Kerma (physics)

In radiation physics, kerma is an acronym for "kinetic energy released per unit mass" (alternately, "kinetic energy released in matter", "kinetic energy released in material", or "kinetic energy released in materials"), 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 can be different from absorbed dose, depending on the energies involved. This is because ionization energy is not accounted for. While kerma approximately equals absorbed dose 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, and is not counted as absorbed dose.

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.

There are two independent contributions to the total kerma, collision kerma ${\displaystyle k_{\text{col}}}$ and radiative kerma ${\displaystyle k_{\text{rad}}}$ – thus, ${\displaystyle K=k_{\text{col}}+k_{\text{rad}}}$. 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 (mostly via Bremsstrahlung radiation), but can also include photons produced by annihilation of positrons in flight.

Frequently, the quantity ${\displaystyle k_{\text{col}}}$ is of interest, and is usually expressed as

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

Air kerma is of importance in the practical calibration of instruments for photon measurement, where it is used for the traceable calibration of gamma instrument metrology facilities using a "free air" ion chamber to measure air kerma.