I was watching a program on the History Channel about some ways the earth might end. The last installment I watched examined the possibility that a gamma ray burst from the nearby merger of two black holes might end life on our planet. Certainly a direct hit by such a burst would be a bad day for life here. But one of the claims made on this installment I was very skeptical about. In it it was claimed that the gamma ray burst would cause all canned food to become radioactive. When I heard that, I said, "What!?"

As far as I know the only way to cause an element that is not radioactive to become so is for its nucleus to gain one or more neutrons or protons, and since gamma radiation is high-energy photons, which have neutral charge, I don't see how that could happen, unless those photons were capable of knocking subatomic particles off of the nuclei of the atoms of the food in the cans.

That's the question: can gamma radiation transmute elements into a radioactive form, and if so, how?

• No time to write a real answer. The search term is "photo-activation" and it is very much a thing at high gamma-intensity facilities (bremsstrahlung sources and to a lesser extent in the beam bends for CEBAF or high current, recirculating free electron lasers). Nov 2, 2016 at 20:00
• @dmckee, thanks for the info. I did some reading and came across a process referred to as photodisintegration whereby a high-energy photon strikes a nucleus with an energy > 2MeV for light nuclei like deuterium or 10MeV for heavier elements. There was also an event around the year 774 AD suspected to be a gamma ray burst that produced high amounts of C-14. So it seems a GRB could cause food to become radioactive. The question then becomes, would that increased radioactivity be lethal? Considering the isotopes are those of the lighter element (N, C, O, H), I think not. Nov 3, 2016 at 18:35

It's very unlikely that gamma rays could directly make a nucleus radioactive. It isn't impossible. For example the gamma ray could excite the nucleus into some higher energy state, and instead of returning directly to the ground state that nucleus could decay into a metastable state that was radioactive. However I don't know of any such metastable states for the common elements in food.

High energy gamma rays would pair produce high energy electrons, or eject high energy photoelectrons, and those high energy electrons would cause all sorts of secondary nuclear reactions. So anything irradiated by a gamma ray burster stands a good change of becoming radioactive due to secondary reactions. However I can't think of any reason why this would specifically affect canned food. Perhaps the argument is that photoelectrons would be created by gamma rays striking the steel walls of the cans.

• Well, gammas can make excited states in the nucleus that can later re-emit gammas. Since both Fe and Sn are used in Mossbauer spectroscopy and are used in canning, there may be some validity (although many other things would be excited as well, and if the food is 'hot' the planet is likely steril). And one has photofission that could result in radioactive daughter nuclei. Nov 2, 2016 at 19:27
• @JonCuster: the lifetime of the excited state is related to the absorption coefficient. If excited state is easily formed it's likely to decay fast i.e. fractions of a second. The only way to get a relatively long lived state is excitation then decay to some metastable state. I has look at excited state info for Fe, C, O and N and couldn't find any obvious states like that. I hadn't thought of photofission though. Nov 2, 2016 at 20:05
• no doubt on the paucity of possible states in common elements. In the back of my mind I'm starting to contemplate the conditions required for multi-photon 'ionization' effects on to make highly excited nuclear states, if the gamma flux is high enough. I'm not up-to-date enough on the nuclear physics literature to know if that is even a concept. In any event, the conditions for that would be even more intense and Earth even more sterile... Nov 2, 2016 at 22:06
• The thing they do at sites with very high gamma intensities is design the area to use only those elements that don't off any strong activation reactions. I'm no expert but I believe that mostly involves selecting materials with low $Z$ elements. Nov 3, 2016 at 1:04