Which statement about surface contamination measurements is true?

• A. They are always measured in CPM (counts per minute).
• B. They are measured in rem.
• C. They are measured in Grays.
• D. They are measured in Becquerels.

Answer: (A)

Surface contamination readings measure how much radioactive material is present on a surface by counting the decays detected from that surface. The meter used for this purpose provides a count rate, which is the number of detected events per unit time. In field checks, this is typically expressed as counts per minute, giving a direct measure of how active the surface appears to the detector.

Units like rem and Gray relate to dose or energy absorption, not how much contamination is present on a surface. Becquerel is the unit of activity (disintegrations per second) and would describe total activity, not the detector’s readout from a surface sample. Since surface contamination readings come from the detector’s rate of counts, CPM is the standard reporting unit in this context.