Irradiation

Shining visible radiation from a torch beam onto a hand lights the hand up because the hand has been exposed to light.

Exposing objects to beams of radiation is called irradiation. The term applies to all types of radiation including radiation from the nuclei of atoms.

Irradiation from radioactive decay can damage living cells. This can be put to good use as well as being a hazard.

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, called a gamma knife, can be used to kill cancerous tumours deep inside the body

These beams are aimed at the tumour from many different directions to maximise the dose on the tumour but to 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.

Person receiving gamma treatment for cancer, with rays being directed to the head.

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