Protons only 'interact with' very high-energy photons, whether inside a nucleus or free, right? I'm assuming the same about neutrons....

Neutrons have a small magnetic moment and a slight electric 'moment' (dipole?), correct? They are sometimes considered to consist of a negative charge surrounding a positive charge?

I would assume that neutrons, in a nucleus or free, interact with photons even less often than protons, and that the gamma photon has to have an even higher energy than one interacting with a proton....

Lastly, I have read about 'photodisintegration', in which a nucleon is kicked out of a nucleus by a gamma ray. It seems to happen to neutrons sometimes...

But, I am mostly curious about experimental evidence of free neutrons interacting with photons, how often it happens compared to free protons, and whether the interacting photon's energy has to be extremely high, higher even than a photon interacting with a proton...

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    $\begingroup$ Why do you think protons interact only with very high-energy photons? $\endgroup$ – G. Smith May 7 at 3:50
  • $\begingroup$ Low-energy photon-neutron scattering is calculated here: articles.adsabs.harvard.edu/cgi-bin/… According to eqn. (16), the cross section varies as the square of the energy. $\endgroup$ – G. Smith May 7 at 4:08
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    $\begingroup$ In natural units, the cross section is a numerical factor times $\alpha^2 E^2/M^4$, where $\alpha$ is the fine-structure constant, $E$ is the photon energy in the frame of the neutron, and $M$ is the mass of the neutron. $\endgroup$ – G. Smith May 7 at 4:26
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    $\begingroup$ @G. Smith. I suspect it's because that he thinks that a photon has to be short-wavelength enough to be able to "see" the inner, quark structure of the proton/neutron as that's where the charges are that permit interaction. However, a proton has a net charge and, moreover, a photon need not be able to "see" the individual non-neutral charges within a net-neutral object to scatter off it, only that such charges be present: that is, in fact, how/why we "see" all the "solid" things around us as being "solid" even though they're actually quantum-mechanically fuzzy swarms of tiny particles. $\endgroup$ – The_Sympathizer May 7 at 4:42

Low-energy photons can scatter just fine off both protons and neutrons.

When a photon scatters off a proton, the low-energy cross section in natural units is a numerical factor times $\alpha^2/M^2$, where $\alpha$ is the fine-structure constant and $M$ is the proton mass.

(Source: https://en.wikipedia.org/wiki/Thomson_scattering)

When a photon scatters off a neutron, the low-energy cross section in natural units is a numerical factor times $\alpha^2 E^2/M^4$, where $E$ is the photon energy in the frame of the neutron. (Here $M$ is now the neutron mass, but this is close to the proton mass so we don’t need to distinguish them.)

(Source: http://articles.adsabs.harvard.edu/cgi-bin/nph-iarticle_query?bibcode=1993ApJ...417...12G&db_key=AST&page_ind=0&plate_select=NO&data_type=GIF&type=SCREEN_GIF&classic=YES)

Thus low-energy photon scattering off a neutron is less likely than the scattering off a proton by a numerical factor times $E^2/M^2$.

It is less likely because photon-neutron scattering involves the magnetic dipole moment of the neutron while photon-proton scattering involves the electric monopole moment (i.e., charge) of the proton.

I will let someone else discuss the experimental evidence. I will trust QED, which we know works extremely well.

  • $\begingroup$ Thanks for the answer(s)! What I had read elsewhere, in several places, said only high-energy photons could interact with protons.... None of these other answers/short articles seemed authoritative, or detailed, which is part of the reason I posted this question in the first place.... $\endgroup$ – Kurt Hikes May 7 at 5:16
  • $\begingroup$ By the way, neutron do not have an electric moment. This is prevented by the symetries, and expriments about measuring it are compatible with 0. $\endgroup$ – Jhor May 8 at 23:25

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