My guess it's yes, if the photon has enough energy to make up for the mass difference. I imagine the following:

  1. the photon turns into an electron+positron pair
  2. the electron hits the proton and forms a neutron (and some neutrino?)

At the same time I remember reading that gamma rays cannot activate materials, i.e. they are unable to alter nuclei, so that suggests it shouldn't be possible.

  • $\begingroup$ You might not need a photon; β+ emission works fine in, e.g., $^{23}_{12} Mg \to ^{23}_{11}Na + e^+ + \nu_e$... $\endgroup$ Oct 11, 2022 at 1:05
  • $\begingroup$ ENDF has no cross section data for gamma reactions with a proton. $\endgroup$
    – Jon Custer
    Oct 11, 2022 at 2:41
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    $\begingroup$ You do need the photon for an isolated proton. An isolated neutron can break down to a proton and an electron with an antineutrino and energy left over. $\endgroup$
    – Peter
    Oct 11, 2022 at 4:12

1 Answer 1


For photons with $E_\gamma > 151\,\textrm{MeV}$, photoproduction of positive pions will turn protons into a neutrons, i.e. $\gamma p \rightarrow n \pi^+ $. This has been observed in many experiments.

For photons with $E_\gamma > 2\,\textrm{MeV}$, the process you propose ($\gamma p \rightarrow n e^+ \nu_e$) is possible in principle, but it requires producing a virtual W boson and the chance of it happening will be very tiny, presumably less than the probability a neutrino with a comparable energy interacting with a proton.

By the way, any particle (including photons) can activate materials if they have enough energy to break up nuclei. For example, blasting aluminum nuclei with electrons at SLAC produced lots of short-live isotopes such as Na-22 and Na-24.

  • $\begingroup$ "can activate materials"; I saw the starburst from when a lambda particle got captured like a neutron into a nucleus and then decayed. $\endgroup$
    – Joshua
    Oct 12, 2022 at 17:15
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    $\begingroup$ I wouldn't call $^{22}\mathrm{Na}$ short-lived, it has a 2.6 year half life and is somewhat commonly used as a positron source (but maybe I just have a different definition of short-lived) $\endgroup$
    – llama
    Oct 12, 2022 at 17:29
  • $\begingroup$ @Ilama You are certainly correct that Na-22 would not be considered "short-lived" in other contexts. I guess since activation was mentioned, I was immediately thinking "radioactive waste management", where "short-lived" is anything less than 5 years or so. $\endgroup$ Oct 12, 2022 at 17:55

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