22 



ENERGY TRANSFER FROM SECONDARY ELECTRONS TO MATTER 



Robert L. Platzman 



I should like to devote the opening portion of this second session to a presen- 

 tation of some speculations with which I have been concerned recently, very 

 closely related to the subject originally assigned for this part of the conference 

 and, indeed, falling somewhere between the second and third topic -- probably 

 right in the middle. 



Those of you who were at Oberlin will perhaps recall that at that time I ex- 

 pressed the idea that those who devoted consideration to problems involving so- 

 called basic mechanisms in radiobiology, very definitely including myself, were 

 working in what might well be called a fool's paradise. This afternoon I should 

 like to lead you briefly into the nether world, in an attempt to be realistic about 

 some of these problems, for it is in the nether world, you will recall, that most 

 of the intellectual treasures are to be found. 



I wish to make it quite clear that the path along which I shall lead you is not 

 likely to be -- indeed, is probably not the correct one. But of one thing I am 

 quite certain, and that is that some of the scenery I shall point out to you is 

 authentic. 



When high-energy radiation* interacts with matter, the sequence of events 

 which ensue may be divided into three general stages. 



First there are produced a number of excited atoms or molecules, positive 

 ions, and swiftly moving electrons. The number of electrons multiplies for a 

 brief time, their energy, on the average, decreasing, and at the end of what I 

 shall call the first stage there are present in the medium a certain number of 

 excited atoms and molecules of various types, a certain number of positive ions 

 of various types, and a number (equal to the number of positive ions) of elec- 

 trons, all having energy lower than the lowest electronic excitation potential of 

 the medium. The energy distribution of the electrons at this stage is still 

 largely unknown; most of the electrons have an energy of the order of 1 to 5 

 electron volts. 



During this first stage the interaction of the radiation -- primary and sec- 

 ondary --is dominantly with electronic systems of atoms and molecules, and 

 at the end of this stage comparatively little of the absorbed energy has been 

 converted into heat. 



The second stage I shall call that in which the electronic kinetic energy is 

 further degraded to thermal energy, and during this period, since the electrons 

 can no longer communicate energy to electronic systems, because of the energy 

 requirement, their interactions must be with atomic motions of various types. 

 Hence, all of this energy loss goes into heat (1). 



Finally, at the end of the second stage, we are where chemistry can take 



