ISOTOPIC TRACERS AND NUCLEAR RADIATIONS 



[Chap. 1 



nuclei of the same A varies as Z 2 and is represented therefore by a parabola 

 about the most stable value of Z located at or near the apex. 



For odd A , Z may be even or odd with equal probability as shown by the 

 nearly equal numbers of these stable nuclei. The only isobar that is stable 



-4 -3 -2 -I Z +1 + 2 +3 +4 



Fig. 1. Schematic diagram representing the cross section of the nuclear energy surface 

 at odd and even isobars, (a) Cross section at odd nuclei. Only the nucleus at Z is stable, 

 all others move toward this value of Z (and N) by /3 _ , /3 + , and K transformation, (b) 

 Cross section at even nuclei. Nuclei at Z + 1 and Z — 1 are stable since transition from 

 one to the other is energetically impossible. All other nuclei are unstable, as indicated 

 by arrows. 



is the one lying nearest the apex of the parabola. All other isobars are 

 unstable and move toward the apex either by K capture or positron or nega- 

 tron emission until the stable Z is reached, as shown in Fig. 1. Stated 

 alternatively, isobars with neutrons in energy states higher than the next 



