Sec. 3.13] 



BETA PARTICLES 



67 



This is due mainly to the more pronounced effect of the coulomb field with 

 increasing Z. The orders of magnitude of FT for allowed, first and second 

 forbidden transitions in light, medium, and heavy nuclei are given in Table 

 5 [15]. 



Table 5. Values of the Product FT ix Beta Decay 



Theoretical selection rules for beta decay are based on the form of the 

 matrix element Q which determines the change in state accompanying beta 

 emission. For many light beta emitters, Q ~ 1 since the residual proton 

 remains in nearly the same state as the initially transformed neutron. This 

 is much less likely in heavy nuclei where the level density is much greater. 

 If the change in state is considerable, Q « 1 and the transition is to some 

 degree, first, second, or higher order, forbidden. The principal changes in 

 state to affect the probability of transition are the change in total angular 

 momentum AJ of the nucleus and the parity change of the proper function. 

 The selection rules originally proposed by Fermi are 



Allowed: 



1st forbidden: 



2d forbidden: 



AJ = no parity change 



AJ = 0, ±1, (0 — > forbidden) parity change 



i/= +1,±2, (loO forbidden) no parity change 



A second set of selection rules proposed by Gamow and Teller [17] and 

 based on a different choice of matrix element Q appears to find better support 

 from the experimental data in certain instances. The simplest results of the 

 G-T rules, without reference to the matrix elements, are 



Allowed : A J 



1st forbidden: A J 



0, ±1, (0 -* forbidden) 

 0, ±1, ±2, (0 ->0 forbidden and 

 }/2 — * }/2 forbidden) 

 2d forbidden: AJ = ±2, ±3, (0 *> 2 forbidden) -> 



no parity change 



parity change 

 no parity change 



3.13. K Capture. The inverse process to normal beta decay is the absorp- 

 tion by the nucleus of an orbital electron accompanied by the emission of a 

 neutrino [18]. Once in the nucleus the electron does not retain its intrinsic 

 form but is essential, as in positron emission, to the transformation of a proton 

 to a neutron. The electron absorbed is one of the two K electrons since they 



