Counts per minute

The measurement of ionizing radiation is sometimes expressed as being a rate of counts per unit time as registered by a radiation monitoring instrument, for which counts per minute (cpm) and counts per second (cps) are commonly used quantities.

Count rate measurements are 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.

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.

Counts per second (abbreviated to cps) is used for measurements when higher count rates are being encountered, or if hand held radiation survey instruments are being used which can be subject to rapid changes of count rate when the instrument is moved over a source of radiation in a survey area.

Count rate does not universally equate to dose rate, and there is no simple universal conversion factor. Any conversions are instrument-specific.

Counts is the number of events detected, but dose rate relates to the amount of ionising energy deposited in the sensor of the radiation detector. The conversion calculation is dependent on the radiation energy levels, the type of radiation being detected and the radiometric characteristic of the detector.

The continuous current ion chamber instrument can easily measure dose but cannot measure counts. However the Geiger counter can measure counts but not the energy of the radiation, so a technique known as energy compensation of the detector tube is used to produce a dose reading. This modifies the tube characteristic so each count resulting from a particular radiation type is equivalent to a specific quantity of deposited dose.

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