DISCUSSION 53 



we might set the effectiveness of the protons as 4 times that of the gamma rays. 

 Accordingly, this factor approximately cancels factor (a). 



(c) The relative fractions of the emitted energy absorbed by the biological object. 

 Since the range of the proton is of the order of 10 microns, the fraction of its 

 energy absorbed is practically 100 per cent even in very small objects. On the 

 other hand, the fraction of gamma-ray energy absorbed depends greatly on the 

 size of the object and is quite low even for an object 1 cm in diameter. 



Accordingly, the relative biological action due to the two capture reactions 

 is the product of the relative number of neutrons absorbed (8 in H to 1 in N) 

 times the fraction of the gamma energy absorbed in the particular biological 

 object. This product, even for an object as large as a mouse, indicates that the 

 N capture predominates radiobiologically over the H capture. In larger objects, 

 of course, the H reaction increases in importance because of increase in the 

 fraction of gamma energy absorbed. 



Kamen : 



The analysis of beta recoil processes based on the simple picture of an isolated 

 nucleus is not adequate for complex molecules. The main difficulty is ignorance 

 about what it is that recoils: thus, a P^^ atom in a nucleoprotein is linked in a 

 variety of ways through oxygen bridges to organic moieties, and it is not certain 

 whether only one or two oxygen atomSj or also a portion of the nucleotide and 

 protein, recoils along with the residual S atom. Thus, the mass effective in the 

 recoil energy is unknown. It must also be noted that the recoiling S atom is 

 imbedded in an atomic matrix with innumerable degrees of freedom, so that by 

 a collision of the second kind, or by internal conversion, a large amount of the 

 initial recoil energy can be degraded or transferred through a large portion of the 

 protein, involving a general excitation of the whole molecule. Another diffi- 

 culty, namely uncertainty about the neutrino distribution, has been mentioned 

 bj' Solomon. 



Perhaps an adequate analysis is not available at the present time. Never- 

 theless, it is badly needed because, as will be pointed out in the panel on bio- 

 chemical processes, data are available which would permit conclusions about 

 the radiosensitivity of specific sites in biologically important molecules to be 

 drawn, provided such an analysis were possible. 



Magee : 



Two questions have particularly bothered me in connection with "hot" atom 

 effects following the (n, 7) process, such as are being studied in our Radiation 

 Chemistry Laboratory at Notre Dame by Hamill and Williams. 



(a) Is the gamma energy given off in one quantum or several? This bears 

 directly on the energy given the recoiling atom, since the resulting momentum 

 of several quanta will result in partial cancellation. The assumption has ap- 

 parently been made in Solomon's paper that the energy is given off in one 

 quantum in all cases. 



