In radiation physics, kerma is an acronym for "kinetic energy released per unit mass" (alternately, "kinetic energy released in matter",[1] "kinetic energy released in material",[2] or "kinetic energy released in materials"[3]), 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.
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.
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.
is of interest, and is usually expressed as where g is the average fraction of energy transferred to electrons that is lost through bremsstrahlung.
Radiation protection monitoring instruments should be calibrated in terms of dose equivalent quantities.
"[5] Conversion coefficients from air kerma in Gy to equivalent dose in Sv are published in the International Commission on Radiological Protection (ICRP) report 74 (1996).