51 



MECHANISMS OF ENERGY DEGRADATION AND CHEMICAL CHANGE: 

 EFFECTS OF SECONDARY ELECTRONS 



John L. Magee 



In this conference we are trying to form a picture as concrete as we can of 

 what happens when high energy radiation is absorbed in matter. I am going to 

 be speaking generally of the case of water, just as Dr. Platzman did yesterday. 

 I don't mean to offend anybody who likes to emphasize the importance of other 

 media, but we have to limit the discussion. The general picture is something 

 like this: for fast particles primary events are rather widely spaced. No one 

 would argue that there is any effect transmitted directly from one of these pri- 

 marily affected regions to another. There will be a distance of several thou- 

 sand Angstroms between successive events for a very fast particle, say a half- 

 million-volt electron. On the other hand, for a very low velocity primary par- 

 ticle, such as an alpha particle, primary events are quite closely spaced and it 

 is generally conceded that the effects overlap. So we have essentially these two 

 extreme cases: one in which a column is formed, and one in which there are 

 isolated events. 



Yesterday Dr. Platzman was telling about an attempt to explain what hap- 

 pens to an ionized electron in water. How far does it go? In the way of a de- 

 tailed picture there is only one thing we have had in the past of which I know -- 

 that is, the only people who have been willing to stick their necks out and give a 

 detailed picture were Lea and Gray (1). They gave a detailed model for the 

 distribution of radicals in space, and we talked about this yesterday; namely 

 that the H2O+ is left behind along the columns or at the centers of isolated pri- 

 mary events and that H 2 + dissociates to form H + and OH. Then, of course, 

 there is electron capture which gives an H atom plus an hydroxyl ion. The idea 

 was that this capture took place at some distance away. This distance, the only 

 value that I have seen given frequently, is 150 A, the idea being that there is a 

 sort of Gaussian distribution 



n H (r) OC exp (-r 2 /b 2 ) 



o 



and the parameter b is about 150 A. Here njj(r) is the number of H atoms 

 formed per unit volume at the distance r from the center of the affected region. 



e 



Essentially the electron gets away to a distance of the order of 150 A. This 

 is the thing that Dr. Platzman was reaching for yesterday, except that he over- 

 stretched, according to his own statements, and wanted the electron to go fur- 

 ther. 



BURTON: Let us get this straight. I thought Dr. Platzman was talking 

 about electrons which came out with about ten volts of energy, and you are 

 talking about electrons which come from the primary column. 



MAGEE: All electrons quickly get down to the ten volt region. Dr. Platz- 

 man was talking about the mechanism which gets the slow electrons away. 



