📡 Which radiation? Gamma
The detector is outside the body, so the radiation has to get out. Alpha and beta would be absorbed by the body and never reach the detector. Gamma rays pass through the body, so a tracer must be a gamma emitter.
Radioactivity » Uses of Radioactivity in Medicine
What you'll learn this session
Study time: 30 minutes
Edexcel spec: 7.14
- How medical tracers let doctors see inside the body
- How radiotherapy uses gamma rays to kill cancer cells
- How gamma rays sterilise medical equipment
- Why each use needs a particular radiation and half-life
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Unlock This CourseChoosing the right radiation
Radioactive sources can help save lives. Doctors choose a source by asking two questions. Which radiation? (alpha, beta or gamma, from Alpha, Beta and Gamma Radiation) and what half-life? (from Activity and Half-Life). The answers are different for each use. This lesson covers the medical uses. Uses in industry come in the next lesson.
Remember: gamma rays are the most penetrating of the three. Alpha is stopped by paper or a few centimetres of air, and beta is stopped by a thin sheet of aluminium. Only gamma can pass out through the body.
Key terms:
- Medical tracer: a radioactive substance that is put into the body so that its path can be followed with a detector.
- Radiotherapy: treating cancer by aiming radiation at a tumour to kill cancer cells.
- Tumour: a lump of cells that grows out of control.
Use 1: medical tracers
A doctor wants to know if a kidney or the heart is working properly, but cannot just cut the patient open. Instead, the patient is given a small amount of a radioactive substance. It is injected or swallowed. It travels around the body, and a detector outside the body picks up the radiation it gives out. A computer turns this into a picture that shows where the substance has gone. A blockage, or a part that is not working properly, shows up as a place where the substance does not go, or where it builds up.
⏳ Which half-life? Short
The tracer stays in the body, so it should stop giving out radiation soon after the scan. A half-life of a few hours is typical. The activity falls away quickly, so the patient is exposed to radiation for a short time only.
Worked example
A tracer has a half-life of 6 hours. A patient is given it at 9 am. What fraction of the activity is left at 9 pm?
Time passed = 12 hours. Half-lives = 12 ÷ 6 = 2.
After 2 half-lives, 1/4 is left.
Answer: one quarter. The patient is soon much safer than if the half-life were many days.
Why not a half-life of a few seconds? It would be gone before the substance reached the part being checked, and before the scan was finished. So the half-life must be long enough to do the scan, but short enough that little is left afterwards. A real example is technetium-99m, a gamma emitter with a half-life of about 6 hours.
Use 2: radiotherapy
Radiation damages living cells, and enough of it kills them. Doctors use this on purpose to treat cancer. In radiotherapy, a narrow beam of gamma rays is aimed at a tumour to kill the cancer cells.
The problem is that the beam also passes through healthy tissue on its way to the tumour. The solution is to aim beams at the tumour from several directions. Picture a ring around the patient. Every beam crosses the tumour, so the tumour gets the radiation from all of the beams added together. Healthy tissue away from the tumour is crossed by only one or two beams, so it gets a much smaller dose.
- Why gamma? The tumour is often deep inside the body. Gamma rays are penetrating enough to reach it from outside. Alpha and beta would be stopped near the skin.
- Why a long half-life? The source is outside the patient, so there is no need for it to fade quickly. A long half-life (cobalt-60 is a typical gamma source, with a half-life of about 5 years) means the activity stays nearly steady for a long time. The treatment dose is easy to predict and the source does not need replacing very often.
Use 3: sterilising medical equipment
Syringes, scalpels and dressings must be free of living microorganisms before they are used. In Uses of Electromagnetic Waves you met gamma rays killing bacteria. Here we look at why gamma is chosen for this job.
- Gamma passes through the packaging. The equipment can be sealed in its plastic wrapping first, then irradiated. The gamma rays pass through the wrapping and kill the microorganisms inside. It stays sterile until the pack is opened.
- No heating needed. Plastic items such as syringes would melt in the heat of other methods of sterilising. Gamma rays do not need high temperatures.
- Long half-life. The source can be kept in the sterilising plant for years. The activity falls only slowly, so each batch gets nearly the same treatment.
Alpha and beta are no use here. They would be absorbed by the wrapping and would never reach the equipment inside.
Summary table
| Use | Radiation | Half-life | Reason |
|---|---|---|---|
| Medical tracer | Gamma | Short (hours) | Gamma leaves the body to the detector. Short half-life means little radiation is left afterwards. |
| Radiotherapy | Gamma | Long (years) | Gamma reaches deep tumours. Steady source that lasts. |
| Sterilising equipment | Gamma | Long (years) | Gamma goes through packaging. Steady source that lasts. |
Common mistakes
- Choosing a long half-life for a tracer. A tracer is inside the patient, so it must fade quickly.
- Choosing alpha or beta for a tracer. They would not get out of the body to reach the detector.
- Saying radiotherapy uses "one strong beam". It uses beams from several directions so healthy cells get a smaller dose.
- Writing "gamma is the strongest" instead of the useful fact: gamma is the most penetrating.
Exam-style question
A hospital gives a patient a tracer to check that a kidney is working.
(a) Describe how the tracer is used to find out whether the kidney is working. [2 marks]
(b) The tracer gives out gamma rays and has a half-life of 6 hours. Explain why each of these is a good choice. [4 marks]
(c) The same hospital uses gamma rays to sterilise syringes in sealed plastic packs. Give one reason why gamma rays are used. [1 mark]
Model answer
(a) The tracer is injected (or swallowed) into the patient (1). A detector outside the body picks up the gamma rays and shows where the tracer has gone, so the doctor can see how the kidney is working (1).
(b) Gamma: it passes out through the body to reach the detector outside (1), whereas alpha and beta would be absorbed in the body (1). Half-life of 6 hours: short, so the activity falls quickly after the scan (1), so the patient is exposed to radiation for only a short time (1).
(c) Gamma rays pass through the plastic pack, so the syringes can be sterilised inside it (1). (Or: no heating is needed.)
Exam tip
In "explain why" questions about a source, give a reason for the radiation AND a reason for the half-life. Each is worth its own mark.