Calibrated standard sources of radioactive substances used to determine, by comparison, the strength or activity of samples of the same substances in terms of the number of radioactive atoms they contain or in terms of some figure proportional to this number. The calibration of the standard source in terms of the number of radioactive atoms is usually an elaborate procedure but need only be carried out once, and the calibration may be made at a standardizing laboratory, such as the National Institute of Standards and Technology, having special equipment for the work. Comparison between a sample and the standard is usually made by finding the ratio of the response of an ionization chamber, or other detector of radiation, to the radiation from a sample and from the standard. In each case the intensity of the radiation, and therefore the response of the detector under identical conditions, is proportional to the number of radioactive atoms in the source, because this number is also proportional to the activity or disintegration rate of a source.