34 RADIATION BIOLOGY 



results when the oscillating currents in various portions of a nucleus or of 

 an atom neutralize one another's radiative effectiveness.) 



By and large, the excitation of an atomic particle does not become 

 easily subdivided, or "degraded," within a single atom or molecule. The 

 excitation of a nuclear particle spreads easily to the other nuclear particles 

 but remains within the nucleus. The excitation of an electron does not 

 even spread easily to other electrons. 



This circumstance, which simplifies the analysis of excitation, relates to 

 the organization of the atomic structure in successive levels or layers. 

 These layers have decreasing rigidity and remain fairly independent of 

 one another according to the following general scheme: 



(a) The nucleus is most rigid and its excitation requires a larger amount 

 of energy. Therefore the intranuclear motions are hardly affected by 

 the extranuclear phenomena. 



(b) The structure of internal electrons has next higher rigidity. It is 

 affected very little by intranuclear phenomena owing to the smallness of 

 the nucleus, and hardly at all by the motion of external electrons which 

 involves small amounts of energy. 



(c) Similarly, the motion of external electrons is affected very little by 

 the events within the nucleus or within the small hard core of internal 

 electrons or by the weaker action of the molecular motion. 



(d) The makeup and the vibrations of molecular structures depend 

 upon the motion of the external electrons, but only upon the average 

 pattern of motion rather than upon the location and velocity of each 

 electron at any particular instant. The influence of the electronic motion 

 upon the attachment and the motion of whole atoms may be described 

 comprehensively as the action of chemical forces exerted by each atom on 

 the adjacent ones. 



The remarkable independence of different layers of the atomic structure does 

 not imply that transmission of energy from an inner layer to a more external one 

 is extremely rare. However, when this transmission occurs, the energy received 

 by the outer layer is quite excessive with respect to the bonds that hold particles 

 within that layer. Therefore the transmission results in the immediate ejection 

 of one particle which carries away from the atom most of the excess energy. 



Nuclei sometimes transfer their energy directly to atomic electrons and thereby 

 eject them from the atom. This phenomenon, called "internal conversion," 

 occurs particularly when the nuclear excitation is small. 



Similarly, a rearrangement of internal electrons may lead to the ejection of an 

 outer electron. This phenomenon, called the "Auger effect," is also the more 

 frequent the lower the energy amount to be transferred. 



Some energy is frequently transferred from an outer electron of a molecule to 

 the interatomic motion at the time the electron radiates a photon. This is to 

 say that the photon gets only a part — though normally a very large part — of the 

 energy released by the electron. Another mechanism is a "radiationless transi- 



