4. Background radiation

We receive a small continuous dose of background ionising radiation due to

- Alpha radiation from breathing in naturally occurring radon gas

- Gamma radiation from natural radioactive isotopes in rocks and soil

- Beta and gamma (mostly) from radioactive isotopes that naturally make up our bodies

- Cosmic rays

Medical procedures like X-rays and cancer treatment are averaged across the population

Nuclear power, nuclear weapons testing, and nuclear waste contribute a tiny amount to background dose

Every living thing is exposed to radiation all the time quite naturally. Background radiation is about the radiation dose a typical person would receive.

Alpha, beta and gamma all cause ionisation of atoms as they pass through them. As we'll see in lesson 7, unwanted ionisations in our cells can upset our cell chemistry and may cause health problems. So other forms of ionising radiation, for example X-rays, also get included when calculating background dose.

Radioactive isotopes have always been part of the Earth. The Sun and planets were formed about 4.5 billion years ago from the dust and gases of long-dead stars. The supernovas that ended these stars lives created the incredibly high temperatures and pressures needed for heavy nuclei to be fused from lighter particles like protons.

We've already met uranium, radium and polonium, but there are less exotic elements like potassium that have natural radioactive isotopes, as well as the stable ones we're used to.

Radioactive isotopes in the rocks and soil around us produce alpha, beta and gamma radiation, but the only form that can really hit people is gamma. An exception to this is naturally occuring radon, which is a gas and so we can breath it in. The alpha particles released by the radon gas can increases the risk of lung cancer. Different parts of the UK have different radon levels. There are regulations and precautions to control our exposure to radon.

Plants are mostly reorganised carbon dioxide with very small amounts of minerals from the soil they grow in. Carbon and some of the minerals have naturally radioactive isotopes. Animals eat plants, and we eat plants and animals, so we're made from very small quantities of radioactive isotopes, too.

One of the most important of these is potassium-40, which is a high energy beta and gamma emitter. We absorb about 3000 beta and gammas per second from potassium-40 in our body.

We also get bombarded with cosmic rays. Fast moving particles from deep space like protons create a shower of exotic particles when they collide with our atmosphere. These cosmic rays are a form of ionising radiation and so get added to our background dose. If you live at high altitude or take lots of flights you're exposed to more cosmic rays.

If you have a medical procedure like an X-ray this is also added to the background dose. But these are averaged across the population, rather than tracked for each person.

Other human activity like nuclear weapons testing, nuclear accidents and nuclear power plants add almost nothing to background radiation.

You can divide background radiation into the radiation from nearby radioactive stuff, and everything else.

As we mention in section 8.1 on nuclear equations, all the naturally occurring radioactive isotopes found on Earth either have a very long half-life - and so survive from the ancestral supernovae that produced them - or are radioactive decay products with those long-lived isotopes as the ultimate parent.

Radioactive isotopes like those of radon, radium and polonium were produced in these ancient supernovae, but had long since decayed to stable isotopes before the Earth was formed. The instances of these isotopes we find on Earth now are all produced as a result of the decay of very long-lived parent nuclei, like uranium-238.