22 SECONDARY ELECTRONS 



loss of the recoil oxygen, nitrogen, and carbon atoms of tissue is negligible. How- 

 ever, in some cases, such as with epithermal neutrons, this may not be true. 



Fang : 



I think that in the epithermal region the secondary reactions are the most 

 important. 



Failla : 



There may be a very narrow region in which the effect discussed by Morgan 

 would be an important factor. I think that, in general, for the very high-energy 

 neutrons most of the energy is transferred to the protons. For very low energies, 

 nuclear reactions which give off radiation probably will set the limit. However, 

 perhaps there is some narrow region in which the effect under discussion might 

 be an important factor. 



Loevinger: 



Morrison has pointed out that ionization may well not be the main mechanism 

 by which radiation produces a biological effect. Yet all dosage computations 

 are based on ionization measurements in air. One uses the average energy per 

 ion pair in air and the relative stopping power to compute energy absorbed by 

 the tissue or organism. Thus, there is the implicit assumption that the biolog- 

 ically important events in tissue are proportional to the ionization in air. Is, 

 then, ionization in air to be considered a satisfactory physical quantity to meas- 

 ure for dosage purposes, or is there hope of finding a better physical quantity to 

 measure for these purposes? 



MORRISGN : 



As I understand it, that was a point of Fano's discussions several years ago 

 which was re-emphasized here; it was just the special property of ionization by 

 glancing collisions in gases that gives a good proportionality between energy 

 lost, the rep, and its measure in the gaseous ionization chamber. I think that 

 it does mean, however, that if we use the very useful parameter of the rep to 

 represent the energy-density distribution for various kinds of radiation, we must 

 expect that 1000 rep may produce very different effects in different biological 

 systems. However, I think that the rep is a very convenient physical unit, 

 primarily because of the excitation-ionization relationship which Fano just 

 showed. 



Failla: 



The way in which the absolute energy is calculated by ionization measure- 

 ments involves the total energy. Since the number of ion pairs is divided by the 

 total energy of the particle, the average value per ion pair is for the total energy 

 lost and not the energy to produce the ion pair alone. Thus the excitation 

 energy and energy lost by other means are averaged into the energy associated 

 with the production of each ion pair. 



