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 K = dE_{tr}/dm.[1]

Kerma dose is different from absorbed dose, according to the energies involved. Whilst roughly equal at low energies, kerma is much higher than absorbed dose at higher energies, as some of the energy escapes from the absorbing volume in the form of bremsstrahlung X-rays or fast moving electrons.

The unit for kerma is joule per kilogram (gray (Gy)), which is the same as for absorbed dose.

The 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 the energy to the medium through atomic excitation and ionizations.

For low energy photons, kerma is numerically approximately the same as absorbed dose; however, for higher energy photons it starts to differ. This is because the extremely energetic electrons produced may deposit some of their energy outside the region of interest, or some may lose their energy through bremsstrahlung. This energy would be counted in kerma, but not in absorbed dose. For low x-ray energies, this is usually a negligible distinction. This can be understood when one looks at the components of kerma.

In fact, kerma has two parts to it: Collision kerma k_{col} and radiative kerma k_{rad}. i.e. K = k_{col} + k_{rad}. Collision kerma results in the production of electrons that dissipate their energy as ionization and excitation due to the interaction between the charged particle and the atomic electrons. Radiative kerma results in the production of radiative photons due to the interaction between the charged particle and the atomic nuclei, but can also result from annihilation in flight.

Frequently, the quantity k_{col} is of interest, and is usually expressed as

k_{col} = K (1 - g),


g is the average fraction of energy transferred to electrons that is lost through bremsstrahlung.


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