO-3 cased transistors to be photodectors, they usually worked great.
I suspect the reason it's not working, is something I don't currently have the tools to measure.
With an OK reflected light microscope I could work out whether there's a glass or clear epoxy coating on the silicon. With an alpha spectroscope, I could characterize the source better. Tools are cheap in Asia, but the space to put them costs a fortune...
So I'm going to shelve this for now and maybe try to build a BJT amplifier for a PIN photodiode detector. I've etched some boards. Fingers crossed.
the smart thing of course would be to buy a scintillator crystal, but I hate the inelegance of it. It shouldn't be necessary.