Closer examination of the atoms that make up a given element has shown that, although they have the same number of protons, they can have differing numbers of neutrons. For example, the element carbon has three naturally occurring atomic forms – 12C, 13C and 14C. These forms, which differ only in the number of neutrons found in the nucleus, are called isotopes. 12C is the most abundant of the three and is a stable isotope.
Some arrangements of the protons and neutrons in atomic nuclei are in a state of high energy and are said to unstable. Rearrangements can occur, leading to the emission of radiation. Of the 3,000 nuclides (distinct kind of atom) known, only about 300 are stable.
Radioisotopes – bad or good?
The more familiar types of this radiation are alpha particles, beta particles and high-energy gamma rays. High-energy gamma radiation is very penetrating and, if the dose is high enough, can damage living cells beyond repair.
There is a lot of low level natural radioactivity around us. For example, our bodies contain radioisotopes, such as potassium-40, which continuously emit radiation, but because the amount is very low, no damage is done.
The world we live in contains C-14 in very small amounts. Unlike the arrangement of protons and neutrons in the nucleus of carbon-12, the arrangement in carbon-14 is unstable. Atoms of carbon-14 undergo a random rearrangement in a process called radioactive decay. For carbon-14, this decay occurs slowly. The time taken for a given amount of carbon-14 to decay so that only half of it remains is called the half-life. For carbon-14, the half-life is 5,730 years. Scientists can use this to calculate the age of very old items of archaeological interest, provided they have a plant or animal origin. Carbon-14 was used by Dr Fiona Petchey to date artefacts of historical importance in an archaeological dig at the Wairau Bar close to the town of Blenheim.
Radioactivity and medicine
Radioisotopes are used in an increasingly large number of diagnostic procedures as well as in the treatment of a range of cancers.
One radioactive substance that is very commonly used in hospitals is called technetium-99m. This emits gamma rays and has a half-life of 6 hours, both of which help to minimise the exposure of patients to radiation. Technetium atoms are easily combined with chemicals that are absorbed by particular parts of the body, allowing doctors to ‘target’ specific organs such as the thyroid, brain and the kidneys.
Cobalt-60, which decays to produce beta particles and gamma rays, is used for the treatment of certain cancers.
Radioactivity in households
Most homes are fitted with smoke detectors. A radioisotope, americium-241, is used as part of the electronic circuitry of one type of smoke alarm. Only a very small amount of americium is used – about 0.2 mg. It has a half-life of 432 years and is an alpha particle emitter. The amount of radiation generated by the smoke detector is very small, and in addition, surrounding air molecules quickly absorb the alpha particles produced.
Radioactivity in industry
In the paper-making industry, the thicknessing of the final paper product is a key step. Strontium-90, which is a gamma ray emitter, is used as part of the control system operating the compression rollers.
Cobalt-60, a gamma ray emitter, is used in the sterilisation of medical dressings and instruments. It is also used in the food industry to delay the ripening of fruits, to inhibit the production of shoots and buds in vegetables such as potatoes and to improve the shelf-life of fruits.
Scientific research often requires the use of radioisotopes as tracers. For example, the movement of nutrients through a living system can be tracked using phosphorus-32.
The student activity Radioactive decay uses coin flips to explore the concepts of half-life and the randomness of radioactive decay.
In Using radiocarbon carbon dioxide data students interpret graphs showing carbon dioxide in the atmosphere of New Zealand and explore how sampling intervals affect the conclusions we are able to make.