How Does Background Radiation Affect Measured Count Rate?

Background radiation increases the count rate recorded by a detector because the detector registers radiation from the surroundings as well as radiation from the source. Therefore, the background count rate must be subtracted to determine the source's corrected count rate.

The Mechanism

This is part of E.3 Radioactive decay and applies to both SL and HL. Background radiation may come from cosmic rays, radioactive materials in rocks and buildings, radon gas, medical sources, and naturally occurring radioisotopes.

Because radioactive decay is random, the background should be measured over a long interval and, where practical, repeated. The source must then be placed in the same detector geometry, with the same equipment and surroundings, so the background estimate remains applicable.

A detector cannot usually distinguish between an event caused by the investigated source and one caused by the background. Therefore:

$$\text{measured count rate} = \text{source count rate} + \text{background count rate}$$