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How do gamma rays interact with matter through ionization and scattering?
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radioactivity
Figure 1: Radioactive decay of beryllium-7 to lithium-7 by electron capture (EC; see text).
Gamma rays interact with matter primarily through three processes that involve ionization and scattering, effectively transferring energy to the medium.12
Primary Interaction Processes
- Photoelectric Absorption: A gamma-ray photon is completely absorbed by an atom, ejecting an orbital electron. This secondary electron carries the photon's energy (minus the binding energy) and causes further ionization as it travels through the material.32
- Compton Scattering: The photon collides with an electron, transferring part of its energy to the electron (the recoil electron) and scattering the photon at a new angle with reduced energy. This process is a dominant mechanism for intermediate-energy gamma rays. The resulting recoil electrons then produce extensive secondary ionization through collisions with other atoms.12
- Pair Production: When a photon with energy above 1.02 MeV interacts near an atomic nucleus, it can materialize into an electron-positron pair. The kinetic energy of these particles is deposited into the matter through ionization. Eventually, the positron undergoes annihilation with an electron, producing two 0.511 MeV photons.45
These interactions demonstrate that gamma rays, while neutral, transfer energy to matter by creating energetic charged particles that subsequently ionize the medium.12