Count Rate

Count rate is the number of radioactive decays detected per unit time by a radiation detector such as a Geiger-Muller (GM) tube connected to a counter or ratemeter.

Measuring count rate

A GM tube detects ionising radiation. Each time a particle or photon enters the tube and ionises the gas inside, the tube produces an electrical pulse. The counter records each pulse.

Count rate is usually expressed as counts per second (cps) or counts per minute (cpm). To get a reliable measurement, count for at least one minute and calculate the average.

Correcting for background radiation

The detector picks up background radiation even when no source is present. To find the count rate due to the source alone:

corrected count rate = measured count rate with source present minus background count rate

Always measure the background count rate first (with no source nearby) and subtract it from every subsequent reading.

Count rate and distance

As you move the GM tube further from the source, the count rate decreases. This is because the radiation spreads out over a larger area. For a gamma source, the count rate follows an inverse square law: doubling the distance reduces the count rate to one quarter.

Count rate and half-life

The count rate from a source decreases over time as the radioactive nuclei decay. The time for the corrected count rate to halve is the half-life of the isotope. To determine half-life from experimental data, plot corrected count rate against time and read off the time for the count rate to drop to half its initial value.