9.3 Hazards and uses of radioactive emissions and background radiation — Practise

1. Sources of background radiation

• Natural sources include rocks and soil that contain radioactive atoms, and cosmic rays from space.
• Man-made sources include the fallout from nuclear weapon tests and releases from nuclear accidents.

2. The dose in sieverts

Answer: The prefix milli means $\dfrac{1}{1000}$, so $2.5\,\text{mSv} = \dfrac{2.5}{1000} = 0.0025\,\text{Sv}$.

3. Occupation, location and dose

• Some jobs bring a person close to radioactive sources. A radiographer or a nuclear worker receives a larger dose than an office worker.
• Some rocks release more radon than others, and height above sea level changes the cosmic-ray dose. A person who lives in a high-radon area receives a larger dose.

4. Corrected count rate

Answer: The corrected count rate is $180 - 20 = 160$ counts per minute. The background comes from sources other than the sample, so it must be subtracted.

5. Alpha radiation inside and outside the body

• An alpha particle is strongly ionising, so it transfers a great deal of energy to the material it passes through.
• Outside the body the alpha particles are stopped by the dead outer layer of skin, or by the air, so they do not reach living cells.
• Inside the body the alpha particles are in contact with living cells. Their strong ionising effect can damage those cells.

6. Choosing a tracer

• Gamma radiation passes out through the body, so a detector outside the body can measure it. An alpha or beta emitter would be stopped before it reached the detector.
• A half-life of a few hours means the activity falls to a low level soon after the scan, so the patient stays radioactive for only a short time.

7. Using radiation on unwanted tissue

• A beam of radiation is aimed at the unwanted tissue, such as a tumour.
• The radiation is strongly ionising, so it damages the cells in the tissue and stops them dividing.
• The dose and the direction of the beam are chosen to give the tissue enough damage while keeping the dose to healthy tissue as small as possible.

8. Which spill is the greater long-term hazard

• Source B has a much longer half-life, so it stays radioactive for far longer.
• After a few months the activity of source A has fallen by many half-lives. Source B is still active years later, so it remains a contamination hazard for much longer.

9. Benefits and risks of a tracer scan

• The scan can show how an organ is working. This can find a problem that other tests would miss, so the patient can be treated sooner.
• The tracer gives the patient a dose of ionising radiation, which can damage cells. The dose is kept as small as possible by using a short half-life.
• The benefit of finding and treating the problem is usually much greater than the small risk from the dose. This is a reasoned judgement, so compare your reasons with a classmate.

10. Number of half-lives

Answer: $\dfrac{800}{50} = 16$. Since $2^4 = 16$, the count rate has halved four times, so four half-lives have passed. This also matches $\dfrac{24}{6.0} = 4$.

11. Choosing a source to explore an organ

• Choose the gamma source. Gamma radiation passes out through the body and can be detected outside it.
• Its half-life of a few hours is long enough for the scan, but short enough that the activity falls to a low level soon afterwards.
• The alpha source is not suitable. Alpha particles are stopped inside the body, so the detector outside could not measure them, and a 10-year half-life would leave the patient radioactive for a long time.

12. Precautions that reduce dose

• Keep as far from the source as possible, using tongs to handle it. The dose falls quickly with distance.
• Spend as little time near the source as possible.
• Put lead or another thick shield between the source and the technician.