Sixth report : nuclear power and the environment : presented to Parliament by command of Her Majesty, September 1976 / Chairman: Sir Brian Flowers.

  • Great Britain. Royal Commission on Environmental Pollution.
Date:
1976
Catalogue details

Licence: Open Government Licence

Credit: Sixth report : nuclear power and the environment : presented to Parliament by command of Her Majesty, September 1976 / Chairman: Sir Brian Flowers. Source: Wellcome Collection.

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    connoting that the strontium nucleus (which has 38 protons) has 52 neutrons, making a total of 90 “‘nucleons’’. Some isotopes are stable, others such as strontium-90 emit radiation and are called ‘“‘radioisotopes’’. A substance con- taining unstable atoms is described as radioactive and, as we have seen, may emit four kinds of radiation: «, B, y, and if fission occurs, neutrons. The indi- vidual processes by which radioactive atoms approach stability by emitting radiation occur completely at random and are independent of all physical and chemical circumstances; the process can neither be advanced nor delayed by man. There is, however, for each particular type of radioisotope, a definite characteristic rate at which the atoms will disintegrate or “decay”: this is measured by its physical “‘half-life’’, that is, the time in which one half of the atoms will decay. The half-life may be a fraction of a second or it may be millions of years, but it is always the same for a given isotope. After one “‘half- life’ the amount of radioactivity from the given nuclei will be halved, and after a second half-life it will be halved again, and so on. Thus the radioactivity will never reach zero, though after sufficient half-lives it will become negligible.+ 33. It follows that for a given number of radioactive atoms the activity (i.e., the rate of radioactive disintegration) is inversely proportional to the half- life. Hence a short-lived substance will be extremely radioactive although for only a short time, while a long-lived substance will be much less radioactive but over a very long period. The unit of measurement of radioactivity is the “curie’, abbreviated Ci. One curie corresponds to the amount of activity displayed by one gram of radium (radium—226, whose half-life is 1602 years), namely 37 billion (3:7 x 10'°) disintegrations per second. After the passage of one half-life, the number of curies associated with the particular radioisotope will be halved. If a radioisotope decays more slowly than radium, that is, it has a longer half-life, then one gram will generally contain only a fraction of a curie. (One thousandth of a curie is a millicurie, mCi, and one millionth, a microcurie wCi.) Short-lived radioisotopes may contain thousands of curies (kCi) or even millions of curies (MCi) per gram: their radioactivity is thus very concentrated. For the purposes of radiation protection it is the radioactivity, rather than the mass, of material that is important. For this reason it is usual to express the amounts of radioactive substances in terms of the curies they contain. 34. Often a radioisotope decays to form a second one, which in turn decays to form a third; the process may continue through many stages, each having its characteristic half-life. The sequence is called a “‘decay chain’, and the subsequent radioisotopes are referred to as “decay daughters” of the first. An example is the decay of uranium—238, the principal constituent of natural uranium, which is found in nature because its half-life (4:5 billion years) is comparabie with the age of the earth. It is w-radioactive and decays to thorium— 234, which is itself unstable. The chain of “decay daughters’? following thorium—234 includes radium—226 (which is found in nature associated with much larger quantities of uranium, as in pitchblende ore) and a number of + Strictly, radioactivity will reach zero when the last nucleus disintegrates. i