radioactivity

radioactivity When Henri Becquerel established the existence of ‘uranic rays’ in March 1896, there was no way of appreciating the far-reaching implications of this discovery. Only 75 elements had been discovered by this time, two of which, uranium and thorium, were radioactive, although this was not known. The periodic table has since been expanded to 81 stable and 31 radioactive elements.

Radioactivity is the process of emission of radiation as a radioactive material changes form, often to a different element. To understand this process, we need to be familiar with a number of concepts and terms. Atoms of each element contain a different and defining number of protons and an equal number of electrons. The nucleus of the atom contains neutrons as well as protons and different numbers of neutrons are present in different isotopes of the same element. Isotopes of an element may be either stable, or unstable and radioactive — radioisotopes. Isotopes of all elements are referred to collectively as nuclides; those that are radioactive, as radionuclides. Radionuclides are specified by the elemental name and the mass number — the combined number of protons and neutrons — for example, carbon-14 (¹⁴C), iodine-131 (¹³¹I), plutonium-239 (²³⁹Pu). Where an element is referred to as radioactive, as in the paragraph above, this is intended to mean that all isotopes of the element are radioactive. Radionuclides differ in their rate of decay as well as the radiation emitted. The rate of decay in a given mass of the radionuclide is measured in units called becquerels (Bq), where 1 Bq equals one transformation per second. Alpha-emitting radionuclides emit alpha particles, each consisting of 2 protons and 2 neutrons. Beta-emissions involve the loss of an electron from the nucleus as a beta particle during the conversion of a neutron into a proton. Gamma rays are high energy photons, often emitted together with beta or alpha radiations when the transformation has left the atom with excess energy. An important characteristic of a radionuclide, as well as the radiation emitted, is its half-life — the time taken for half the atoms present to decay to the daughter nuclide. Thus ¹³¹I is a beta-emitter with a half-life of 8 days, while ²³⁹Pu is an alpha-emitter with a half-life of 24 000 years.

We are exposed to radionuclides throughout our lives, mainly from natural sources. The greatest exposures are due to inhalation of radon-222 (²²²Rn) gas, present in the atmosphere due to the decay of uranium-238 contained in rocks and soil. Artificial sources include the medical use of radiopharmaceuticals and small amounts released by the nuclear industry. There is, of course, the potential for greater exposures from nuclear installations if accidents occur, the most noteable example being the accident at Chernobyl in the former Soviet Union in 1986.

The health risk associated with exposure to a particular radionuclide will depend on the radiation emitted and its chemical behaviour. Beta and gamma radiations can penetrate through the skin and may pose an external radiation hazard, but the main concern generally is the entry of radionuclides into the body by inhalation and ingestion. Intake will lead to dose delivery to the respiratory and alimentary tracts, and absorption into the blood followed by entry into other organs and tissues. Depending on their chemical behaviour, some radionuclides concentrate in specific organs and tissues. For example, iodine-131 concentrates in the thyroid gland because iodine is an essential constituent of the hormone, thyroxine. Consequently, the dose to the thyroid is much greater than doses to other tissues, presenting a potential risk of thyroid cancer. Plutonium-239 is deposited mainly in the skeleton and liver and presents a potential risk of liver and bone cancer and leukaemia. Doses are calculated for intakes of radionuclides, taking account of their distribution and retention in the body and the pattern of deposition of radiation energy in different tissues. These calculations are done primarily by the International Commission on Radiological Protection, and the calculated values of dose per unit intake (Sv per Bq) are used as a basis for restrictions on radionuclide exposure in legislation in Europe, the UK, and elsewhere.

John D. Harrison

See also imaging techniques; radiation, ionizing; radiology; radiotherapy.