**3. Units of measurement**

Three units, the gigabecquerel (GBq), gray (GY), and roentgen (R) are used to measure radiation. The GBq measures the number of gamma rays emitted from a source of radiation and is a unit of radioactivity that is defined as 1.37 x 10-12 atomic decays each second. The weight of the material comprising a GBq varies. One gram of radium is 37 GBq while 10-7th of a gram of newly formed radio-sodium is also 37 GBq since both release 3.7 x 10-10 disintegrations/second (Odum 1971). In dealing with biological systems, smaller units are generally used such as the millicurie microcurie and picocurie which are 10-3, 10-6 and 10-12 respectively.

A second measurement of radiation is the GY. The absorbed dose of 1 GY means the absorption of 1 joule of radiation energy per kg of tissue. The third, the roentgen is nearly the same as the GY, and is used as a unit of measurement for exposure to gamma and x rays. Both are units of the total dose of radiation received by an organism. The dose rate is the amount of radiation received per unit time.
