Re: If exposed to radiation, does gold become radioactive?

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!