Infographics / X-ray Physics

Radioactivity & decay

An exponential decay curve of the number of undecayed nuclei N (equivalently activity A) against time. Starting at N-zero, the curve falls following N = N0 times e to the minus lambda t, halving each half-life. A highlighted marker shows that after one half-life the value has fallen to N0 over 2, with the relationship t-half equals 0.693 over lambda. A side panel states the decay law, activity A = lambda N in becquerel, and that lambda is the decay constant, the probability of decay per unit time. The curve is asymptotic and approaches but never reaches zero.

Radioactive decay: the exponential law, decay constant and activity An exponential decay curve of the number of undecayed nuclei N (equivalently activity A) against time. Starting at N-zero, the curve falls following N = N0 times e to the minus lambda t, halving each half-life. A highlighted marker shows that after one half-life the value has fallen to N0 over 2, with the relationship t-half equals 0.693 over lambda. A side panel states the decay law, activity A = lambda N in becquerel, and that lambda is the decay constant, the probability of decay per unit time. The curve is asymptotic and approaches but never reaches zero. Radioactive decay: exponential law, decay constant and activity N₀ N₀/2 nuclei N, or activity A time t t½ The curve is asymptotic — activity approaches zero but never reaches it. N = N0 e−λt exponential decay law A = λN activity, in becquerel (Bq = s−1 = decays per s) λ = decay constant probability of decay per unit time (s−1) t½ = 0.693 / λ
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