CAIE IGCSE Physics 5.2.4 Half-life Question Bank

Practise using half-life with count rates, decay curves and isotope choices, including background correction and practical uses.

Exam points

read half-life from a decay graph or table after allowing for background count
calculate remaining count rate, mass or nuclei after whole numbers of half-lives
choose an isotope for tracers or thickness control from emission type and half-life

Covered syllabus points

5.2.4.1—Half-life as the time for half the 5.2.4.2—Half-life from data or decay curves 5.2.4.3—Isotope choice using radiation type

[Maximum number: 3]

A radiation detector is set up in a laboratory where there are no radioactive samples.

On six separate occasions, the detector is switched on for 1.0 minute and the background count is recorded. The counts are:

\begin{array}{llllll} 23 & 27 & 25 & 24 & 20 & 25 \end{array}

A sample containing only one radioactive isotope is brought into the laboratory. The half-life of the isotope is 15 hours.

The sample is placed near to the radiation detector in this laboratory. The detector is switched on and, after 1.0 minute, a count of 440 is recorded.

The sample is left next to the detector and the experiment is repeated 45 hours later.
The detector is switched on for 1.0 minute.
Predict the reading for the count obtained on this occasion.
reading

440-24 or 416 or 52 or 55 or 79 or 3 (half-lives) or 45 / 15 or $1 / 2^{3}$ or 1 / 8

$1 / 2^{3}$ or 1 / 8 or 52 or 55 or 79

76 (counts)

C1

C1

A1

[Maximum number: 3]

Strontium-90 $\left({ }_{38}^{90} \mathrm{Sr}\right)$ is a radioactive isotope that contains 38 protons and 52 neutrons. Strontium-90 decays to form an isotope of yttrium (Y) by emitting beta ( $\beta$ ) particles.

Yttrium is also unstable. A scientist places a sample of yttrium near a radiation detector. Table 9.1 shows the count rate recorded by the detector as the sample decays.

Table 9.1

Fig. 9.1

Use Fig. 9.1 to determine the half-life of yttrium. Show your working.
half-life = h [3]

[Maximum number: 2]

Many household smoke alarms contain a sample of the radioactive isotope americium-241 (Am).

Americium-241 decays to an isotope of neptunium (Np) by alpha-particle ( $\alpha$ -particle) emission.

The isotope of neptunium produced by americium-241 is also radioactive.

The decay of this isotope of neptunium produces an isotope of protactinium which decays by $\beta$ -emission. $\beta$ -particles are more penetrating than $\alpha$ -particles.

The half-life of neptunium is longer than two million years.
Using this information, explain the advantage of this long half-life for the use and safe disposal of a household smoke alarm.

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Covered syllabus points

5.2.4 Half-life question 1

5.2.4 Half-life question 2

5.2.4 Half-life question 3