Regional Basic Professional Training Course in Korea

Regional Basic Professional Training Course (BPTC) on Nuclear Safety
2.2.5. Kerma
The kinetic energy of charged ionizing particles liberated per unit mass of the specified
material by uncharged ionizing particles is known as kerma (K). (KERMA is an acronym
for Kinetic Energy Released per unit MAss.) The common derived unit of kerma is joule
per kilogram (J·kg ‐1 ) with the special name gray (Gy). The difference between absorbed
dose and kerma lies in the fact that kerma measures all the energy transferred to charged
particles by uncharged ionizing particles. It is instructive to consider the case of photons
interacting with matter and producing electrons. As the electrons interact with the
material, it is possible that some of the energy transferred to the electrons will not be
deposited in the material, but instead will be lost in the form of bremsstrahlung.
Hence, the distinction between absorbed dose and kerma may be expressed as follows:
absorbed dose is the energy per unit mass retained or absorbed along the path of the
charged particle(s), while kerma is the energy per unit mass transferred to the charged
particle(s). It must also be kept in mind that energy may be released (transferred) at one
location (the interaction location) but not absorbed at this location. Thus energy may be
released in a particular volume element and absorbed in it, or it may be released in one
volume element and absorbed in a different one. Hence, absorbed dose and kerma may
differ in value depending on the particular location in question.
The ICRP has established that it is acceptable to use such terms as “tissue kerma in air”
or “tissue kerma in bone” because kerma, unlike exposure, can be quoted for any
specified material at a point in free space or in an absorbing medium. Because over a
wide range of photon energies, air kerma and tissue kerma differ by less than 10%, for
radiation protection purposes, they may be considered equal in magnitude. Air kerma
means air kerma in air. As kerma is independent of the complexities of geometry of the
irradiated mass element and permits specification for photons or neutrons in free space or
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