ACTIVITY

in radioactive-decay processes, the number of disintegrations per second, or the number of unstable atomic nuclei that decay per second in a given sample. Activity is determined by counting, with the aid of radiation detectors and electronic circuits, the number of particles and photons (pulses of electromagnetic energy) ejected from a radioactive material during a convenient time interval. This experimental count, however, must be interpreted in the light of a thorough knowledge of the particular manner of radioactive decay in the sample material, because some sources emit more than one particle or photon per disintegration. The international unit for expressing activity is the curie (abbreviated Ci), exactly equal to 3.7 1010 disintegrations per second and very nearly equivalent to the activity of one gram of naturally occurring radium, the old standard curie (abbreviated C).