at Chicago during the past few years. For instance, one of their most recent 

 papers (13) treats the phenomena of capture and loss of orbital electrons by 

 helium ions as they pass through various gases. In previous years several 

 similar studies for hydrogen ions were completed (14), (15), (16). This group 

 of investigations really appears to polish off the experimental problem --a 

 problem, of about forty years' standing, the literature of which had been marked 

 by a staggering amount of discordance. We now have at least empirical knowl- 

 edge of all of the main features of the phenomena which have been studied. It is 

 truly beautiful work. 



The fourth topic --and we are gradually moving, with this point, at least, 

 to items of more immediate interest in radiobiology -- I have arbitrarily called 

 the phenomena of low energy: the behavior, in passing through matter, of par- 

 ticles, the velocity of which is not great enough to permit the application of 

 simple theories -- e.g. , the Born approximation. Here we have what I con- 

 sider to be one of the two major advances in the experimental investigation of 

 penetration phenomena of interest to radiobiology. This is the work carried 

 out at the California Institute of Technology with Lauritsen's electrostatic gen- 

 erators: the study of the loss of energy of beams of hydrogen ions passing 

 through gases at low pressure. So far there has not been a detailed publication 

 but one will appear shortly (17). An abstract of a paper presented at a meeting 

 of the American Physical Society has already been published, and I have some 

 of the data here. 



The experiments were performed by Reynolds, Dunbar, Wenzel, and 

 Whaling, and involved measurement of the stopping power for protons of energy 

 between 0.6 Mev and 0.03 Mev in a remarkable variety of gases. In this low- 

 energy region, precise stopping- power data were previously virtually unavail- 

 able. These workers found, for instance, that the Bragg rule, about which we 

 all have heard a great deal of discussion, is satisfied, within an uncertainty of 

 several per cent, for proton energies between 600 and 150 kev, with the impor- 

 tant exception of the case of NO, a notorious case because the molecule con- 

 tains an odd electron. Here, just as anticipated from theoretical considera- 

 tions, there is a departure from the Bragg-rule prediction of about 4 per cent. 

 For energies below 150 kev, all compounds show departures from the predic- 

 tions of the Bragg rule; these may be as great as 10 per cent. This energy 

 region is quite obviously of great importance in radiobiology. It is extremely 

 important, for instance, for experiments on the effects of "fast" neutrons, 

 which manifest themselves in tissue chiefly through proton recoils. 



Similar experiments, in substantially the same energy region, for just a 

 few gases but with several different kinds of penetrating ions, have been car- 

 ried out in Chicago (18). It is highly satisfying that, for protons in air, the 

 results of the two laboratories are in splendid agreement. Several other in- 

 vestigators have also studied these phenomena recently. 



Although the primary absorption of electromagnetic radiation is of no 

 direct fundamental interest to us in the present conference, I may just mention 

 the enormous practical help provided by the recent compilations of X ray and 

 gamma ray absorption coefficients by White, at the National Bureau of Stand- 

 ards (19), and by Davisson and Evans, at M.I.T. (20). The latter is already 

 published, and the former will be shortly, I understand. These studies con- 

 stitute a remarkable improvement in the status of aggregate information on 

 this topic. In addition, Dr. Fano's attention to the problems of diffusion and 

 attenuation of gamma ray beams continues. 



I have, finally, a number of miscellaneous topics. Because the physics of 



