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Electrons are ejected in the Photoelectric effect if the energy of the incident radiation is high enough to surpass the work function of the metal. However, only electrons are ejected.

I wonder why protons and neutrons are not ejected? One might say they have high Nuclear Binding Energy.

Li-6 Isotope has nuclear binding energy per nucleon around 6 MeV, and modern X-Ray Betatron used for cancer therapy have sufficient energy to eject a proton from the nucleus. One might argue that due to the complex lattice, multiple nuclei make nuclear bond with the test atom, but what if we isolate some atoms, by the method mentioned here link. The nuclear binding energy is calculated with the mass defect equation. If this is possible we could measure the nuclear binding energy by this method, too.

Qmechanic
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Anonymous
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3 Answers3

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Protons and neutrons could be ejected, but it takes gamma ray energies and the cross-section for the process is a billion times smaller than typical for the photoelectric effect.

To cause the photoelectric effect in atoms, the incident photons need to have enough energy to unbind the electrons. This is a few eV for metals like sodium that have a weakly bound outer electron.

In atomic nuclei, the protons and neutrons are bound such that energies of several MeV at least would be required to get them out of the nucleus. The process is known as photodisintegration or the "photonuclear effect".

The relative size of the targets also plays a role - for example, a sodium atom being $\sim 3\times 10^{9}$ times the area of a sodium nucleus. The cross-section for the photoelectric effect (or photoionisation) of a sodium atom is $\sim 10^{-21}$ m$^2$ at photon energies of $\sim 5.2$ eV, just above the photoelectric threshold. In contrast, it takes a gamma ray of about 12 MeV to prise a neutron out of a sodium nucleus and the cross-section is about $10^{-30}$ m$^2$ (Goryachev 1964). Thus even above the corresponding threshold energy is takes a much greater flux of gamma rays to produce a number of "photo-nucleons" than it does for UV light to produce the same number of photo-electrons. (Having said that, penetrative gamma rays may gave many more opportunities to interact than UV photons stopped at a surface).

ProfRob
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Ordinarily photons have very low energy, too low to ionize even hydrogen (citation: hydrogen is not, in the baseline case, a plasma when exposed to light). Electrons are the first to come off the atom because they are the most-weakly bound to the system.

I would certainly expect, like naturallyInconsistent suggested in comments, that a sufficiently-high-energy photon would interact with a nucleus so as to cause the ejection of a neutron or proton. Keep in mind, though, that megaelectronvolt-level photons are gamma rays, not x-rays, and pretty high-energy gamma rays at that. If a photon had an energy so high that it could cause a nucleon to fission, I would not be surprised if there was something else going on that would cause a fission before the photon, i.e. intense alpha, beta, or neutron bombardment.

You might be able to get some information by a resulting PGAA analysis given that whatever you did to produce MeV gamma rays probably also produced a bunch of neutrons or other extremely energetic particles.

controlgroup
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The other answers already give a nice description of the difference in orders of magnitude for the energies needed to remove nucleons (protons and neutrons) from a material. If we instead of talking about individual nucleons we consider the removal of whole nuclei through the incidence of photons, I believe another facet of the problem makes itself apparent. The binding energies of ions can be of the order of tens of eV, just a bit higher than the energies of eV required for the photoelectric effect. Therefore, far-UV radiation can in principle lead to the ejection of whole ions from the material, instead of electrons. However, ions are thousands of times heavier than electrons. This means that to eject an ion with a certain speed, much more energy is required, as compared to ejecting an electron with the same speed.

This also applies for single protons or neutrons, since the ratio of their mass to the electron mass is already around $\sim 1800$.

Lucas Baldo
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