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Irradiation for sterilisation

Irradiation can be used to preserve fruit sold in supermarkets by exposing the fruit to a radioactive source - typically cobalt-60. The gamma rays emitted by the cobalt will destroy any bacteria on the fruit but will not change the fruit in any significant way. The process of irradiation does not cause the irradiated object to become radioactive.

Medical irradiation

Doctors also use radioactive sources for a number of reasons, eg:

• Sterilisation of surgical instruments.
• Beams of gamma rays can be used to kill cancerous tumours deep inside the body. The beams are aimed at the tumour from many different directions to maximise the dose on the tumour but minimise the dose on the surrounding soft tissue. This technique can damage healthy tissue, so careful calculations are done to establish the best dose - enough to kill the tumour but not so much so that the healthy tissue is damaged.

A gamma knife is a specific type of gamma irradiation used to kill cancerous cells in the brain. The head is clamped into a special helmet to make sure it stays perfectly still during the irradiation.

In medical applications that involve using radioactive sources, efforts are made to ensure that irradiation does not cause any long-term effects. This is done by considering:

• the nature of decay (alpha, beta or gamma)
• the half-life (long enough for the isotope to produce useful measurements, but short enough for the radioactive sources to decay to safe levels soon after use)
• toxicity

If the half-life chosen is too long, the damaging effects of the radiation would last for too long and the dose received would continue to rise.

Advantages and disadvantages of irradiation

Advantages

• Sterilisation can be done without high temperatures.
• It can be used to kill bacteria on things that would melt.

Disadvantages

• It may not kill all bacteria on an object.
• It can be very harmful - standing in the environment where objects are being treated by irradiation could expose people's cells to damage and mutation.

Medical contamination

In some cases, injected radioactive sources (such as technetium-99) can be used as tracers to make soft tissues, such as blood vessels or the kidneys, show up through medical imaging processes. An isotope emits gamma rays that easily pass through the body to a detector outside the body, for example an X-ray machine or a 'gamma camera'. In this way, the radioactive isotope can be followed as it flows through a particular process in the body.

Changes in the amount of gamma radiation emitted from different parts would indicate how well the isotopes are flowing, or if there is a blockage.

In medical applications that involve injecting radioactive sources, efforts are made to ensure that contamination does not cause any long-term effects. This is done by choosing isotopes that:

• have very short half-lives - sources used typically have half-lives of hours, so after a couple of days there will hardly be any radioactive material left in a person's body
• are not poisonous

Contamination to check for leaks

Water supplies can be contaminated with a gamma-emitting radioactive isotope to find leaks in pipes. Where there is a leak, contaminated water seeps into the ground, causing a build-up of gamma emissions in that area. The build-up of gamma emissions can be found using a Geiger-Müller tube. This makes it easier to decide where to dig to find the leak.

The isotope used for this purpose must:

• be a gamma emitter
• have a half-life of at least several days to allow the emissions to build up in the soil
• not be poisonous to humans as it will form part of the water supply