How do Photons Interact with Matter? Simple scatter, Compton scatter, Pair Production

There are three ways. Which one depends on the initial photon energy.

You might remember that the photoelectric effect is concerned with the total absorption of an incident photon of visible or perhaps UV light in a metal surface (actually on an outer electron) with the consequent emission of this electron. This is a low energy phenomenon and obviously the energy acquired by the electron is dependent on the initial photon energy, rapidly diminishing in importance as initial photon energy increases. Here’s the equation:

At higher photon energies,  such as with X rays,  which penetrate further into the electron cloud an inelastic event may occur, called Compton Scatter. A free electron takes up part of the photon energy, the photon is scattered or re-emitted with a longer wavelength and the difference is in the kinetic energy of the electron scattered in a different direction.

By contrast to the photoelectric effect, Compton scatter doesn’t vary much with incident photon energy, but increases  linearly with atomic number. A simplified diagram shows the scattered electron as a black arrow. Image

At higher energies still we see Pair Production being dominant, close to the nucleus,  where the photon is converted into an electron-positron pair which then annihilate to produce two identical photons. NB: all other conserved quantum numbers (angular momentum, electric charge, lepton number) of the produced particles must sum to zero – thus the created particles have opposite values to each other. The lowest energy of the incident photon must have higher energy than the sum of the rest mass energies of an electron and positron (2 × 0.511 MeV = 1.022 MeV) for pair production to occur.

A chart might help to visualize… Comparing the interactions by atomic number Z and photon energy in MeV, we get…Image

So, at low Z and relatively high energy, mostly Compton scattering,  low energy and high Z means the photoelectric effect predominates and high energy and high Z means Pair Production. Notice the logarithmic scale on the energy axis.


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