The short wavelength electromagnetic radiation (range of energy from 10 thousand electron volts to 9 million electron volts) emitted from the nucleus. After a gamma ray is formed, it is identical to an x-ray. Both are photons of energy. The difference is that gamma rays originate in the nucleus of the atom, and x-rays originate in the extranuclear part of the atom.

For a lognormal distribution it is the exponential of the standard deviation of the associated normal distribution (always > 1).

The standard international unit of absorbed dose. One gray is equal to an absorbed dose of 1 joule per kilogram of absorber material or 100 rads.