506 ISOTOPIC TRACERS AND NUCLEAR RADIATIONS [Chap. 24 



isotopes omitted from the list will have no biological application. It is too 

 early to predict what will and will not be practical in the tracer and radio- 

 biological work of the future. In general, isotopes have been selected from 

 the Seaborg chart on the basis of half-life, with the assumption that species 

 with long half-lives (weeks to years) will be of most general value but that 

 shorter lived species (hours, or even minutes, to days) may be desirable in 

 certain cases, as, for example, for tracer and therapeutic uses in humans. 

 Some of the isotopes in Table 48 may never find biological application by 

 reason of some practical difficulty, as discussed in Sec. 24.3 d. 



There are now known over 700 radioactive isotopes, of whicn, however, 

 only about 233 have properties suggesting value in biological research. A 

 total of 92 radioactive isotopes (of 56 elements) have been used in biological 

 and medical studies, 82 (of 54 elements) of them in tracer studies. Thus, 

 somewhat less than half of the potentially applicable species have found use 

 in biological experimentation. Unquestionably, many, if not most, of those 

 as yet unemployed will find experimental application in the near future. 

 The majority of studies with radioactive isotopes have been in the field of 

 tracer work. A number of studies, however, have been on the biological 

 effects of radiation; and a few radioisotopes now have well-established 

 applications in clinical medicine. 



Nine stable isotopes have been applied to biological problems, and the use 

 of a tenth (H 1 as proton beams) has been suggested. Most work with stable 

 isotopes has been with the rarer species of elements existing in more than one 

 stable form. Of 278 known stable isotopes, 193 are less abundant species of 

 the 60 elements having two or more naturally occurring isotopes; but of the 

 193, only 7 have known biological applications. Most of the work with these 

 rarer species has been in tracer studies. However, not all the rarer stable 

 species so far used have been applied in this manner, for certain isotopes have 

 peculiar properties that have made possible, or suggest, their use in radiation 

 studies. For example, both lithium and boron have rarer isotopes that split 

 when bombarded with accelerated particles. The specific ionization of the 

 radiation released is considerably greater than that attending the accelerated 

 particles themselves. Thus, selective localization of lithium and boron 

 compounds has made possible selective radiation of certain tissues. How- 

 ever, these properties are not necessarily restricted to the rarer species of the 

 elements concerned. Beryllium, consisting of a single isotope (Be 9 ), has 

 given off fast neutrons when bombarded with protons or deuterons; the 

 resulting neutron beams have in turn been used for experimental therapeutic 

 purposes. Alpha particles, the nuclei of ordinary helium (a or He 4 ), and 

 deuterons, the nuclei of heavy hydrogen (d of H 2 ), have been used to irradiate 

 living cells. Finally, the more common of the two isotopes of hydrogen 



