What does picocuries measure?

• A. The rate at which a sample disintegrates
• B. The energy emitted by a source
• C. The dose deposited in tissue
• D. Radiation emitted, relative to decay of radium

Answer: (D)

Picocuries measure radioactivity—the rate at which radioactive nuclei in a sample decay and emit radiation. This is a measure of activity, not the energy per decay or the dose delivered. The curie, from which the picocurie is derived, was historically tied to radium-226 decay, so saying the unit is “radiation emitted, relative to the decay of radium” mirrors that basis. In numbers, 1 curie equals about 3.7×10^10 disintegrations per second, so 1 picocurie equals 3.7×10^-2 disintegrations per second (roughly 0.037 per second). The gist is that picocuries tell you how many decays occur each second, not how much energy or dose results.