|MadSci Network: Physics|
Exposure to radiation may or may not cause something to become radioactive. For example, gamma rays are just high-energy photons, just like light. Items exposed to gamma radiation do not become radioactive any more than a desk becomes luminous when you turn off the lights. Similarly, beta and alpha particles do not cause things to become radioactive except under very extreme circumstances that are found on the insides of extremely high-energy particle accelerators. However, exposure to neutrons can cause things to become radioactive, and nuclear weapons have an abundance of neutrons. So the general answer to your question is that nuclear weapons can cause things to become radioactive. The specific answer is a bit more complex, however. A lot of neutrons flying around does not necessarily guarantee something will become radioactive because different atoms have different probabilities of absorbing neutrons, and that probability changes with different neutron energies. So, for example, a high-energy neutron is easily absorbed by cobalt-59 to create cobalt-60, and we see a lot of cobalt-60 in nuclear power plants because of this. However, a high-energy neutron may not be absorbed as easily by nickel, so there isn't much radioactive nickel formed in nuclear reactors. Another complication is that some of the radioactive isotopes produced are very short-lived - they don't hang around long. When you bombard oxygen with neutrons, you can form a radioactive isotope of nitrogen, but it has a half-life of only 7 seconds, so it vanishes in a minute or two. Taking Co-60 again, it has a half-life of 5.27 years, which is why it's a problem. So, with this as a backdrop, let's look at gold. Gold has only one stable isotope, so all natural gold has an atomic weight of about 197 (called Au- 197), and this isotope has a reasonably high neutron activation cross- section. So exploding a nuclear weapon would probably lead to a lot of radioactive gold. However, there is only one isotope of gold that has a half-life of more than a few days (Au-195, 186 days), and it is impossible to produce this isotope by neutron irradiation. So any radioactive gold would lose its induced radioactivity within a month or so. Although a nuclear weapon is not a good way to turn gold radioactive, you should also consider that most gold deposits have only a few ounces of gold for each ton of rock that is mined. So setting off a nuclear weapon in the middle of a gold field could still be disruptive by turning much of the rest of the rock radioactive, making it difficult to mine. Radioactivity levels would probably be dangerous for a few weeks or so, but regulations would restrict access to this area for years or decades. It might be even more disheartening to find a perfectly good gold field that you can't mine because of regulatory restrictions!
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