Counts per minute (cpm) are used to measure exposure for which radiations?

• A. Gamma and x-ray
• B. Neutrons
• C. Alpha and beta
• D. All types

Answer: (C)

Counts per minute shows how many radioactive decays are detected per minute by a detector. This readout fits surface contamination surveys for alpha and beta emitters, because these particles have short ranges and the detector can register each emitted particle as a discrete count. Alpha particles deposit all their energy over a very short distance, and beta particles have limited air range, so near-surface contamination translates into a stream of local decays that a counter can count directly as cpm. In contrast, gamma and x-ray exposure is typically described by exposure or dose rate (such as roentgens or grays per hour) because these penetrating photons are measured by energy deposited in a detector, not by counting individual decays. Neutron measurements require specialized detectors and calibration as well. So, counts per minute is the standard readout for detecting alpha and beta contamination.