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betavoltaic cell anyone? Answered

Has anyone built a betavoltaic cell? Tell us about it! I could imagine someone collecting the isotopes from old smoke detectors,painting them onto an old solar cell and making electricity in the dark. I'd try it myself if I wasn't such a lazy old fart.


Won't work. Why not? The rate is way too low. Photovoltaic cells work (as you clearly know from your idea) by having visible or UV photon "liberate" electrons in the material (technically, they raise bound electrons above the bandgap energy). You'd think that a radiation source would do that as well, and you'd be right.

There are two problems with it. First, the energy scale -- a semiconductor bandgap is roughly 1 eV (the exact value depends on the material, doping, etc.; I'm just doing order-of-magnitude), which is similar to the energy of a visible or UV photon. The betas or gammas emitted by radioactive materials are MeV. A thin p-n junction just isn't going to absorb all that energy -- the particle will transfer a few eV to a few electrons, and then just pass right through.

The second problem is rate. Solar irradiance is roughly watts per m2, which corresponds to 1018-ish photons. Consumer-scale sources (alphas, betas, whatever) are extremely small (µCi or less), so just a few thousand particles per second.

There aredevices (RTGs) which convert radioactivity to electricity, which are used on spacecraft, but you're not going to convert a solar cell into one :-)

Dangerous business, also if it's beta radiation you're after smoke detectors use an alpha emitter... It does seem like a cool project, could you not just throw a bunch of solar cells down mines that have been mostly depleted but still emit radiation... It's maybe not that efficient a process though, granted my guess would be that most ionizing radiation is a great deal more energy dense than light and UV, so would you get more power from it...

I also want to make something like that. I made a forum on nuclear reactors and batteries and a betavoltaic cell is a type of nuclear battery. I'd probably have made one by now but my parents probably wouldn't want me playing around with nuclear or radioactive materials.