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.

  • Saigonauticon@voltage.vnOP
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    1 year ago

    OK, I repeated the experiment with a new transistor (which tested OK after modification).

    Sadly, the results are the same. Oh well!

    • 0x4E4F@lemmy.fmhy.ml
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      1 year ago

      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.

      • Saigonauticon@voltage.vnOP
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        1 year ago

        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.