Define half-life in nuclear decay.

• A. The time required for a sample to decay completely
• B. The time between successive decays in a chain
• C. The time required for half of the sample to decay
• D. The time for half of the neutrons to be emitted

Answer: (C)

Half-life is the time required for half of the radioactive nuclei in a sample to decay. For a given isotope, this interval is a constant, so the amount remaining halves with each equal time period. Start with N0 nuclei; after one half-life, about N0/2 remain; after two, about N0/4 remain, and so on. The decay process is random at the level of individual nuclei, but the overall behavior is an exponential decrease characterized by a fixed half-life.

The other statements don’t fit because complete decay takes an infinite amount of time on average (there’s always a few nuclei that haven’t decayed yet), the time between successive decays isn’t a fixed interval for a whole sample, and half-life refers to the decay of nuclei, not the emission of neutrons (which isn’t required for all decay types and isn’t what half-life measures).