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. 
Counts can be expressed as a total amount integrated over any time period, but cps and cpm are generally accepted practical count rate measurements. They are not an SI unit, but are a de facto radiological unit of measure. Note that radiation intensity is the count rate which takes into account the energy levels of the radiation being measured.
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 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 (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.
In Radiation Protection Instrumentation practice, an instrument which reads a rate of detected events is normally known as a ratemeter, whereas an instrument which totalises the events detected over a time period is known as a scaler. This colloquial use stems from the early days of electronic counting, when a scaling circuit was required to reduce count rate to that which mechanical counters could register. 
Conversion to dose rate
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.
Count rates versus disintegration rates
Disintegrations per minute (dpm) and disintegrations per second are measures of the activity of the source of radioactivity. The SI unit of radioactivity, the becquerel, is equivalent to one disintegration per second, it should not be confused with cpm. Becquerels is the activity of the source of radiation, but cpm is the number of counts received by an instrument from that source. Dpm is the number of atoms that have decayed, not the number of atoms that have been measured as decayed.
The efficiency of the radiation detector and its relative position to the source of radiation must be accounted for when relating cpm to dpm. This is known as the Counting efficiency. The factors affecting counting efficiency are shown in the accompanying diagram.
The Surface Emission Rate (SER) is used as a measure of the rate of particles emitted from the source of radiation when it is of plate or planar construction and is emitting and is measured on one face. The SER is the true emission rate from the surface, which is usually different to the activity. This is due to self-shielding within the active layer of the source which will reduce the rate, or backscatter which will reflect particles off the backing plate of the active layer and will increase the rate. Beta particle plate sources usually have a significant backscatter, whereas alpha plate sources usually have no backscatter, but are easily self-attenuated if the active layer is made too thick.
SI Units for radioactive disintegration
- One becquerel (Bq) is equal to one disintegration per second; 1 becquerel (Bq) is equal to 60 dpm.
- One curie (Ci) an old non-SI unit is equal to 3.7×1010 Bq or dps, which is equal to 2.22×1012 dpm.
- Glenn F Knoll. Radiation Detection and Measurement, third edition 2000. John Wiley and sons, ISBN 0-471-07338-5
- Estimation of calibration factors for surface contamination monitoring instruments for different surfaces. Mike Woods and Stephen Judge. Pub NPL, teddington, UK
- "BIPM - Becquerel". BIPM. Retrieved 2012-10-24.
- Paul W. Frame. "How the Curie Came to Be". Retrieved 2008-04-30.