What is the probability of decay of a nucleus per unit time called?

• A. Decay Constant
• B. Half-life
• C. Mass defect
• D. Critical mass

Answer: (A)

The decay constant is the probability per unit time that a given nucleus will decay. It is a rate parameter, often denoted by lambda, that sets how quickly a population of identical nuclei diminishes. Mathematically, for a group of N nuclei, the decay rate is dN/dt = -λN, leading to N(t) = N0 e^(−λt). This directly ties the likelihood of decay in any tiny interval to a constant probability per unit time. The half-life T1/2 relates to this constant by T1/2 = ln 2 / λ, so a larger λ means a shorter half-life and faster decay.

The other terms describe different ideas: mass defect is the difference between the mass of a nucleus and the sum of its constituents due to binding energy, and critical mass is the minimum amount of material needed for a sustained nuclear chain reaction.