Uses of Radioactive Isotopes: GCSE Physics

How is a radioactive isotope chosen for a given use?

Three factors determine which isotope is appropriate for a given task:

1. Type of radiation emitted: alpha (stopped by skin), beta (stopped by a few mm of aluminium), or gamma (penetrates many cm of lead).
2. Half-life: must match the timescale of the application — short half-life for medical use (limits patient dose); long half-life for industrial gauges (avoids frequent source replacement); specific half-life for dating (must match the age of the sample).
3. Chemical behaviour: for medical tracers, the element must be one that the body naturally takes up in the organ of interest (e.g. iodine concentrates in the thyroid gland).

How are radioactive isotopes used in medicine?

Medical tracers and diagnostic imaging

A radioactive tracer is a small amount of a radioactive isotope introduced into the body (by injection, swallowing, or inhalation). The patient's body handles it chemically the same way it would the stable version of the element, so the tracer travels to the organ under investigation. The radiation it emits passes out of the body and is detected externally.

• Technetium-99m (⁹⁹ᵐTc): the most widely used medical tracer. It emits gamma radiation (detected by a gamma camera outside the body), has a half-life of 6.0 hours (short enough to limit patient dose but long enough to complete the scan), and can be attached to a range of carrier molecules to target different organs (bones, heart, lungs, kidneys). The "m" means metastable — it decays to the ground state of ⁹⁹Tc by emitting gamma rays, not a new element.

• Iodine-131 (¹³¹I): used to treat thyroid disease. Iodine is naturally concentrated by the thyroid gland. ¹³¹I emits beta radiation, which irradiates and destroys thyroid tissue (overactive thyroid, or thyroid cancer) from within. Half-life: 8 days (long enough to deliver treatment; short enough that activity falls to safe levels within a few weeks).

• PET scanning: uses short-lived positron-emitting isotopes such as fluorine-18 (¹⁸F, half-life 110 minutes) attached to glucose (FDG). Active tissues (e.g. tumours, active brain regions) take up the glucose; the positron annihilates with an electron producing two gamma rays detected simultaneously, producing a 3D image.

Radiotherapy

Cobalt-60 (⁶⁰Co) emits high-energy gamma rays and is used in external beam radiotherapy to destroy cancer tumours. Beams are aimed at the tumour from multiple angles, concentrating the dose on the tumour while minimising damage to surrounding tissue. Half-life: 5.27 years (requires infrequent source replacement in the machine).

Sterilisation of medical equipment

Gamma radiation from ⁶⁰Co or caesium-137 (¹³⁷Cs) penetrates sealed packaging to kill bacteria and viruses on surgical instruments, syringes, and dressings. The equipment does not become radioactive itself. This method is cheaper and safer than chemical sterilisation for certain items.

How are radioactive isotopes used in industry?

How is carbon-14 used in archaeological dating?

Carbon-14 (¹⁴C) is a radioactive isotope of carbon produced continuously in the upper atmosphere when nitrogen-14 is bombarded by cosmic ray neutrons:

¹⁴N + ¹n → ¹⁴C + ¹H

The ¹⁴C becomes incorporated into CO₂ and enters living organisms through photosynthesis (and then food chains). While an organism is alive, the ratio of ¹⁴C to stable ¹²C in its tissues matches that in the atmosphere.

When the organism dies, it stops taking in carbon. The ¹⁴C present at death decays at a known rate (half-life = 5,730 years) while the ¹²C remains. By measuring the current ratio of ¹⁴C to ¹²C in a sample and comparing it to the atmospheric ratio, the time since death can be calculated.

Worked example: A wooden artefact has 25% of the ¹⁴C activity of a modern sample of the same mass.

• After 1 half-life (5,730 years): 50% remains
• After 2 half-lives (11,460 years): 25% remains

Age ≈ 11,460 years

Carbon-14 dating is reliable for organic materials up to about 50,000 years old. Older materials use isotopes with longer half-lives (e.g. potassium-40, half-life 1.25 billion years, for dating rocks).

Frequently asked questions

Why is a short half-life preferred for medical tracers?

A short half-life means the radioactive material decays quickly, reducing the total radiation dose received by the patient over time. Technetium-99m (half-life 6 hours) delivers most of its radiation during the scan, and its activity drops to negligible levels within a day. If a long-lived isotope were used as a tracer, the patient would continue to receive radiation dose for weeks or months after the procedure, increasing the risk of radiation damage to healthy tissue. The half-life must still be long enough to complete the scan — a few hours is the practical minimum for most diagnostic procedures.

Why is alpha radiation used in smoke detectors but not in medical tracers?

Alpha radiation has very short range (a few centimetres in air, stopped by skin). In a smoke detector, this short range is exactly what is needed — the alpha source ionises air in a small chamber without irradiating anything outside it, and smoke particles absorb the alpha within the chamber. In the body as a tracer, alpha would be absorbed within a few micrometres of the emitting atom and would deliver a highly concentrated radiation dose to nearby tissue without reaching an external detector — making it useless for imaging and dangerous as a localised irradiator. Gamma radiation, which passes through the body to an external detector, is needed for diagnostic tracers.

What are the risks of using radioactive isotopes and how are they managed?

Radiation can ionise molecules in cells, potentially causing mutations in DNA. High doses can cause radiation sickness or cancer. Risks are managed by: using the minimum necessary activity (ALARA principle — As Low As Reasonably Achievable); using isotopes with appropriate half-lives so that activity is as low as possible consistent with the task; shielding (lead aprons, lead-lined rooms, remote handling equipment); distance (inverse square law — doubling distance reduces intensity by a factor of four); and time (minimising time of exposure). Regulated by the Health and Safety Executive in the UK and internationally by the IAEA.

Can carbon-14 dating be used on bones or only on charcoal and wood?

Carbon-14 dating works on any material that was part of a living organism and therefore incorporated atmospheric carbon: charcoal, wood, textiles (wool, linen), seeds, parchment, shells (with careful calibration), antler and bone collagen (the organic protein component of bone, not the mineral component). Bone mineral (calcium phosphate) contains very little carbon, but the collagen within bone contains carbon that can be dated. Archaeological bone samples are therefore treated to extract the collagen fraction before measurement. Inorganic materials such as stone, metal tools, or fired pottery cannot be carbon-14 dated (they contain no organic carbon), though associated organic material from the same context can be.

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