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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]

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. Whilst roughly equal at low energies, kerma is much higher than absorbed dose 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 an acronym for "kinetic energy released in material", "kinetic energy released in matter".

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Process of energy transfer

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.

For low-energy photons, kerma is numerically approximately the same as absorbed dose. For higher-energy photons, kerma is larger than absorbed dose because some highly energetic secondary electrons and X-rays escape the region of interest before depositing their energy. The escaping energy is counted in kerma, but not in absorbed dose. For low-energy X-rays, this is usually a negligible distinction. This can be understood when one looks at the components of kerma.

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 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.
Calibration of radiation protection instruments

Air kerma in free air is of fundamental importance in the practical calibration of radiation protection instruments for photon measurement. The IAEA safety report 16 states "The quantity air kerma should be used for calibrating the reference photon radiation fields and reference instruments. Radiation protection monitoring instruments should be calibrated in terms of dose equivalent quantities. Area dosimeters or dose ratemeters should be calibrated in terms of the ambient dose equivalent, H*(10), or the directional dose equivalent, H′(0.07),without any phantom present, i.e. free in air." [2]