ASSAY OF RADIOACTIVITY 



TABLE 1 

 Characteristics of the Radioactive Isotopes of Greatest Interest in Biological Research 



B.C. = orbital electron capture by nucleus 

 I.T. = isomeric transition 



In cases where positrons are emitted, annihilation radiation (0-5 MeV y) is always observed, For further details 

 of the more complicated decay schemes see A.E.R.E. Catalogue No. 4 of Radioactive Materials, or Kamen^, etc. 



atoms that they approach sufficiently close, causing them to dissociate 

 into an ion pair — a heavy positive ion and a negatively charged electron. 

 Each such event or 'ionization' robs the moving particle of some of its 

 energy, so that eventually it comes to rest. In air, roughly 30 electron volts 

 are needed for the formation of a single ion pair, so that a /9-particle emitted 

 by ^H will be brought to a standstill in air after producing about 600 ioniza- 

 tions while one from *^K will give rise to no less than 120,000 ion pairs. In 

 the energy range with which we are concerned here the efficiency of the 

 ionization process is inversely proportional to the square of the velocity 

 of the /9-particle, so that most of the ionization produced by a ^^-particle 

 occurs near the end of its path, and the ionization per unit length of path is 

 initially much less for an energetic particle than for a slowly moving one. The 

 total distance traversed by a /9-particle before it comes to a stop, that is to 

 say its penetrating power, increases with its initial energy. In solid matter 

 /3-particles will not, of course, travel as far as in a gas, since interactions with 



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