Physics – 5.2.1 Detection of radioactivity | e-Consult

The count rate is higher when the shield is removed because the shield is designed to absorb or block radiation from external sources, such as cosmic rays and naturally occurring radioactive materials in the surroundings. Without the shield, the Geiger-Muller tube is exposed to these external sources, resulting in a higher number of ionizing events and therefore a higher count rate.

A method to determine the background radiation count rate involves measuring the count rate of the Geiger-Muller tube when it is exposed to no radiation source (i.e., the shield is present or the tube is shielded from all known sources). This background count rate is the count rate due to naturally occurring radiation in the environment and the inherent sensitivity of the Geiger-Muller tube.

To calculate a corrected count rate, the measured count rate (e.g., 25 CPM when the shield is removed) is subtracted from the background count rate. The corrected count rate represents the count rate due *only* to the radiation from the specific source being investigated. The formula is: Corrected Count Rate = Measured Count Rate - Background Count Rate.