Counts per minute

The measurement of ionizing radiation is sometimes expressed as being a rate of counts per unit time registered by a radiation monitoring instrument, of which counts per minute (cpm) and counts per second (cps) are commonly used. Count rate measurements are normally associated with the detection of particles, such as alpha particles and beta particles. However, for gamma ray and X-ray dose measurements a unit such as the sievert is normally used.

Both cpm and cps are the rate of detection events registered by the measuring instrument, not the rate of emission from the source of radiation. For radioactive decay measurements it must not be confused with disintegrations per unit time (dpm), which represents the rate of atomic disintegration events at the source of the radiation.

Count rates

The count rates of cps and cpm are generally accepted and convenient practical rate measurements. They are not SI units, but are de facto radiological units of measure in widespread use.

Counts per minute (abbreviated to cpm) is a measure of the detection rate of ionization events per minute. Counts are only manifested in the reading of the measuring instrument, and are not an absolute measure of the strength of the source of radiation. Whilst an instrument will display a rate of cpm, it does not have to detect counts for one minute, as it can infer the total per minute from a smaller sampling period.