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

1. One half of the original count rate remains.
2. Alpha radiation is stopped by paper.

If question 1 went wrong, look again at the definition of half-life in Section 9.2. If question 2 went wrong, look at the table of penetration in Section 9.2.

Natural sources have always been present. Man-made sources began in the last century.

For the natural pair, name one source from the ground and one from space. For the man-made pair, name one from weapon tests and one from an accident.

Natural sources: radioactive rocks and soil, and cosmic rays from space. Man-made sources: fallout from nuclear weapon tests, and releases from nuclear accidents.

Location affects the rocks and the height above sea level. Occupation affects how close a person works to a source.

Give one example of a place that gives a larger dose, and one job that gives a larger dose.

Some rocks contain more radioactive atoms than others. A person who lives in an area with more radon receives a larger dose. A person who lives high above sea level receives more cosmic rays. For occupation, a radiographer or a nuclear worker spends time near sources, so receives a larger dose than an office worker.

The detector measures the source and the background together.

Subtract the background count rate from the measured count rate.

Think about how far each type of radiation travels, and how strongly it ionises the material it passes through.

Say which one is more strongly ionising, then say where it would need to be to become a serious hazard.

Alpha radiation is strongly ionising but has a short range. Outside the body it is stopped by the skin, so it is a small hazard. Inside the body it transfers a lot of energy to a short range of living cells, so it is a serious hazard.

Gamma radiation is weakly ionising but has a long range. Outside the body it can pass through the body and reach living cells, so a strong gamma source is a serious external hazard.

The radiation must reach a detector outside the body. The patient should not stay radioactive for a long time.

Choose the isotope whose radiation escapes the body, then say why its half-life is suitable.

Isotope A is suitable. Its gamma radiation passes out through the body, so a detector outside the body can measure it. Isotope B emits alpha radiation, which would be stopped inside the body. Isotope A also has a short half-life of a few hours, so its activity falls quickly after the scan and the patient receives a small dose. Isotope B would leave the patient radioactive for many years.

Think about how long the contaminating atoms keep decaying.

Say what happens to the activity of the short-half-life source after a few half-lives, then compare it with the long-half-life source.

A source with a long half-life keeps decaying for a long time, so the contamination stays radioactive for years. A source with a short half-life falls to a very low activity after a few half-lives, so the hazard does not last as long.

The benefit is what the scan finds. The risk comes from the dose of ionising radiation.

Write one sentence for the benefit, one for the risk, and one judgement that gives a reason.

Benefit: the scan can show how an organ is working, and can find a problem that other tests would miss, so the patient can be treated sooner. Risk: the tracer gives the patient a dose of ionising radiation, which can damage cells and, in the long term, cause a cancer. Judgement: the dose is kept small by choosing a short half-life and a small amount of the isotope. For most patients the benefit of finding a serious problem is much greater than the small risk, so the scan is worth having.