NUCLEAR PHYSICS DUBRIDGE 233 



It is possible that this property of neutrons of converting certain 

 elements into their radioactive isotopes may be turned to good use. 

 It might, for example, prove useful to direct a slow neutron beam at 

 certain regions in the body and convert some of the elements present 

 there into radioactive isotopes, whose radioactivity would remain 

 for several hours after the Deutron exposure had ceased. Since 

 almost every element may be made radioactive by neutrons there 

 are many possibilities to be considered. And this brings us directly, 

 then, to a discussion of the possible uses in biological work of artificially 

 produced radioactive materials. 



INDUCED RADIOACTIVITY 



It was in January 1934 that the Curie-Joliots reported that certain 

 targets bombarded by alpha-particles continued to emit positrons for 

 some time after the bombardment ceased. This positron activity 

 was found to decay exponentially with time, just as in the case of the 

 natural radioactive materials. By 1934 nuclear-physics technique 

 was quite well developed so that further studies of this induced radio- 

 activity were undertaken at once by many laboratories. And now, 

 a little over 3 years after the first announcement, over 220 radio- 

 active isotopes are known, including practically every element of the 

 periodic table and some beyond the end of the table. (Radioactive 

 isotopes of elements of atomic number 94, 95, and 96 were once 

 indicated but are ruled out by recent work.) 



The existence of these radioactive isotopes is easily understood. 

 Each element in the periodic table has only a limited number (be- 

 tween 1 and 1 1) of stable isotopes^ which, are found in nature. Thus flu- 

 orine has but one stable isotope, F 19 ; oxygen has three with weights, 16, 

 17, 18; and so on throughout the table, the heavier elements in general 

 tending to have a larger number of stable isotopes. It is easy to see 

 that many nuclear disintegration processes may give rise to isotopes 

 of a particular element which are not found naturally and hence are 

 presumably unstable. Thus one can produce F 17 , F 18 , and F 20 , all of 

 which are unstable. Such atoms can in general convert themselves 

 into stable ones by the ejection of a nuclear particle, and they will 

 do so in the course of time. In most cases this is accomplished by 

 the ejection of a positive or negative electron. 6 These electrons are 

 expelled usually with coasiderable energy and the ionization they 

 produce is readily measured. Each unstable atom has a certain 

 probable lifetime, which is shorter the more unstable its nuclear 

 structure. So, just as in the case of a large population of individuals, 

 the number "dying" per unit time is proportional to the total num- 



» Electrons and positrons do not exist as such in tho nucleus, but they may be created by the conversion 

 of a proton into a neutron and a positron, or by the conversion of a neutron into a proton and an electron, 

 the proper amount of energy being supplied or released. 



