Particle induced Ionization

Alpha particles are very energetic little buggers. In fact, energies of 20MeV per particle are not uncommon. While this makes them incredibly dangerous if you happen to ingest something that emits them [radon, polonium] it can also make make for some rather interesting macroscopic phenomena.

As most [some?] of you may know, smoke alarms contain a small amount of americium 241, usually along the lines of 1 or two microcuries’ worth. (37 to 74 GBq). There, a small charge is placed across two plates and the voltage across them is taken note of by either an analog network or a micro controller. This voltage is not set in stone though.

Inside, that small button of 241Am is working some magic between those plates. Normally we don’t think of air as conductive, and for good reason; it’s not. But when we introduce a cloud of alpha particles we change this, and a very small current is able to flow across the plates. This brings down the potential difference to something lower than it ought to be, and we make an ionization chamber.

But then, someone burns popcorn and the room fills with smoke. The smoke particulates find their way into the ionization chamber and begin to screw everything up. The energetic particle cloud is absorbed by the smoke, current ceases to flow and the voltage rises to some arbitrary, yet abnormal value, and an alarm is triggered.

What’s interesting though, is that this effect becomes highly visible when a high voltage is placed across theses plates. Air as we know is an insulator, [to a certain extent], but once we add some alpha particles to the mix everything begins to change.

Neat, isn’t it?

By no means  though, is it a very good radiation detector. A device such as this is completely insensitive to anything but alpha radiation, and very high particle fluxes are needed to see any effect whatsoever. (That’s a 75uCi source!)

Still, it demonstrates the principle of a geiger-muller tube quite well, even without a low pressure gas fill.

Carl Willis’ apparatus is much better than this, so it might be worth your time to check out his video.

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