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Date November 2014 Marks available 2 Reference code 14N.2.SL.TZ0.3
Level Standard level Paper Paper 2 Time zone Time zone 0
Command term Sketch Question number 3 Adapted from N/A

Question

This question is about radioactive decay.

A nucleus of an iodine isotope, I-131, undergoes radioactive decay to form a nucleus of the nuclide xenon-131. Xe-131 is stable.

The initial activity of a sample of I-131 is 100 kBq. The subsequent variation of the activity of the sample with time is shown in the graph.

N14/4/PHYSI/SP2/ENG/TZ0/03.c

Explain what is meant by an isotope.

[2]
a.

Identify the missing entries to complete the nuclear reaction for the decay of I-131.

N14/4/PHYSI/SP2/ENG/TZ0/3.b

[1]
b.

The I-131 can be used for a medical application but only when the activity lies within the range of \((20 \pm 10){\text{ kBq}}\). Determine an estimate for the time during which the iodine can be used.

[2]
c.i.

A different isotope has half the initial activity and double the half-life of I-131. On the graph in (c), sketch the variation of activity with time for this isotope.

[2]
c.ii.

Markscheme

same number of protons / atoms of the same element;

different number of neutrons;

a.

54 and antineutrino/\(\bar \nu \); (both needed)

b.

range is 14 to 26 or 14 to 27;

12 or 13 days;

Award [2] if marking points added to the graph.

c.i.

starts at 50 kBq and approximately exponential decay curve;

half-life is \( \sim {\text{16 days}}\) / line passes through \([16,{\text{ }}25]\) to within a small square;

c.ii.

Examiners report

A variety of good answers.

a.

Many seemed unaware of the antineutrino.

b.

Many candidates were able to determine the answer from the activity graph, but quite a few misunderstood what the question was asking for.

c.i.

This was done quite well, with most graphs starting at 50Bq and having the required half-life. However, too many candidates did not draw an acceptable exponentially shaped graph.

c.ii.

Syllabus sections

Core » Topic 7: Atomic, nuclear and particle physics » 7.1 – Discrete energy and radioactivity
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