Suppose that a raidoactive brick of an unknown element with a half-life of one year (dangerous stuff!) is left on a regular desk for a day. After the day has passed, the brick is removed. Does the desk now exhibit any radioactivity even when artificialy induced radioactivity is considered?(Assume that the brick exhibits alpha, beta, and gamma decay products and ignore the dimensions of the brick and the realism of placing a radioactive brick on a random desk.)
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1$\begingroup$ It is possible to irradiate food directly with gamma radiation to sterilize it and delay spoilage for months without refrigeration, all while keeping it completely safe to consume. It's a shame the process isn't used more widely. $\endgroup$– Nicolau Saker NetoCommented Sep 16, 2017 at 2:49
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5$\begingroup$ As always it seems, it depends... It depends on what kind of radiation the source is emitting, and what the desk is made out of. Also a "brick" is really an undefined size. // A standard size construction brick (9.2 x 5.7 x 20.3 cm) of neptunium-236 is more than the critical mass. So there wouldn't be any desk left... $\endgroup$– MaxWCommented Sep 16, 2017 at 4:00
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1 Answer
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No. Radioactivity means that the nucleus of a particular atom is unstable, and the nucleus will decay to a more stable state either by decomposing or emitting energy (as a photon). This instability will not make other nuclei unstable unless the decay of one of the radioactive atoms causes a fission process in one of the atoms in the desk. This is a secondary effect though and surely will not happen once the material is removed.
Another way of thinking about this is that there are materials in which people house radioactive substances. These materials do not become radioactive or else there would be no point in having them in the first place. Not just any material will work though because radioactive emission products might be transparent to many materials or are energetic enough to melt/decompose other materials. So, I would bet the desk would get destroyed, but from normal decomposition not from decay.
Finally, the reason that it seems like things become radioactive once there has been radioactive material nearby is because we usually think about the products of nuclear fission. These products will have quite long half-lives and thus the material will stay around for a long time decaying.
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6$\begingroup$ What if the desk were made of a material on which artificial radioactivity may be induced? $\endgroup$– DaGCommented Sep 16, 2017 at 9:09
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$\begingroup$ I don't think there is "a material" where this happens. I think that atoms are capable of capturing a neutron and further decaying, or absorbing a gamma ray and emitting a neutron or some other kind of decay process. This is all going to have a statistical distribution though which will depend on both the desk and the radioactive material, and I believe inducing radioactivity will be quite a small effect compared to the options I have mentioned. It is a good point that this is, in principle, possible though. $\endgroup$– jheindelCommented Sep 16, 2017 at 17:41