19 



well in hand. There are some beginnings. For instance, just to quote one 

 figure, it turned out that if a million-volt photon strikes a water surface, the 

 probability of its coming back is of the order of one-third. This is something 

 about which, until three months ago, we had absolutely no idea whatsoever. 

 It might have been ten or it might have been eighty per cent. 



MAGEE: Is this backscattering essentially the result of the first Compton re- 

 coil ? 



FANO: I mean the probability of the photon going around in the water and 

 eventually coming back out. 



PLATZMAN: It is for a thick absorber. 



MAGEE: This includes all backscattering to get out? 



FANO: Yes. Many photons come out with low energy; on an energy basis, 

 the average energy fraction coming out is not one- third, but only five or six 

 percent. This is very preliminary. 



MAGEE: Has each photon been scattered about three or four times? 



FANO: Very often more. A few photons, one or two times. Many even ten 

 times. 



Regarding the penetration and diffusion of electrons, we are breaking the 

 problem down into a number of partial problems and making some progress. 

 It is the sort of thing that ought eventually to be of very considerable interest 

 to radiobiologists because the problem of where the energy of the electrons is 

 dissipated is a fairly wide open question, and we are going at it systematically. 

 We don't have anything very striking to report. The main thing is to point out 

 that this work is in progress. 



PLATZMAN: There has been some new theoretical work on the photoelec- 

 tric effect by Meyerott (32). Do you know about that? 



FANO: On absorption coefficients? Yes, that was reported the other day 

 in Washington. 



PLATZMAN: I went over some of this ground with him, and it is shocking 

 how little experimental material there is with which to compare it. For .in- 

 stance, he calculates a partial cross section for different electron shells at a 

 wavelength at which several shells contribute to photoelectric absorption, and 

 there have been absolutely no experiments that we could find that give the sep- 

 arate contributions. Again that is something that would not be hard to measure. 



BURTON: If there is no other discussion of this point, I should like to re- 

 turn to what Dr. Boag had to say about water and what we were all talking 

 about, particularly what Dr. Kasha was saying. 



In a complicated organic system excitation to an energy state sufficiently 

 high for dissociation is not necessarily followed by an observable chemical 

 change. Various aspects of the Franck-Rabinowitch effects are involved. For 

 example, a relatively small amount of energy may be tapped, leaving the mole- 

 cule in a highly excited state which may persist for a long time prior to disso- 

 ciation, for example by a rearrangement process. In that time, complete de- 

 activation or many other competing processes might occur. That situation 

 simply does not exist in water. 



KASHA: Why doesn't it exist? 



