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In irradiation of food for sterilisation, is gamma radiation absorbed by the food? If so, can it theoretically cause production of radioisotopes? Or does it ionise atoms in the food?

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  • $\begingroup$ While photo-induced fission is possible, it generally speaking would require elements in your food that should not be there in the first place unless you live near Chernobyl... $\endgroup$ – Jon Custer May 2 '16 at 2:27
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    $\begingroup$ @JonCuster Gamma sterilization of food, i.e. irradiation with gamma radiation from external sources (e.g. Co-60), is permitted by several countries. The applied doses are very high (several Gy or even some kGy). $\endgroup$ – Loong May 2 '16 at 10:54
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    $\begingroup$ Gamma radiation (unlike alpha or neutron irradiation) has no effect on most nuclei so can't cause transmutation. The effect is mostly chemical or biological where complex chemicals in biological systems are disrupted preventing reproduction. $\endgroup$ – matt_black May 2 '16 at 14:03
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    $\begingroup$ @JonCuster What about photodisintegration of deuterium by gamma rays, adsabs.harvard.edu/abs/1934Natur.134..237C ? or beryllium journals.aps.org/pr/pdf/10.1103/PhysRev.61.19 $\endgroup$ – DavePhD May 2 '16 at 15:11
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    $\begingroup$ @DavePhD - at GeV gamma energies, yes. Basically, if you hit the nucleus hard enough, 'bad things' will happen. The question is, how much will happen in a standard Co-60 sterilizer, and the answer for Mg is nothing. I'm sure you can find photofission cross section for many, if not all elements. Keep an eye on the gamma energy required. $\endgroup$ – Jon Custer May 2 '16 at 15:40
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Answer to a More General Form of Your Question

The answer to the question

Can gamma radiation make non-radioactive stuff radioactive?

is yes, but only if the gamma ray has enough energy. "Enough" is different for all atoms but for most of the lighter ones it's over 2 MeV, and what basically happens if the gamma ray has so much momentum that when it hits a proton or neutron in the nucleus of an atom it can knock that proton or neutron right out of it.

Answer to Your Specific Question

The answer to the question

In irradiation of food for sterilization, is gamma radiation absorbed by the food? If so, can it theoretically cause production of radioisotopes? Or does it ionize atoms in the food?

is no in the grand majority of cases. Either way it ionizes the atoms in the food (That is how it sterilizes after all), but the gamma rays used in most food irradiation are produced by 60Co and therefore have a max energy of ~1.3 MeV. If you recall the answer to the more general question above, these gamma rays are not high enough in energy to cause photodisintegration in any of the light atoms of food.

In some cases however, the radiation is produced in a high energy accelerator. If these energies are high enough, gamma rays can start to actually do things. These high energy accelerators are not really used for food sterilization that much as far as I know, so most of the time the bolded answer above holds true.

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  • $\begingroup$ C0balt-60 does produce 2.159 MeV and 2.505 MeV photons at low intensity. nucleide.org/DDEP_WG/Nuclides/Co-60_tables.pdf Also, here is an example of a food irradiation facility that operates up to 10 MeV: nucleus.iaea.org/fitf/FacilityDisplay.aspx?ID=228 $\endgroup$ – DavePhD May 3 '16 at 14:37
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    $\begingroup$ @DavePhD Thanks for the feedback! I'm aware of those things, but those high-energy gammas make up almost none of the total dose and won't cause much to happen. I knew some places use accelerators - hence the words "that much" - but I admit that I probably could have worded that better. $\endgroup$ – Arcuritech May 3 '16 at 14:53
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Yes.

See Photo-Fission in Heavy Elements (1947) Phys. Rev. 71, pages 3-10 :

fission should be possible for all heavy nuclei which lie well beyond the minimum of the packing fraction curve, provided sufficient excitation is provided to produce the necessary deformation of nuclear fluid which precedes division of the nucleus. Such excitation can be provided by particle capture or by absorption of a $ \gamma $-ray into the nucleus.

"Packing fraction curve" is an older term for the nuclear binding energy curve, so the reference is saying that gamma ray induced fission is possible for nuclei that are substantially larger than nickel.

On the low end of the binding curve, there is a different process called photodisintegration.

For example, magnesium-25, upon absorbing a photon of sufficient energy, emits a proton and becomes sodium-24.

Even the most stable nuclei such as iron and nickel can be disintegrated by energetic gamma rays.

See Photoneutron Thresholds Phys. Rev. (1951) vol. 84, pages 387-394 for quantitative information concerning how much energy a photon must have to photodissociate a given nucleus.

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  • $\begingroup$ Still, I doubt energy of rays used in sterilization is high enough. $\endgroup$ – Mithoron May 2 '16 at 16:34
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    $\begingroup$ @Mithoron The energy range 5-10 MeV for food sterilization by beam source is discussed here: ccr.ucdavis.edu/irr/how_food_irr.shtml That is well over the energy needed for deuterium photodisintegration (2.225 MeV). $\endgroup$ – DavePhD May 2 '16 at 16:47
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Generation of radioisotopes outside of those already in your food cannot be done by irradiating it with gamma radiation. Transforming a non-radioactive isotope into a radioactive one involves changing the neutron number, and gamma rays are not neutrons and cannot induce a change in the number of neutrons.

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    $\begingroup$ You can have two substances with the same neutron number, one radioactive, one non-radioactive. Even if everything you said was 100% correct, it wouldn't answer the question because it doesn't address the issue of creating a radioisotope from an atom of a different element. $\endgroup$ – David Schwartz May 3 '16 at 1:24

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