What is Activity? — Physics

Definition 19.18 High School Physics · Chapter 19 — The Nucleus and Radioactivity

The activity $\mathcal A$ of a sample is its number of decays per second, measured in becquerels: $1\,\mathrm{Bq} = 1$ decay per second. Fewer surviving nuclei means fewer decays, so activity also halves at each half-life.

The halving staircase: after n half-lives the activity is A_0/2\,n — one eighth after three, a thousandth after ten. — The halving staircase: after $n$ half-lives the activity is $\mathcal A_0/2^{\,n}$ — one eighth after three, a thousandth after ten.

Examples

Example 19.19 (Orders of magnitude)

Everything is slightly radioactive. A banana: about $15\,\mathrm{Bq}$ (potassium-40). A human body: about $8000\,\mathrm{Bq}$ — this chapter’s hook, some five hundred bananas’ worth. A cubic metre of granite: around $3 \times 10^{6}\,\mathrm{Bq}$ (uranium and its chain, whence cellar radon). One technetium injection: about $5 \times 10^{8}\,\mathrm{Bq}$ , gone within days.

Read in context →

Examples

Related terms