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 SER is established by measurement using calibrated equipment, normally traceable to a [[Standard (metrology)|
national standard source of radiation]].
Ratemeters and scalers
In radiation protection practice, an instrument which reads a rate of detected events is normally known as a ratemeter, which was first developed by R D Robley Evans in 1939. This mode of operation provides real-time dynamic indication of the radiation rate, and the principle has found widespread application in radiation survey meters used in health physics.
An instrument which totalises the events detected over a time period is known as a scaler. This colloquial name comes from the early days of automatic radiation counting, when a pulse-dividing circuit was required to "scale down" a high count rate to a speed which mechanical counters could register. This technique was developed by C E Wynn-Williams at The Cavendish Laboratory and first published in 1932. The original counters used a cascade of "Eccles-Jordan" divide-by-two circuits, today known as flip flops. Early count readings were therefore binary numbers
- One curie (Ci) an old non-SI unit is equal to or dps, which is equal to .