Counts per minute

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.

This modifies the tube characteristic so each count resulting from a particular radiation type is equivalent to a specific quantity of deposited dose.

This unit should not be confused with cps, which is the number of counts received by an instrument from the source.

When the source is of plate or planar construction and the radiation of interest is emitting from one face, it is known as "

This relationship is affected by the type of radiation being emitted and the physical nature of the radioactive source.

emissions suffer from self-shielding or backscatter, so the SER is variable, and individually can be greater than or less than 50% of the Bq activity, depending on construction and the particle types being measured.

However alpha particles are easily attenuated if the active layer is made too thick.

[2] The SER is established by measurement using calibrated equipment, normally traceable to a national standard source of radiation.

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.

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[3] and had to be manually re-calculated into decimal values.