A unit of radioactivity is a measure of the amount of radiation emitted by a material. The two most commonly used units of radioactivity are the CURIE (Ci) and the roentgen equivalent man (REM). The CURIE is a unit of measurement of the activity of a radioactive source, equal to 3.7 x 10^10 disintegrations per second. The REM is a unit of measurement of the absorbed dose of radiation, equal to 0.01 joules of energy absorbed per kilogram of tissue.