CHAPTER 7 



RADIOACTIVITY 



7.1. Summary of Radioactivity. Radioactivity refers only to those proc- 

 esses by which unstable nuclei decay by loss of their excitation energy to 

 form known stable nuclear species. These processes are distinct from the 

 primary processes of nuclear interaction in which unstable or excited nuclei 

 are formed. Radioactive decay proceeds at a rate proportional only to the 

 number of unstable nuclei present and is wholly independent of external 

 influences, whereas the primary process of formation depends upon the type 

 and energy of the bombarding radiation. In addition, the primary process of 

 formation of an excited nucleus takes place in a time of the order of 10 -10 

 sec or less, while the probable time for decay of an unstable nucleus is enor- 

 mously longer and varies over the range from 10 -7 sec (RaD) to 4.4 X 10 17 

 sec (thorium). 



All known unstable nuclei decay only by emission of one or more of the 

 following radiations: negatron, positron, neutrino, gamma ray, alpha particle, 

 and neutron. In general, decay proceeds by several discrete energy steps or 

 transitions to lower nuclear quantum levels until the ground state of a stable 

 nuclear species is reached. In most cases this involves the emission of a 

 charged particle and one or more gamma rays. In many instances, particu- 

 larly in medium and heavy nuclei, decay may occur by one of several alter- 

 native processes involving different sets of quantum levels and sometimes 

 different radiations. The probabilities for the alternative processes is given 

 directly by the observed fractions of different radiations when the decay 

 scheme is known. 



A summary of the processes that are known to occur in radioactive decay 

 is given below. More detailed discussions will be found in those sections 

 describing the specific radiations. 



a. Negatran Emission (Beta Decay). A beta particle and neutrino are 

 emitted simultaneously and share in any proportion the total fixed energy 

 corresponding to a discrete level transition in the nucleus. The observed 

 beta kinetic energy varies from zero to a well-defined maximum corresponding 

 to the total transition energy. The residual or daughter nucleus is greater in 

 charge by one electronic unit, and the exact atomic weight is diminished by 

 only the mass equivalence of the maximum kinetic energy of the beta particle 

 emitted. The statistics of the nucleus remain unchanged, but the nuclear 

 spin is altered by an integral multiple of h/2ir. 



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