108 PRIMARY PROCESSES 



the reabsorption of the Auger electron (s) as distinct events: these proc- 

 esses may in fact be coupled together to some extent, at least for that 

 portion of the energy reabsorbed by an adjacent atom. In this connec- 

 tion it is of interest to note that an Auger electron of 100-ev energy has 

 a speed of about 10^ cm per sec, and therefore is "reabsorbed" by the 

 molecule in a time of the order of 10""^^ sec or less. This is very much 

 shorter than the time required for molecular rearrangement. Regretta- 

 bly little is known about Auger transitions in very light atoms, experi- 

 mental investigation being hampered by extreme difficulties of a practi- 

 cal nature. The structural changes consequent to Auger effects in light 

 atoms bound in molecules can, however, be investigated more readily. 

 Such studies have only recently been initiated. They should ultimately 

 provide much information of interest.) 



Although the exact series of mechanisms cannot be mapped with cer- 

 tainty, there can be no doubt that, wherever a K ionization occurs, a 

 relatively great amount of energy is communicated to a small region of 

 space — perhaps a single molecule or a portion of a very large molecule. 

 The "primary process" embraces the effects of the initially ejected K 

 electron, of the Auger electron (s), and of the multiple ionization, all 

 centering at the atom originally affected. This energy, transferred in a 

 single primary encounter, will soon be converted to molecular jwtential 

 energy by electronic "rearrangements" (including internal conversion) 

 and will then usually shatter the molecule by a complex polyatomic dis- 

 sociation, the latter occurring rather slowly, that is, not as a direct pri- 

 mary effect. (Indeed, the process could result in the disordering of a 

 solid — even by electron bombardment, for which the amount of dislo- 

 cation by direct momentum transfer is very small.) The concentration 

 of absorbed energy will in fact exceed that arising on the average from 

 direct energy transfer to electrons in the penetration of the medium by 

 a beta or gamma ray, and may even exceed that sustained in penetra- 

 tion by radiation of such high specific ionization as an alpha particle. 

 Its possible importance for effects not induced by small energy transfer 

 is therefore impressive. Such effects are customarily ascribed exclusively 

 to delta rays, which transfer relatively much energy to a small volume. 

 The relative importance of the two mechanisms cannot be elucidated 

 without a much more detailed analysis; however, an approach to this 

 analysis could readily be based on a simplified model, and such a study 

 would doubtless prove most interesting. It is evident that the relative 

 contribution differs for different types of irradiation and different media. 

 The mechanisms are intrinsically distinct from the physical view: in one 

 case energy is transferred only as direct momentum transfer to an elec- 

 tron; in the other, it is transferred to the electronic system of a single 



