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Spec links: 4.4.2.1 Radioactive decay and nuclear radiation; 4.4.3.2 Different half-lives of radioactive isotopes (Physics only); 4.4.3.3 Uses of nuclear radiation (Physics only); 4.6.1.2 Properties of waves · Reading time: about 4 minutes

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A glowing skeleton

Sam is 15 and plays rugby. After a hard tackle, his ankle still hurts weeks later, but an X-ray shows nothing. So the doctor sends him for a bone scan. A nurse gives him a small injection. A few hours later, he lies under a large camera. On the screen, his whole skeleton appears as a glowing shape. One small spot in his ankle shines more brightly than the rest. It is a tiny stress fracture.

How did a camera see inside his bones? The answer is a radioactive tracer.

How a tracer works

A tracer is a radioactive isotope that is put into the body, often by injection. It is attached to a chemical that collects in a particular organ or tissue. In a bone scan, the chemical gathers where bone is repairing itself. As the tracer decays, it gives out radiation. A gamma camera outside the body detects this radiation and builds up a picture.

The most widely used tracer is technetium-99m. It gives out gamma rays and has a half-life of about 6 hours.

Choosing the right isotope

Doctors choose tracers carefully. Two things matter most.

First, the type of radiation. Gamma rays are very penetrating and weakly ionising. They pass out of the body easily to reach the camera, and they cause little damage on the way. Alpha particles would be useless. They would be stopped inside the body and would be very strongly ionising, harming the cells around them.

Second, the half-life. It must be long enough for the tracer to spread through the body and for the scan to be done. But it must be short enough that the activity falls quickly afterwards. With a 6-hour half-life, after one day (4 half-lives) only 1/16 of the technetium-99m is left.

Gamma rays are electromagnetic waves, so they obey the wave equation: wave speed = frequency × wavelength, or v = f λ. They travel at 3 × 10⁸ m/s. The gamma rays from technetium-99m have a wavelength of about 9 × 10^-12 m. Rearranging, f = v ÷ λ = (3 × 10⁸) ÷ (9 × 10^-12) ≈ 3.3 × 10¹⁹ Hz. That is an extremely high frequency.

Using radiation to treat cancer

Radiation can also be used to destroy unwanted tissue. This is called radiotherapy. Beams of gamma rays or X-rays can be aimed at a tumour from outside the body, from several directions, so that the tumour gets a high dose and healthy tissue gets less. Sometimes a small radioactive source is placed inside the body, right next to the tumour. Iodine-131 is used to treat some thyroid problems, because the thyroid gland absorbs iodine.

Weighing up the risks

Every scan gives the patient a radiation dose. This slightly increases the risk of cancer later in life. Doctors only use a scan when the benefit is greater than the risk. For Sam, finding the fracture meant the right treatment. His tracer was almost all gone within a day or two.

Key words

• Radioactive tracer: a radioactive isotope put into the body so that its radiation can be detected outside, to explore internal organs.
• Gamma ray: electromagnetic radiation from the nucleus.
• Half-life: the time it takes for the number of nuclei of an isotope in a sample to halve, or for the count rate from a sample to fall to half its initial level.
• Radiotherapy: the use of nuclear radiation to control or destroy unwanted tissue, such as a tumour.
• Ionising: able to knock electrons out of atoms, turning them into ions.
• Penetration: how far radiation can travel through a material before it is absorbed.

Questions

1. Name the most widely used medical tracer. [1 mark]
2. What type of radiation does this tracer give out, and what is its half-life? [2 marks]
3. What does a gamma camera detect? [1 mark]
4. Explain why an alpha emitter would not be suitable as a medical tracer. [3 marks]
5. Explain why a tracer should have a half-life of a few hours, rather than a few seconds or many years. [2 marks]
6. A patient is injected with technetium-99m with an activity of 480 MBq at 8 am.
   a) Calculate the activity at 8 pm the same day. [2 marks]
   b) Calculate the time it takes for the activity to fall to 30 MBq. Give your answer in hours. [2 marks]
7. X-rays used in radiotherapy have a frequency of 6.0 × 10²⁰ Hz. They travel at 3.0 × 10⁸ m/s. Calculate their wavelength. Give your answer in metres. [3 marks]
8. A hospital is choosing between three isotopes for a new tracer:
   Isotope A: alpha emitter, half-life 4 hours
   Isotope B: gamma emitter, half-life 6 hours
   Isotope C: gamma emitter, half-life 30 years
   Evaluate which isotope is the most suitable. Give reasons for your choice and against the others. [6 marks]

Show answers

1. Technetium-99m (1)
2. Gamma rays (1); about 6 hours (1)
3. Gamma rays (radiation) given out by the tracer inside the body (1)
4. Alpha particles have low penetration / are stopped inside the body (1); so they would not reach the camera / could not be detected outside (1); alpha is very strongly ionising so it would damage cells near the tracer (1)
5. Long enough for the tracer to spread through the body and for the scan to be completed (1); short enough that the activity falls quickly so the patient receives a low dose (1)
6. a) 12 hours = 2 half-lives (1); 480 → 240 → 120 MBq (1)
   b) 480 → 240 → 120 → 60 → 30 MBq = 4 half-lives (1); 4 × 6 = 24 hours (1)
7. λ = v ÷ f (1); λ = (3.0 × 10⁸) ÷ (6.0 × 10²⁰) (1); λ = 5.0 × 10^-13 m (1)
8. Indicative content:
   • A: alpha is stopped inside the body, so it would not reach the camera
   • A: alpha is very strongly ionising, so it would damage nearby cells; unsuitable
   • C: gamma passes out of the body and can be detected
   • C: half-life of 30 years means it stays radioactive in the body for a very long time, giving a large dose; unsuitable
   • B: gamma is penetrating and weakly ionising, so it can be detected outside the body and does little damage
   • B: 6-hour half-life is long enough to do the scan but short enough for activity to fall quickly
   • Conclusion: B is the most suitable (similar to technetium-99m)
   Level 3 (5–6 marks): clear choice of B, with reasons linking both type of radiation and half-life, and reasons for rejecting both A and C; Level 2 (3–4 marks): chooses B with some reasons, and rejects at least one other isotope with a reason; Level 1 (1–2 marks): simple statements about one isotope, or a choice with little reasoning.

More reading: See all 30 GCSE Physics comprehension articles