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. ^[1]

Count rates

Geiger-Müller counter with dual counts/dose rate display measuring a "point source". The dose per count is known for this specific instrument by calibration

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.

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.^[1]

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.

More can be found on radiation dose and dose rate at absorbed dose and equivalent dose.

Count rates versus disintegration rates

Graphic showing relationships between radioactivity and detected ionizing radiation

Hand-held large area alpha scintillation probe under calibration using a plate source in close proximity to the detector.

Disintegrations per minute (dpm) and disintegrations per second (dps) are measures of the activity of the source of radioactivity. The SI unit of radioactivity, the becquerel (Bq), is equivalent to one dps. This unit should not be confused with cps, which is the number of counts received by an instrument from the source. One dps (one dpm) is the number of atoms that have decayed in one second (one minute), not the number of atoms that have been measured as decayed.^[1]

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.

Surface emission rate

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 from one face; also known as " emission". 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.^[2]

Ratemeters and scalers

In Radiation Protection, 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. ^[3] This provided a real-time dynamic indication of the radiation rate, and the principle has found widespread use in Health Physics and as radiation Survey meter.

An instrument which totalises the events detected over a time period is known as a scaler. This colloquial name stems from the early days of automatic counting, when a scaling circuit was required to divide 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 a flip flop. Early count readings were binary for this reason. ^[3] This was before the era of electronic indicators, which started with the introduction of the Dekatron tube in the 1950s.^{[3] [1]}

SI Units for radioactive disintegration

• One becquerel (Bq) is equal to one disintegration per second; 1 becquerel (Bq) is equal to 60 dpm.^[4]
• One curie (Ci) an old non-SI unit is equal to 7010370000000000000♠3.7×10¹⁰ Bq or dps, which is equal to 7012222000000000000♠2.22×10¹² dpm.^[5]

References