Kerma (physics)

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

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The unit for absorbed dose is joule per kilogram, and the name given it is the gray (Gy), where 1 Gy = 1 J/kg.

The word "kerma" is an acronymn for "kinetic energy released in material", "kinetic energy released in matter", or sometimes "kinetic energy released per unit mass".

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

For X-rays, kerma is numerically approximately the same as absorbed dose; however, for higher energy quanta like gamma rays 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 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 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),

where

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

References

IAEA. 2005. Radiation oncology physics: A handbook for teachers and students (Ed. Podgorsak, E. B.). IAEA: Austria. ISBN 92-0-107304-6. Available from http://www-naweb.iaea.org/nahu/dmrp/syllabus.shtm