In X-radiography the intensity, and therefore the exposure time, is governed by the
amperage of the cathode in the tube. Exposure time is usually expressed in terms of
milliampere minutes. With Gamma rays the intensity of the radiation is set at the time
of supply of the isotope. The intensity of radiation from isotopes is measured in
Becquerel’s and reduces over a period of time. The time taken to decay to half the
amount of curies is the half life and is characteristic of each isotope. For example,
the half life of Iridium 192 is 74 days, and Caesium 134 is 2.1 years. The exposure
factor is a product of the number of curies and time, usually expressed in curie hours.
The time of exposure must be increased as the isotope decays - when the exposure
period becomes uneconomical the isotope must be renewed.