106 PRIMARY PROCESSES 



tinction, which fact — together with the small total energy loss by this 

 mechanism — will make the process unimportant in the interpretation of 

 most chemical and biological effects. 



A possible exception to the last conclusion might arise in the case of 

 irradiation with rather slow neutrons, for which the recoil ions will often 

 have velocities in the capture-loss region. Nevertheless even here the 

 spatial distribution of ions will not be very abnormal, because just for 

 those particle velocities for which capture and loss predominates, the 

 cross sections for the two processes approximate geometrical cross 

 sections, so that positive ion and electron originate at almost the same 

 place. The total ionization in this velocity region, however, may differ 

 considerably from that anticipated on the basis of extrapolation of 

 knowledge for high velocities (the partition of the total energy loss 

 between excitation and ionization may be quite different at low from 

 that at high energies, and nuclear collisions have an important effect). 

 There is as yet very little information on this question, and the pos- 

 sibility of peculiar effects should not be discounted. 



Auger Disruptions 



Although most of the energy lost by high-energy radiation is trans- 

 ferred to valence electrons of molecules of the medium, a not inappreci- 

 able portion is absorbed by inner electrons of the atoms. The remark- 

 able effects to be anticipated for this part of the energy loss apparently 

 have not been pointed out before. 



The media of importance in radiobiology, and in much of radiation 

 chemistry as well, are composed almost entirely of two types of atoms: 

 hydrogen, having only a valence electron, and carbon, nitrogen, and 

 oxygen, which have inner (K) electrons in addition to the outer ones. 

 (Presence in the medium of small amounts of heavier atoms will not 

 affect any of the conclusions to be drawn.) An important and repre- 

 sentative medium is water, and, as an example, Fig. 2 illustrates the 

 total fraction of the energy of penetrating protons which is transferred 

 in primary collisions to the K electrons of oxygen atoms when the pro- 

 tons are completely absorbed in water. (Figure 1 shows the manner in 

 which this portion of the energy loss is distributed along the range of 

 the protons.) The fraction, although small, is by no means negligible. 

 In the case cited, for example, it is 4 per cent at 3 Mev and increases 

 with the proton energy. Transfer of energy to a K electron almost in- 

 variably ejects that electron from its atom, an ion with an inner vacancy 

 thus being formed. The average energy transferred in such processes 

 is several times the K ionization energy (which is 531 ev for oxygen) 



