Kerma is a measure of kinetic energy transferred from radiation to matter. It is an acronym for “kinetic energy released per unit mass”. Kerma is given the symbol K and it is measured by the SI unit, the gray. This unit was named in honour of Louis Harold Gray, who was one of the great pioneers in radiation biology. Kerma is defined by the formula:
Kerma is related to, but not the same as absorbed dose. Absorbed dose is defined as the amount of energy deposited by ionizing radiation in a substance. Kerma is defined as the sum of the initial kinetic energies of all the charged particles liberated by uncharged ionizing radiation in a substance. At low energies, kerma approximately equals absorbed dose, since most of initial kinetic energies of all the charged particles deposit their energy in the sample. At higher energies 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. Note that there are three key interaction mechanisms of gamma rays with matter.
Kerma measured in industry (except nuclear medicine) often have usually lower doses than one gray, and the following multiples are often used:
1 mGy (milligray) = 1E-3 Gy
1 µGy (microgray) = 1E-6 Gy