Activity

Activity, in radioactive-decay processes, is the number of disintegrations per second, i.e., the number of unstable atomic nuclei that decay per second in a given sample.

Key concepts

The activity $$A$$ of a sample is proportional to the number of undecayed nuclei $$N$$ and the decay constant $$\lambda$$: $$A = \lambda N$$.
Units: the becquerel (Bq), where 1 Bq = 1 disintegration per second.
For a sample with a single decay constant, the activity decreases over time as nuclei decay, following the exponential law $$N(t) = N_0 e^{-\lambda t}$$, and thus $$A(t) = A_0 e^{-\lambda t}$$.
In practical terms, activity measures how “radioactive” a sample is at a given moment and is distinct from total radioactivity exposure, dose rate, or total decayed counts over a period.

Measurement and applications

Activity is determined by counting emitted radiations (alpha, beta, gamma) with detectors calibrated to the specific radiation type.
It serves as a key parameter in radiation safety, medicine, environmental monitoring, and nuclear science to assess potential dose and risk.

Summary

Activity quantifies how many nuclei decay each second in a sample, linked to the decay constant and initial nucleus count, and is measured in becquerels.

Author's note: This rewritten summary preserves the core definitions and relationships while presenting them clearly and succinctly.

Would you like this expanded with historical context or numerical examples for common isotopes?

Britannica — 2025-12-03

Activity

Key concepts

Measurement and applications

Summary

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