The unstable atom

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Radiation comes from atoms, the basic building blocks of matter.

Most atoms are stable; a carbon-12 atom for example remains a carbon-12 atom forever, and an oxygen-16 atom remains an oxygen-16 atom forever, but certain atoms eventually disintegrate into a totally new atom. These atoms are said to be 'unstable' or 'radioactive'. An unstable atom has excess internal energy, with the result that the nucleus can undergo a spontaneous change towards a more stable form. This is called 'radioactive decay'.

Each element exists in the form of atoms with several different sized nuclei, called isotopes. Unstable isotopes (which are thus radioactive) are called radioisotopes. Some elements, eg uranium, have no stable isotopes.

When an atom of a radioisotope decays, it gives off some of its excess energy as radiation in the form of gamma rays or fast-moving sub-atomic particles. If it decays with emission of an alpha or beta particle, it becomes a new element. One can describe the emissions as gamma, beta and alpha radiation. All the time, the atom is progressing in one or more steps towards a stable state where it is no longer radioactive.

Another source of nuclear radioactivity is when one form of a radioisotope changes into another form, or isomer, releasing a gamma ray in the process. The excited form is signified with an "m" (meta) beside its atomic number, eg technetium-99m (Tc-99m) decays to Tc-99. Gamma rays are often emitted with alpha or beta radiation also, as the nucleus decays to a less excited state.

Apart from the normal measures of mass and volume, the amount of radioactive material is given in becquerel (Bq), a measure which enables us to compare the typical radioactivity of some natural and other materials. A becquerel is one atomic decay per second *.

* A former unit of (radio)activity is the Curie - 1 Bq is 27 x 10-12 curies.

Radioactivity of some natural and other materials

1 adult human (100 Bq/kg) 7000 Bq
1 kg of coffee 1000 Bq
1 kg superphosphate fertiliser 5000 Bq
The air in a 100 sq metre Australian home (radon) 3000 Bq
The air in many 100 sq metre European homes (radon) 30 000 Bq
1 household smoke detector (with americium) 30 000 Bq
Radioisotope for medical diagnosis 70 million Bq
Radioisotope source for medical therapy 100 000 000 million Bq
1 kg 50-year old vitrified high-level nuclear waste 10 000 000 million Bq
1 luminous Exit sign (1970s) 1 000 000 million Bq
1 kg uranium 25 million Bq
1 kg uranium ore (Canadian, 15%) 25 million Bq
1 kg uranium ore (Australian, 0.3% 500 000 Bq
1 kg low level radioactive waste 1 million Bq
1 kg of coal ash 2000 Bq
1 kg of granite 1000 Bq

NB. Though the intrinsic radioactivity is the same, the radiation dose received by someone handling a kilogram of high grade uranium ore will be much greater than for the same exposure to a kilogram of separated uranium, since the ore contains a number of short-lived decay products (see section on Radioactive Decay).

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