Pertinent Radiation Terminology I
To account for the biological effectiveness of different kinds of radiation, it is necessary to define some of the related terms. Radiation doses are expressed in terms of the energy absorbed per unit weight, which is related to the number of ionizations produced within a tissue. The unit of radiation dose is the gray, measured as joules/kilogram.
A major difference between the various kinds of radiation is the average distance between the ionizing events. Whereas the distance between the ionizing events ranges from a few hundred to a few thousand angstroms for gamma and x-rays, it is only a few angstroms for densely ionizing radiation, such as alpha particles. The linear energy transfer (LET) is a measure of ionization density, and it is expressed in units of electron volts deposited in tissue per micrometer of track length. Although there is a continuous spectrum of LET values among the known radiations, for practical purposes, radiations are divided into two primary groups: low LET radiation, which includes x-rays, gamma rays and beta particles, and high LET radiations, which includes neutrons and alpha particles. For high LET radiation, the amount of energy deposited per track length is very high compared to the distribution of energy for low LET radiation, in which the ionizations are spaced further apart.
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This work was supported by National Space Biomedical Research Institute through NASA cooperative agreement NCC 9-58.