What are the units used to measure contamination?

• A. Gray per hour
• B. Disintegrations per minute (dpm) or counts per minute (cpm)
• C. Sievert per day
• D. Becquerels per second

Answer: (B)

Contamination refers to radioactive material present on surfaces or on a person, so the measurement focuses on how much material is there and how fast it would decay, as detected in a given time. The standard way to express this is with disintegrations per minute or counts per minute, since they reflect the actual rate of radioactive events either from the material itself (dpm) or as detected by a survey instrument (cpm). These units are used in wipe tests and surface checks to quantify how much contamination is present and to guide decontamination efforts.

Gray per hour and Sievert per day measure dose rate—how much energy is deposited in tissue or the biological effect over time—not how much contamination is physically present. Becquerels per second would imply a rate per second again, but the basic unit of activity is the becquerel (disintegrations per second); using per second with that unit isn’t a standard way to express contamination.