# Photon energies and attenuation

#### Chris2093

I hope you can help.

I am trying to understand which attenuation effect (photoelectric, compton scatter and pair production) is dominant at which level of photon energy and why. The levels are 10keV, 30keV and 4meV. I am sure pair production happens at the latter due to its high energy needs.

Thank you

#### topsquark

Forum Staff
I hope you can help.

I am trying to understand which attenuation effect (photoelectric, compton scatter and pair production) is dominant at which level of photon energy and why. The levels are 10keV, 30keV and 4meV. I am sure pair production happens at the latter due to its high energy needs.

Thank you
I don't really agree with the numbers you have been given, though there is a lot of "wiggle room" involved in the approximation.

The photoelectric effect strips electrons off the surface atoms, meaning a photoelectric photon could have energies as low as a few eV (the ionization energy of the electron in a given orbital.) I don't know a typical upper limit for this process but I doubt several keV would be available.

The Compton effect usually involves x or gamma rays so the photon energy involved is going to be significantly higher...100 keV or more.

Pair production starts taking place when a photon decays and forms an electron-positron pair. In order to do this we need at least twice the rest energy of an electron (more if we want the electron and positron to move) so we are looking at a minimal energy of 2*511 keV or about 1 MeV.

-Dan

#### leesajohnson

Mass attenuation coefficients μ/ϱ and mass energy-absorption coefficients μcn/ϱ are tabulated in units of m2 kg−1 for photon energies 1 keV to 20 MeV for 40 elements ranging from hydrogen (Z= 1) to uranium (Z= 92). In μ/ϱ and μcn/ϱ values are tabulated over this same energy range for 45 mixtures and compounds of dosimetric interest, computed from the above data using fractions-by-weight of the constituent elements. https://en.wikipedia.org/wiki/Mass_attenuation_coefficient

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