104 PRIMARY PROCESSES 



too large molecule is of the order of 10~^^ sec, while roughly 10~^^ sec 

 is required for significant internal reorganization of atoms in a molecule, 

 such multiple excitations are also effectively "simultaneous." This dis- 

 tinction, although not always well defined, is a useful one. Either type 

 of multiple excitation could be important if more energy than is provided 

 by a single excitation is required to disrupt the molecule.) 



There does exist some empirical information on multiple ionization 

 by impacts of electrons of low and intermediate speeds, almost all of it 

 for monatomic gases. (Slowly moving charged particles are known, on 

 general grounds, to be much more effective than rapidly moving parti- 

 cles in producing multiple processes, the yield of the latter being weighed 

 relative to that of single processes.) The results on the noble gases, 

 which are the simplest to interpret, indicate a yield for the production 

 of an (n + 1) positively charged ion roughly one-tenth that of the cor- 

 responding -{-n ion for electron energies from several times the ioniza- 

 tion potentials up to 500 ev, the upper limit of the experiments. (For 

 helium the relative yield of He"^"*" to He"*" is much smaller — of the order 

 of 10~^.) The yields of multiply charged ions are, however, very much 

 lower at smaller energies (and are, of course, zero below the respective 

 ionization potentials), and this is the region in which most of the second- 

 ary electrons ejected by high-energy particles fall. For the few other 

 gases investigated, the relative yields, although not exactly similar, are 

 at least of the same order of magnitude. Thus the over-all yield of 

 initially doubly charged ions produced by high-energy radiation would 

 be expected to be very small — less than 10~^ and perhaps less than 10~^ 

 of all ions. The contribution of the primary ionization should not alter 

 this conclusion, although experimental information on the question is 

 unsatisfactory and is in fact entirely lacking for beta or gamma radi- 

 ation. Experiments performed some 30 years ago failed to detect multi- 

 ple ions, either primary or secondary, in yields greater than about 1 

 per cent, produced by alpha particles in a variety of gases. (Some re- 

 sults indicating 5-15 per cent yield of He"*""^ relative to He"'' in helium 

 gas, by alpha particles at various speeds, attracted much attention at 

 the time. They have been interpreted as proving that in helium half of 

 the primary ionization acts near the maximum of the Bragg curve create 

 double ions. If true these results are remarkable and merit further 

 study. They are not relevant in the present considerations, however, 

 and the evidence on other gases is clear.) 



Virtually nothing is known about multiple processes in molecular sys- 

 tems of chemical or biological importance. A conservative estimate 

 suggests 10~^ as an upper limit for the relative yield of doubly ionized 

 atoms or small molecules produced directly in single events by most 



