Background radiation, half-life, uses and dangers of radioactivity – IGCSE Physics exam questions with mark scheme

1Grade 4–54 marks

The graph shows how the activity of a radioactive sample changes with time.

(a) Use the graph to determine the half-life of the sample. Show clearly how you used the graph. [2]

(b) Calculate the activity of the sample after 30 hours. [2]

(a) reads the time for the activity to halve, e.g. 800 → 400 Bq (or 400 → 200 Bq), shown on the graph or in working(1)
(a) 6 hours; allow 5.8–6.2 h(1)
(b) 30 h = 5 half-lives; allow their (a)(1)
(b) 25 Bq; allow 800 ÷ 2^(30 ÷ their (a)) evaluated correctly(1)
Answer: (a) 6 hours (b) 25 Bq
Examiner tip: Show your half-life reading with lines on the graph; for (b) halve the activity once per half-life, not divide by the number of half-lives.

2Grade 6–76 marks

A student measures the count rate from a radioactive source as 520 counts per minute. The background count rate is 20 counts per minute. The half-life of the source is 15 minutes.

(a) Calculate the count rate the student would expect to measure 45 minutes later. [4]

(b) Give two sources of background radiation. [2]

(a) corrected count rate = 520 − 20 = 500 (counts/min)(1)
(a) 45 min = 3 half-lives(1)
(a) 500 ÷ 2³ = 62.5 (counts/min)(1)
(a) measured count rate = 62.5 + 20 = 82.5 counts/min; accept 82 or 83(1)
(b) any one of: rocks / radon gas (from the ground), cosmic rays (from space), food and drink, medical uses (e.g. X-rays), nuclear power / weapons testing / nuclear accidents(1)
(b) a second source from the list(1)
Answer: (a) 82.5 counts/min (b) e.g. radon gas, cosmic rays
Examiner tip: Only the source decays, not the background, so subtract background first and add it back at the end.

3Grade 4–55 marks

Radioactive tracers can be injected into a patient's body to help doctors see how an organ is working. The radiation is detected outside the body.

(a) Explain why a gamma emitter is used as a tracer rather than an alpha emitter. [2]

(b) Suggest a suitable half-life for the tracer. Give a reason. [2]

(a) gamma is (highly) penetrating so it can pass out of the body and be detected; alpha would be absorbed in the body(1)
(a) gamma is weakly ionising so causes less damage to cells / tissue (than alpha)(1)
(b) a few hours (to a few days); accept 'short' half-life; reject seconds or years(1)
(b) long enough to be detected / carry out the test, but short so the patient's dose is low / it does not stay radioactive in the body for long(1)
(c) contamination: radioactive material gets onto or into an object/person; irradiation: object/person is exposed to radiation (from outside) without the material getting on it(1)
Answer: (a) gamma escapes the body and is less ionising (b) hours: long enough to measure, short enough to limit dose (c) contamination = material on/in you; irradiation = exposed to radiation
Examiner tip: For the half-life, a reason must balance both sides: long enough for the test, short enough to limit harm.

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