31 



ASSAY OF RADIOACTIVITY 



R. D. KEYNES 



Reference books 



It is impossible in a brief chapter to present more than an outline of the 

 methods used for determination of radioactivity. There are a number of 

 standard textbooks which cover the whole field of tracer technique, both 

 in general and as applied to biological problems, among which those by 

 Kamen\ Sacks^'^, Siri*, Whitehouse and Putnam^ Taylor^ and Sharpe' may 

 be recommended to the reader seeking a more complete account than can 

 be given here. The Atomic Energy Research Establishment's catalogue of 

 radioactive materials and stable isotopes (Catalogue No. 4 was issued in 

 January 1957) also contains much useful information. 



Isotope characteristics 



The characteristics of the radiation emitted when radioactive atoms decay 

 differ widely from one isotope to another, and play an important part in 

 determining the most appropriate technique to be used in assaying any given 

 isotope. Three types of radiation may be encountered: a-particles, which 

 are hehum nuclei with a double positive charge moving relatively slowly 

 (order of 10^ cm/sec), /5-particles, which are electrons (or positrons) with a 

 single negative or positive charge moving about ten times faster, and y-rays, 

 which are uncharged quanta of electromagnetic radiation moving with the 

 speed of light. The various isotopes differ not only in the type of radiation 

 that they emit (many of them produce (i- and y-radiation at the same time, 

 though some are pure /?- or y-emitters) but also in the energy of their 

 radiations, i.e. in the amount of energy dissipated when the process of radio- 

 active disintegration of their nuclei takes place. Table 1 lists the characteristics 

 of some of the radioactive isotopes most likely to be needed in biological 

 tracer experiments, and it will be seen that /^-particle energies range from the 

 very weak 0-018 MeV radiation of tritium (^H) to the powerful 3-6 MeV 

 /5-particles given by ^^K, while y-ray energies are distributed over a some- 

 what narrower range. The rate at which radioactive isotopes decay, in 

 other words their half-lives (the time taken for half the atoms initially present 

 to disintegrate), also varies widely. Some are so unstable that their half-life 

 is a small fraction of a second, while at the other end of the scale *°K, the 

 naturally occurring radioactive isotope of potassium, has a half-hfe estimated 

 as 4-5 X 10^ years. 



INTERACTIONS OF RADIATIONS WITH MATTER 



In penetrating through matter the charged particles (a and /?) interact 

 electrostatically with the orbital electrons, and less often the nuclei, of those 



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