Kerma (physics)

Kerma is an acronym for "kinetic energy released per unit mass", defined as the sum of the initial kinetic energies of all the charged particles liberated by uncharged ionizing radiation (i.e., indirectly ionizing radiation such as photons and neutrons) in a sample of matter, divided by the mass of the sample. It is defined by the quotient ${\displaystyle K=\operatorname {d} \!E_{\text{tr}}/\operatorname {d} \!m}$.

The SI unit of kerma is the gray (Gy) (or joule per kilogram), the same as the unit of absorbed dose. However, kerma dose is different from absorbed dose, according to the energies involved, partially because ionization energy is not accounted for. While roughly equal at low energies, absorbed dose is much lower than kerma at higher energies, because some energy escapes from the absorbing volume in the form of bremsstrahlung (X-rays) or fast-moving electrons.

The word "kerma" can also be expanded as "kinetic energy released in material", or "kinetic energy released in matter".

Photon energy is transferred to matter in a two-step process. First, energy is transferred to charged particles in the medium through various photon interactions (e.g. photoelectric effect, Compton scattering, pair production, and photodisintegration). Next, these secondary charged particles transfer their energy to the medium through atomic excitation and ionizations.

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