INITIAL ENERGY TRANSFER FROM INCIDENT RADIATION TO MATTER 



Robert L. Platzman 



For this first session I have comparatively little to say. And the word "in- 

 formal" in the title of this Conference will be taken very literally --in my ad- 

 herence to the topic of this morning, at least. 



This topic is the one about which we know more, I think, then about any oth- 

 er of the topics listed, and it is the one, the vagaries of which cause us the 

 least concern; therefore, it is actually of somewhat less urgency to us at the 

 moment. Since we have very little time this morning, I propose to give just a 

 brief resume of the general progress, which is likely to be of interest in radio- 

 biology, in the general field of study of the penetration of high- energy particles 

 through matter, and allow some time for those of you who have items of special 

 interest to introduce to do so. This, then, will be an attempt to list a few of 

 the very high points, with only a few moments devoted to each item. I shall 

 cover, approximately the past three years -- that is, the period which has 

 elapsed since the Oberlin Symposium, when many of these topics were re- 

 viewed from the standpoint of radiobiology. It may be worth mentioning that 

 such a procedure automatically omits topics of great importance which are 

 poorly understood, if they have not been studied recently. An example is the 

 vital problem of the energy distribution of secondary electrons. 



First I might mention the experimental facilities that have recently become 

 available. The most sensational new facility is the high-energy cyclotron 

 ("synchro-" or "FM" cyclotron), which has given us, for example, protons of 

 energy up to four hundred Mev. And potentially the most important, in my 

 opinion, is the facility which provides intense sources of ions of high energy- - 

 heavy ions bearing high charge. The two places, of which I am aware, that 

 have such sources now are Birmingham, England, and Berkeley, California. 

 From both of them there have recently appeared brief notes (1), (2) giving an 

 idea of the type of ions accelerated, the currents available, and the sort of 

 experiments performed thus far, or contemplated. I think that this work holds 

 promise of providing extremely helpful information. The work with the high- 

 energy cyclotron is perhaps somewhat less interesting to us, although of the 

 greatest importance in physics, of course. 



In addition we now have a variety of sources of electrons of extremely great 

 energy. An importance of these sources to fundamental radiobiology is, how- 

 ever, rather unlikely. 



Next let us turn to the progress in understanding of the basic parameter 

 measuring penetration -- the stopping power. I have about a half-dozen items 

 listed under this. In theory, in the first place, there has been some progress 

 made by the major contributor to this field, Bethe, and a few of his stu- 

 dents, chiefly Walske (3), (4), (5), (6). They continue to interpret stopping 



