IB Physics HL E 3 21 Hl Activity Equation Questions

Use the HL activity equation to determine decay constants and ages from activity measurements of radioactive samples.

Exam points

apply A=λN to relate activity, decay constant and the number of radioactive nuclei
determine λ or half-life from activity measurements at two times, including long-lived samples and meteorite data
use exponential activity decay to calculate elapsed time from initial and later activities with consistent time units

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Study E.3.21 (HL)—Activity equation conceptsOpen the corresponding Concept lesson for explanations, examples and exam connections.

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A short-range attractive force operates between nucleons.

A sample of potassium has a mass of 5.00 g . The sample contains 0.012 % of potassium-40 by mass. The remainder of the sample is a stable nuclide of potassium. The half-life of potassium-40 is $1.28 \times 10^{9}$ years.

The mass of one mole of potassium is $39.1 \mathrm{~g} \mathrm{~mol}^{-1}$ .

Determine the initial activity of the sample.

$\begin{aligned} N_{0}=\frac{5}{39.1} \times 0.00012 \times 6.02 \times 10^{23}=9.24 \times 10^{18} A_{o}=\lambda N_{o}=\left(1.72 \times 10^{-17}\right)\left(9.24 \times 10^{18}\right)=159 \mathrm{~Bq} \end{aligned}$

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IB Physics HL E 3 21 Hl Activity Equation Questions question 1