It shows that most of the Radon-222 that is generated from Radium-226 decay escapes the dials before it disintegrates into non-volatile daughters. * this assumes that most of the radon-222 escapes from the dial surface before it disintegrates into polonium-218, which is nonvolatile.Īn excellent article. However, we can pretty accurately estimate the number of radon-222 atoms produced in the three-month period just by measuring the current activity ("activity" = number of disintegrations per unit time) of the dial, as the total number of radium disintegrations in a period equals the number of radon atoms produced in the same period. This fraction might be modestly increased by removing the bezels. Whereas the detector might be very efficient at registering the passage of nearly all radon-222 disintegrations that occur within it, the fraction of radon-222 atoms that ever reach the detector from inside the watches before they disintegrate is likely quite small, and could be comparable or even smaller than background Rn-222 levels in the test environment. We need to know how efficient the alpha detection scheme is to know whether there is much of a chance of detecting anything. So if we measure the activity of the dials, which will be approximately unchanged since they were painted*, we can determine the number of radon-222 atoms produced in a three-month period, which will be nearly equal to the number of alpha particles produced by radon-222 decay in the same period. Nearly all the radon-222 produced in that three-month period will have disintegrated by the end of it. The half-life of the parent nuclide, radium-226, is 1,599 years, so only 27 ppm of the pre-existing radium charge will disintegrate to radon-222 in a 3 month period. Still, I don't think you would find much. Of course, there might be some radon in your environment anyway, so you should do a background control run too. It might be interesting to redo the experiment with the bezels removed. So that means, depending on how much radon-222 escapes the sample over its 3.8 day half-life, the total activity of the paint sample ("activity" is the total number of disintegrations of all kinds, per unit time) may actually increase for the first hundred years or so, as the concentrations of the first six radium-226 decay products build towards "secular equilibrium."Ĭlick to expand.I would imagine that much of the radon produced in the dial, which is all radon-222 with a half-life of 3.82 days, has already disintegrated before it leaks out of the watch cases into the surrounding air. Only one of those short-lived isotopes, radon-222, is a volatile. The first six disintegrations after Ra-226 have a combined half life of about 22 years, dominated by the polonium-210 half-life. The decay chain continues on from there, but it hardly matters. But I have always shied away from uranium and thorium minerals, because I just don't want any more alpha emitters around me than the americium-241 in my smoke detectors.īy the way, Ra-226 undergoes a sequence of seven consecutive disintegrations with short half-lives before reaching Bismuth-210 with a half-life of 3 million years. I'm a physicist, I occasionally teach the Applied Nuclear Physics course to the Navy engineers who come to my lab for training, and I also collect minerals. (The gamma's put out by the radium dials are not worth worrying about.) And that tends to be much more carcinogenic than the same whole body dose of gamma radiation. (Your outermost layer of dead skin will stop nearly all of them!) However, for similar reasons, if you inhale or ingest a tiny, dust mote sized piece of alpha emitter, and it gets lodged in the lining of your lung or your intestine, it will deliver a large local dose of radiation to a very small volume of tissue. Alpha particles are completely harmless when their source is outside the body, because they are heavy, slow charged particles (i.e., they have a small charge-to-mass ratio) that give up their kinetic energy and stop in a very short distance when traveling through matter. If you're going to collect radium watch dials, I would wash my hands after handling them, just out of an abundance of caution.
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