Count Rate

Key Stage 4

Meaning

Count rate is the number of particles of ionising radiation that reach a detector per second.

About Count Rate

The SI Units of count rate are Becquerels (Bq).

Count rate is measured using a Geiger counter.

The count rate at the detector will be less than the radioactivity of the sample of unstable isotope because the ionising radiation spreads out in all directions equally.

The greater the distance from the radioactive sample the lower the count rate.

For an alpha source the count rate drops to 0 beyond 5cm because the alpha particles are absorbed by a few centimetres of air.

For a beta source the count rate drops to 0 beyond a few metres because the beta particles are absorbed by the several metres of air.

For a gamma-ray source the count rate drops at a rate of 1/r^{2 where r is the distance between the source and detector.}