9.3 Hazards and uses of radioactive emissions and background radiation — Learn
Prerequisites: Section 9.2, especially the three types of radiation, their penetration and ionising power, and half-life.
By the end of this section you can: describe background radiation and where it comes from; explain why the dose a person receives varies; describe the hazards of ionising radiation; and describe and evaluate the medical uses of nuclear radiation.
1. Background radiation
Background radiation is the ionising radiation that is around us all the time. It comes from several sources.
- Natural sources. Rocks and soil contain radioactive atoms, and cosmic rays reach the Earth from space.
- Man-made sources. Fallout from nuclear weapon tests, and releases from nuclear accidents, add to the background.
The level of background radiation, and the radiation dose a person receives, depend on where they live and on their job. A radiographer works near sources, so receives a larger dose than an office worker. A person who lives in an area with more radon in the rocks also receives a larger dose. A person who lives high above sea level receives more cosmic rays.
Radiation dose is measured in sieverts (Sv). A thousand millisieverts (mSv) make one sievert.
2. Measuring a count rate
A detector measures a count rate in counts per second. The detector also records background radiation, so to find the count rate from a source alone you subtract the background count rate.
Worked example 1. A detector measures 260 counts per minute near a source. The background count rate is 20 counts per minute. Calculate the corrected count rate from the source.
3. The hazards of ionising radiation
Ionising radiation can knock electrons away from the atoms it passes through. Inside a living cell, this can damage the DNA. Damaged cells may die, or they may grow in an uncontrolled way and start a cancer.
The size of the hazard depends on two things: how ionising the radiation is, and whether the source is inside or outside the body.
- Alpha radiation is the most strongly ionising, but it is stopped by paper or by the outer layer of skin. Outside the body 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.
- Beta radiation penetrates further and is less ionising. It can pass into the body and can burn the skin.
- Gamma radiation is the least ionising, but it passes through the body. A strong gamma source outside the body is the main external hazard.
The half-life also matters. A source with a long half-life stays radioactive for a long time, so it remains a hazard in the environment or in the body. A source with a short half-life becomes less active quickly, which can make it easier to work with and to dispose of.
4. Using radiation in medicine: exploring the body
A tracer is a small amount of a radioactive isotope that is taken into the body. It moves through the body in the same way as the substance it is attached to. A detector outside the body follows the radiation and builds up an image of an internal organ.
A tracer should:
- emit gamma radiation, because gamma rays pass out through the body and can be detected outside it;
- have a short half-life, so that the activity falls to a low level soon after the scan and the patient receives a small dose.
An alpha or beta emitter would be stopped inside the body, so its radiation could not reach a detector outside.
5. Using radiation in medicine: treating tissue
A strong beam of radiation can also be used to control or destroy unwanted tissue, such as a tumour. The radiation damages the cells in the target tissue and stops them dividing.
The beam is aimed carefully from several directions, so that the target tissue receives a large dose while the healthy tissue around it receives as small a dose as possible. This use needs a larger dose than a tracer scan, because the aim is to damage the cells.
6. Choosing the right source
The best source for a job depends on what the radiation has to pass through and what the detector has to measure.
| Use | Best radiation | Why |
|---|---|---|
| Tracer to image an organ | Gamma | Passes out through the body to a detector |
| Treatment of a tumour | Gamma or beta | Strongly ionising, so it damages the target cells |
Watch out. Irradiation does not make an object radioactive. An irradiated object only receives radiation; when the source is taken away, it is not radioactive itself. Contamination is different, because radioactive atoms are left on or in the material. Check by asking: has any radioactive material been left behind, or has radiation only passed through?
7. Try it: stopping radiation
The figure lets you choose a source and see how far its radiation travels. The barriers are paper, aluminium and lead.
Predict first. Do not press the buttons yet. Write down your answers.
- Which radiation passes through paper?
- Which radiation is stopped by a few millimetres of aluminium?
- Which radiation needs thick lead to reduce it?
Test. Choose each source in turn. Read the barrier that stops it, its range in air, and its ionising power. Then read the ionising-power bar for each source and compare the three.
Explain. Alpha radiation is the most ionising but the least penetrating. Explain why strong ionising power makes a particle give up its energy in a short distance. Write your answer in two or three sentences.
8. Weighing risk against benefit
A medical use of radiation gives a patient a dose, which carries a small risk. The benefit is a diagnosis or a treatment that can be much more valuable. When you evaluate a use, give both sides.
- Benefit: the scan can find a problem early, or the treatment can destroy unwanted tissue.
- Risk: the dose can damage healthy cells, and in the long term it can cause a cancer.
- Judgement: describe how the dose is kept small, using a short half-life or a carefully aimed beam. Then say whether the benefit is worth the risk, giving a reason.
9. Progress check
Return to the chapter problem. The tracer emits gamma radiation and has a half-life of 6.0 hours. Explain why gamma radiation is the right choice for a tracer, and why a half-life of a few hours is better than a half-life of many years.
You should now be able to
- name natural and man-made sources of background radiation;
- describe how location and occupation change the radiation dose a person receives;
- correct a measured count rate for background radiation;
- describe the hazards of alpha, beta and gamma radiation, inside and outside the body;
- explain how half-life affects the hazard of a source;
- describe how tracers and beams of radiation are used in medicine;
- evaluate the risks and the benefits of a medical use of radiation.