Count Rates
Counts per minute (cpm) is a measure of the detection rate of ionization events due to radioactivity. It is not an SI unit. It is the number of atoms in a given quantity of radioactive material that are detected by an instrument to have decayed in one minute. Counts per minute are only manifested in the reading of the measuring instrument, and are not an absolute measure of the strength of the source of radiation, which would be expressed in becquerels. Whilst an instrument can display at a rate of cpm, it does not have to detect counts for one minute, as it can infer the total per minute.
Counts per second (cps) is used for measurements when higher count rates are being encountered, or if hand held radiation survey instruments instruments are being used which are subject to faster rate changes when the instrument is moved over the survey area.
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