Contamination

Contamination occurs if an object has a radioactive material introduced into it. An apple exposed to the radiation from cobalt-60 is irradiated but an apple with cobalt-60 injected into it is contaminated.

As with irradiation, contamination can be very useful as well as being potentially harmful.

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 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-Muller tube. This makes it easier to decide where to dig to find the leak.

Gamma rays concentrated at the pipe leak, underneath the a normal residential road.

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
Advantages of contaminationDisadvantages of contamination
Radioactive isotopes can be used as medical and industrial tracersRadioactive isotopes may not go where they are wanted
Use of isotopes with a short half-life means exposure can be limitedIt can be difficult to ensure that the contamination is fully removed so small amounts of radioisotope may still be left behind
Imaging processes can replace some invasive surgical proceduresExposure to radioactive materials can potentially damage healthy cells

Irradiation versus contamination

The two processes of irradiation and contamination are often confused. However they are very different and useful in their own right.

IrradiationContamination
Occurs when an object is exposed to a source of radiation outside the objectOccurs if the radioactive source is on or in the object
Doesn’t cause the object to become radioactiveA contaminated object will be radioactive for as long as the source is on or in it
Can be blocked with suitable shieldingOnce an object is contaminated, the radiation cannot be blocked from it
Stops as soon as the source is removedIt can be very difficult to remove all of the contamination