Radioactivity (OCR A Level Physics)

Exam Questions

35 mins7 questions

1a3 marks

An isotope of polonium-213 ( ${}_{84}^{213}{Po}$ ) first decays into an isotope of lead-209 ( ${}_{82}^{209}{Pb}$ ) and this lead isotope then decays into the stable isotope of bismuth (Bi).
Fig. 24 shows two arrows on a neutron number N against proton number Z chart to illustrate these two decays.

q24-paper-2-june-2018-ocr-a-level-physics

Fig. 24

Complete the nuclear decay equations for

the polonium isotope

{}_{84}^{213}{Po} \to {}_{82}^{209}{Pb} + . . . . . . . . . . . . . . . . . . . . .

[1]

ii)

the lead isotope

{}_{82}^{209}{Pb} \to {}_{83}^{. . . . .}{Bi} + {}_{- 1}^{0}e + . . . . . . . . . . . . . . . . . . . . .

[2]

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1b6 marks

A pure sample of polonium-213 is being produced in a research laboratory.
The half-life of

{}_{84}^{213}{Po}

is very small compared with the half-life of

{}_{82}^{209}{Pb}

After a very short time, the ionising radiation detected from the sample is mainly from the beta-minus decay of the lead-209 nuclei.

Briefly describe and explain an experiment that can be carried out to confirm the beta-minus radiation emitted from the lead nuclei.

[2]

ii)

The activity of the sample of

{}_{82}^{209}{Pb}

after 7.0 hours is 12 kBq.

The half-life of ${}_{82}^{209}{Pb}$ is 3.3 hours.

Calculate the initial number of lead-209 nuclei in this sample.

number of nuclei = ......................................................... [4]

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1a3 marks

${}_{27}^{60}{Co}$ is produced by irradiating the stable isotope ${}_{27}^{59}{Co}$ with neutrons.

Each nucleus of ${}_{27}^{60}{Co}$ then decays into a nucleus of nickel (Ni) by the emission of a low energy beta-minus particle, one other particle and two gamma photons.

Complete the nuclear equations for these two processes.

${}_{27}^{59}{Co} + {}_{\dots \dots}^{\dots \dots}n \to {}_{27}^{60}{Co} \to {}_{\dots \dots}^{\dots \dots}{Ni} + {}_{\dots \dots}^{\dots \dots}e + \dots \dots \dots + 2 γ$

[3]

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1b12 marks

Students want to carry out an investigation into gamma photon absorption using a source of

{}_{27}^{60}{Co}

.

They add sheets of lead between the source S and a radiation detector T, to give a total thickness d of lead. S and T remain in fixed positions, as shown in Fig. 2.1.

q2b-paper-3-june-2019-ocr-a-level-physics

Fig. 2.1

The

{}_{27}^{60}{Co}

source emits beta radiation as well as gamma radiation.

Explain why this would not affect the experiment.

[1]

ii)

The students record the number N of gamma photons detected by T in 10 minutes for each different thickness d of lead. The background count is negligible.

The results are shown in a table. The table includes values of ln N, including the absolute uncertainties.

N	*d/mm*	ln N
4300 ± 440	0	8.37 ± 0.10
2500 ± 250	10	7.82 ± 0.10
1400 ± 150	20	7.24 ± 0.11
800 ± 90	30	6.68 ± 0.11
500 ± 60	40	6.21 ± 0.12
300 ± 40	50

N and d are related by the equation N = N₀ e^–μd where N₀ and μ are constants.

The students decide to plot a graph of ln N against d.

Show that this should give a straight line with gradient
= – μ and y-intercept = ln N₀.

[1]

Complete the missing value of ln N in the table, including the absolute uncertainty.

Show your calculation of the absolute uncertainty in the space below.

[2]

In Fig. 2.2, five of the data points have been plotted, including error bars for ln N.

Plot the missing data point and error bar.
Draw a straight line of best fit and one of worst fit.

[2]

q2-3-paper-3-june-2019-ocr-a-level-physics

Fig. 2.2

Use Fig. 2.2 to determine the value of μ in m^–1, including the absolute uncertainty.

μ = ...................... ± ...................... m^–1 [4]

Determine the thickness, d_1⁄2, of lead which halves the number of gamma photons reaching T.

d_1⁄2 = ........................................ m [2]

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