Half-life and activity

1. Radioactive decay is random, so you cannot predict when any particular unstable nucleus will decay.
2. In a large sample, the same fraction of the remaining unstable nuclei decays in each half-life, so the activity falls as the number of undecayed nuclei falls.
3. The half-life tells you how quickly this happens, and each radioactive isotope has its own half-life.
4. Half-lives range from fractions of a second to billions of years, so the hazard from different radioactive materials varies enormously.
   1. Technetium-99m has a half-life of about 6 hours.
   2. Iodine-131 has a half-life of about 8 days.
   3. Caesium-137 has a half-life of about 30 years.
   4. Plutonium-239 has a half-life of about 24 000 years.
5. The hazard from a radioactive material depends on two things linked to half-life: how many decays happen each second, and how long the material keeps decaying.

Short half-life

1. A material with a short half-life has nuclei that decay quickly, so a large fraction of them decays every second.
2. For two samples with the same number of unstable nuclei, the one with the shorter half-life has the higher activity.
3. A high activity means many particles or rays are emitted each second, so anyone nearby receives a large dose of radiation in a short time.
4. The activity also falls quickly, so after a few half-lives only a small fraction of the original unstable nuclei is left.
5. After ten half-lives the activity has fallen to less than a thousandth of its starting value, because ${\left(\frac{1}{2}\right)}^{10}=\frac{1}{1024}$.
6. A material with a half-life of a few hours is therefore close to harmless within a few days.
7. The hazard from a short half-life material is intense but brief.

Long half-life

1. A material with a long half-life has nuclei that decay slowly, so only a small fraction decays each second.
2. Its activity is lower than that of a similar sample with a short half-life, so the dose received each second is usually smaller.
3. Its activity stays almost the same for years, so the material remains hazardous for a very long time.
4. Radioactive waste from nuclear power stations contains isotopes with half-lives of thousands of years, so it must be kept safe for many generations.
5. This waste is sealed in strong containers, kept away from water supplies and often stored deep underground, so that it cannot contaminate people or the environment while it slowly decays.
6. Land and buildings contaminated with long half-life material can stay unsafe for decades, which is why the area around the Chernobyl power station is still closed to the public.
7. The hazard from a long half-life material is weaker at any moment but lasts far longer.

Judging the overall hazard

1. To judge how hazardous a radioactive material is, you need to consider both how strongly it emits now and how long it will keep emitting.
2. A short half-life is the bigger concern for a brief, close exposure, such as handling a freshly prepared source.
3. A long half-life is the bigger concern when material might escape, be swallowed or be left in the environment, because it keeps emitting radiation for years.
4. Half-life combines with the type of radiation, so an alpha emitter with a long half-life is especially dangerous if it is breathed in, because it keeps irradiating nearby lung cells for many years.
5. The half-life of an isotope is therefore one of the main factors used to decide how it is stored, transported and disposed of.