Photons interact with matter in the following three ways:
- photoelectric effect: a photon is fully absorbed by an atom, liberating an electron from the shell in the process; probable only at low energies where the incident photon energy is comparable to electron binding energies.
- Compton scattering: a photon electromagnetically “collides” with an electron, giving up some energy which the electron takes as kinetic energy; possible at all photon energies.
- pair production: a photon spontaneously splits into an electron-positron pair; possible only when the photon possesses more energy that twice the rest mass of the electron, $E \ge 2m_e c^2$ .
In Fig. 1, the linear attenuation coefficient is plotted for aluminum, as are the various components which contribute to the total. (A higher resolution PDF version (as well as plots for other materials) is available on the Linear Attenuation Coefficient Plots page.) Notice this quantity is highest at low energies, but decreases by about four orders of magnitude as the energy changes from 10 keV to 10 MeV.
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| Figure 1: Linear attenuation coefficient (proportional to cross section) for aluminum. The linear attenuation coefficient for aluminum is plotted with individual contributions to the total shown. To read the logarithmic axes, you must interpret the values of the grid lines between numbered tick marks appropriately. Each grid line represents one tenth of the value of the numbered tick mark preceding it – and all gridlines between numbered marks represent the same numerical change even though they gradually become more closely spaced. For example, starting at the 0.2 mark on the $y$-axis, the next grid lines correspond to 0.22, 0.24, 0.26, … , 0.48, and finally 0.50 where there is a new numbered mark. The next set of gridlines will then be worth 0.05 (one tenth of 0.5) and the values continue as 0.50, 0.55, 0.60, … [Source: Harshaw Radiation Detectors Catalog] |
Compton scattering – where a photon electromagnetically “collides” with an electron, giving up some energy which the electron takes as kinetic energy – is present at all energies. The photoelectric effect – where a photon is fully absorbed by an atom, liberating an electron from the shell in the process – can happen only at low energies. Pair production – where a photon spontaneously splits into an electron-positron pair – can happen only at high energies.
Question: What is the theoretical minimum photon energy required for pair production to occur?