18 



hard thing to believe is the intensity figure they quoted, which is comparatively 

 great. 



ALLEN: There was no radiation which could get through quartz that Miller 

 could detect by scintillation. 



PLATZMAN: Dee and Richards claimed that the amount of energy in the 

 radiation was a substantial fraction of the incident energy. This is the point 

 that is hard to believe. Franck has pointed out a mechanism whereby some 

 energy may be radiated. In the alpha-particle track there is something like a 

 plasma for a brief time, with a great charge density, and such a system is 

 known to radiate in the far ultraviolet. 



HOCHANADEL: According to Spence, Richards, who is now at Harwell, is 

 still working on this problem of light emission and is not yet ready to retract 

 his previously reported observations. 



PLATZMAN: That is fine. I am sure we are glad to hear that. We don't 

 know enough to say that it is wrong; it is just that if somebody comes along with 

 a hair-raising conclusion, the burden of proof is on him. It is not on everybody 

 else. 



FANO: I might describe the state of a few programs at the Bureau of Stand- 

 ards to which Dr. Platzman has referred. One is the study of the absorption 

 coefficient of gamma rays in narrow beams. It is what is usually called the ab- 

 sorption coefficient. The work was undertaken quite a few years ago on a some- 

 what more ambitious basis than the Davisson-Evans work (20), essentially to 

 cover a much wider range. The report on this work by Miss White (19) has 

 been circulated informally and its supply is exhausted. It is planned to put it 

 out in the form of a circular, but some difficulties have to be settled, especial- 

 ly in the low energy range; to be specific, below 100 kv. and especially under 

 10 kv. The situation is that both theory and experiment have an uncertainty of 

 ten or twenty per cent, which is more than I had previously thought. Miss 

 White, after long cogitation, decided to stick to theory which, however, in the 

 present state of art could be only essentially hydrogen-like theory. Good ex- 

 perimentalists, like Parratt at Cornell, to whom I have talked, did not attach 

 more reliance to the experiments than ten or twenty per cent either. Miss 

 White landed ten or twenty per cent lower than the experiments, as, for ex- 

 ample, embodied in the Victoreen semi-empirical formula. The situation six 

 months ago was regarded as open. One could see flaws in the theory, and one 

 could see flaws in the experiments. It would be very desirable and rather dif- 

 ficult to improve the experiments performed some 15 or 20 years ago. At the 

 Bureau we ran only one preliminary test on the absorption coefficient of carbon 

 with copper R-alpha, and that fell near the Victoreen formula. So the best we 

 can say is that White is likely to be low and Victoreen might be better. How 

 far we will have to revise we don't know. 



At higher energies, above 100 kv. the data are much more dependable, to 

 within two or three per cent, and often, especially in light elements, much 

 better than that -- one per cent. 



This covers the narrow beam work. Much of our work has been on attempts 

 to determine by theory the penetration and diffusion of gamma or X rays through 

 large masses of material. The gamma ray problem we regard as essentially 

 finished. But essentially we figure we have it in hand for all the effects where 

 the medium can be regarded as infinite. Surface effects, which effect, of 

 course, the lower energy scattered components particularly, we do not have 



