power essentially in terms of the individual electrons of the atom. Work in 

 this field has recently become less active because, paradoxically enough, in- 

 terest in the subject has slackened. 



A very interesting paper by Lindhard and Scharff (7) in Copenhagen will ap- 

 pear shortly. It presents a rather different approach to the theory of the stop- 

 ping power, departing from that adopted by Bethe and his school and consider- 

 ing not individual electrons, but the electrons in the atom as a whole, using a 

 Thomas-Fermi model with appropriate modification where demanded. This 

 approach, I think, should be quite useful in some areas, although in radiobiolo- 

 gy, because of the peculiarity that we are concerned, chiefly, with atoms of 

 low atomic number, it probably will not be very relevant. Another analysis of 

 conventional stopping- power theory has been published by Neufeld (8). 



There has been little experimental work in the last few years on stopping 

 power at high energies. Since the first burst of enthusiasm over the high- 

 energy protons at Berkeley (cf. the work of Bakker and Segre (9) and Mather 

 and Segre (10) ) very little, as far as I know, has beer done with them, in the 

 present connection. I think that the reason can be attributed to the shift of in- 

 terest among physicists from penetration of radiations through foils and gases, 

 the old-fashioned type of absorber, to the one which is of chief interest at the 

 present time -- and a nasty one it is -- namely, the photographic emulsion. 

 Thus it appears that physicists are experiencing some of the difficulties that 

 radiobiologists have long known, in working with a highly complex medium. 



A good deal of experimental work has been done on the effect of the nature 

 of the medium on stopping power at relativistic velocity (Fermi effect). I might 

 mention in concluding this section that the problem of a possible great anomaly 

 in the stopping power of liquid water is apparently still unresolved. 



FANO: Do you know about Thompson's work (11) on the effect of chemical 

 binding on stopping power? 



PLATZMAN: I have heard of this work, but have not yet seen the results. 



TOBIAS: I have a copy of the report here. 



PLATZMAN: When I have finished, perhaps you would give us a brief 

 summary of it. This work is at high energy, is it not? 



TOBIAS: Yes. 



PLATZMAN: The third topic which I shall discuss is the process of charge 

 exchange in the impact of two atomic or molecular systems. In recent years 

 a good deal of experimental work, and also some theory, have been devoted to 

 this topic. Much of this was summarized at the symposium which took place 

 at the University of Notre Dame in June, 1952 and has now been published in the 

 Journal of Physical Chemistry (12). In addition, there have appeared numerous 

 papers distributed through the journals, chiefly of physics. 



This is a subject which is not yet of great applicability in radiobiology, but 

 I venture to predict that in years to come, when we know more about the basic 

 processes, we shall have much use for this information. The physical problem 

 is very difficult, both experimentally, where there are any number of compli- 

 cations, and theoretically, where the problem is extraordinarily difficult. Still 

 a good deal of progress has been made, and as chief among the contributions I 

 might mention the work accomplished by Allison and a number of his students 



