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.
I once got inspired by this: https://scoollab.web.cern.ch/diy-particle-detector It uses photodiodes instead of BJTs. Advantage is that you can get some which are easy to decap. I did not get it to work :/ but also did not spend so much time. I think what is really important is to properly shield the circuit from electromagnetic radiation, use a battery (low-noise) and also shield the detector from light.
I’ve gotten a similar circuit to work. Good shielding on the preamp was indeed key.
That was like 12 years ago though. Back then I used a battery. I probably know enough to get it working with a switched power supply now, which would be way more convenient.
The PIN diodes aren’t cheap though! Also some are export controlled. Not the one from that project though. I have a few around that I’ll use if I can’t get this to work.
The BJT method is attractive due to really low cost. I never managed to get it working though. There are enough independent reports of the method working online that I think it’s possible, but the documentation hasn’t been sufficient to easily replicate it.
It might be something boring like some manufacturers put a clear coating (e.g. glass) on the internals of a type of transistor, and others don’t.