PRINCIPLES OF RADIOLOGICAL PHYSICS 



35 



tion" in which an outer electron changes its motion and transfers the excess 

 energy directly to the interatomic motion. This process occurs more frequently 

 in larger atomic aggregates than in smaller ones. It constitutes nearly the sole 

 channel for the conversion of electronic energy into interatomic, i.e., thermal, 

 agitation. 



2-ld. Nuclear Disintegrations. The neutrons and protons which con- 

 stitute atomic nuclei tend to escape, singly or even in small aggregates, 

 whenever a nucleus is excited by a sufficiently large amount of energy. 



When the substitution of a neutron with a proton or of a proton with a neutron 

 yields an energetically more stable nuclear aggregate, this substitution takes 

 place spontaneously through the process of /3 disintegration (see Sect. 1-lb). 



When different modes of disintegration are possible, each of them may 

 take place in a nucleus with a characteristic probability per unit time. 

 If the rates are comparable, part of the nuclei disintegrate in one way 

 and part in another. For example: 65 per cent of the nuclei of Bi'-^^ 

 (commonly called thorium C) 

 experience a ^ disintegration to 

 Po2i2 (Th C), whereas the re- 

 maining 35 per cent disintegrate 

 to TP«« (Th C") by a-particle 

 emission. 



_] 

 O 



2 X 



< o 



CENTER EDGE 

 OF NUCLEUS 



Fig. 1-23. Diagram of the electric barrier 

 effect around a nucleus. 



uj O 



As mentioned before, neutrons be- o o 

 come attached to nuclei more easily ^ 



than protons or other positively o 



charged particles because the ap- 

 proach of a neutron is not hindered 

 by electric repulsion. Similarly, the 

 ejection of neutrons is favored over 

 the ejection of charged particles. All 

 around a nucleus there is a zone in which a positively charged particle experiences 

 a strong electric repulsion but is beyond the reach of attraction of nuclear forces 

 (see Fig. 1-23). The particle finds it difficult to penetrate this zone, whether it is 

 on its way in or out of the nucleus. Therefore the zone is called the "potential 

 barrier" of the nucleus. It may be said that the capture or the ejection of a 

 charged particle requires that a certain activation energy be available to traverse 

 the potential barrier. The range of "activation energy" is approximately 1-13 

 Mev for the capture or ejection of a proton, depending on the atomic number of 

 the nucleus. 



Aside from the barrier effect, the ejection of an a particle generally requires the 

 least amount of excitation energy because the two neutrons and two protons 

 which form an a particle become attached to each other far more strongly than to 

 the rest of the nucleus. Therefore the ejection of a particles results quite fre- 

 quently from the excitation of light nuclei, in which the barrier effect is small. 



Nuclear disintegration by the emission of an a particle does not require any 



