Half-Life & Risk (Oxford AQA IGCSE Physics)

Half Life & Risk

• The half-life is the average time it takes for the number of nuclei in a radioactive source to decrease to half of its original value

• Different radioactive isotopes can have very different half-lives

• For example:

  • Francium-218 has a half-life of only 1 millisecond (0.001 seconds)

  • Polonium-210 has a half-life of about 140 days

  • Uranium-235 has a half-life of about 700 million years

Short half-life values

• If an isotope has a short half-life, the nuclei will decay very quickly

  • This means that the isotope will emit a lot of radiation in a short amount of time

• The most unstable nuclei have the shortest half-lives

• If only a small amount of the isotope is used, having a short half-life can be advantageous, as the material will quickly lose its radioactivity

• If a large amount is used, however, the levels of radiation emitted could make handling the isotope extremely dangerous

Long half-life values

• If an isotope has a long half-life then a sample of it will decay slowly

  • Although it may not emit a lot of radiation, it will remain radioactive for a very long time

• The least unstable nuclei have the longest half-lives

• Sources with long half-life values present a risk of contamination for a much longer time

• Radioactive waste with a long half-life is buried underground to prevent it from being released into the environment

Disposal of radioactive waste

Uses of radioactive isotopes

• There are many different uses of radioactive isotopes

• The radioactive isotope used, and its associated hazards, depends on the type of radiation it emits and its half-life

Medical tracers

• A tracer is a radioactive isotope that can be used to track the movement of substances, like blood, around the body

• Gamma emitters are usually used for this purpose

  • Gamma rays are highly penetrating and so will be able to pass through the body and be detected outside the body

• This allows an internal image of the body to be created

Using a medical tracer

• Since gamma rays are less ionising than some other forms of radiation, the harm caused to the patient is also minimised

• As well as choosing a gamma emitter:

  • The amount of isotope used is kept to a minimum to reduce people’s exposure to radiation

  • Isotopes are chosen that have short half-lives of around a few hours: Long enough to carry out the procedure, but not so long that they cause long term harm