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ANNUAL, REPORT SMITHSONIAN INSTITUTION, 1938 



ber "living." And if none were being born the total population would 

 decrease exponentially with time. The period, or half-life, is the 

 time required for the number living (and also the death rate) to de- 

 crease to half its original value. This is just the law which governs 

 the radioactivity of the natural radioelements, such as radium. 



As an example, if a C 12 nucleus should capture a proton the result- 

 ing nucleus would be N 13 , the charge and mass both being increased 

 by 1. But N 13 is not among the stable isotopes of nitrogen, though 

 it could convert itself into the stable C 13 by emitting a positron. 

 This is what happens and it is found that N 13 has a half-life of about 

 11 minutes. That is, if a large number of N 13 atoms are formed at 

 a given time the number of positrons emitted per second by the whole 

 group falls to half its value in each succeeding 11 minute interval. 

 It is in general possible by some nuclear process to produce from one 

 to six or more radioactive isotopes of every element in the periodic 

 table (except hydrogen), and in many cases a particular isotope may 

 be produced by several different nuclear reactions. As an example 

 the radioactive Na 24 with a period of 14.8 hours may be produced by 

 bombarding the stable Na 23 by either neutrons or deuterons, by 

 bombarding Al by neutrons, or by bombarding Mg by neutrons or 

 deuterons; five reactions in all. The properties of Na 24 are the same 

 no matter how it is produced. On decay the Na 24 atom emits a 

 negative electron and becomes stable Mg 24 . 



A few of the radioactive isotopes of some of the elements which 

 occur commonly in biological materials are listed in table I. The 

 last column indicates how the isotopes are produced. For example 

 the notation B-<2-n opposite C u indicates the bombardment of boron 

 (B) by deuterons (d) with the ejection of a neutron (n), the reaction 

 being 



B 10 +d 2 -^C u +n\ 



Table I. — Some typical radioactive isotopes. 1 



1 For a complete tabulation, see Livingston and Bethe, Rev. Mod. Phys., vol. 9, p. 359, 1937. 



