BASIC RADIATION BIOCHEMISTRY 271 



BIOCHEMICAL EFFECTS OF ALPHA RADIATION 

 COMPARED WITH THOSE OF X RADIATION 



So far only X-radiation effects have been considered. It is of special 

 interest to compare the effect of radiations differing widely in the density 

 of ionization occurring along their tracks. There are only a few papers 

 dealing with this question in isolated systems, in contrast with a much 

 greater number of investigations on the effects of a and neutron radiation 

 on biological subjects generally, i.e., on growth, mitosis, structural 

 changes in cell nuclei, mutations, etc. A full treatment of this aspect 

 will be found in other chapters of this book. 



It is found for nearly all reactions that a radiation in aqueous solutions 

 is less efficient than X radiation in causing chemical change and especially 

 so for organic solutes. Nurnberger (1937) has examined the oxidation 

 of tyrosine by a radiation from radon and found the very low ionic yield 

 of 0.003. Even the small effect observed could have been due to /3 rays 

 for which no allowance was made in his calculations. Dale, Gray, and 

 Meredith (1949) have compared the efficiencies of a- and 500-kv X radia- 

 tion over a range of concentrations of the enzyme carboxypeptidase. 

 Radon was the source of the a particles, and special attention was given 

 to the effects of the accompanying /3 rays. Again, the results show that 

 a radiation is very much less efficient than X radiation, but that the effi- 

 ciency of a radiation is slightly dependent on the concentration of enzyme, 

 increasing fourfold as the concentration of enzyme is raised from 6 X 10"^ 

 to 6 X 10~^ g/ml. The increase in efficiency is not proportional to 

 concentration. 



The explanation for this difference in relative efficiency of the two radia- 

 tions is thought to lie in their very different ionization densities, a rays 

 producing an ion density 100 times greater than do 60-kev electrons. 

 (Secondary electrons produced by X rays generated at 500 kv have an 

 average initial energy of 60 kev.) Consequently the OH radicals will be 

 formed in very close proximity along the a-ray track, forming a column 

 within which rapid interaction between radicals occurs with the formation 

 of some hydrogen peroxide (Lea, 1946). There is, therefore, a smaller 

 chance for reaction between radicals and solute molecules than in the case 

 of the more randomly distributed ionizations of 60-kev electrons. Dale, 

 Gray, and Meredith were inclined to attribute most of the low a-ray effect 

 to ionizations caused by 5 rays which ionize at a density much nearer that 

 of' the 60-kev electrons and are sufficiently energetic to travel clear of the 

 column and its surrounding sheath of negative ions. 



A slightly higher ionic yield was obtained in an investigation (Dale, 

 Davies, and Gilbert, 1949b) of a- and X-ray efficiency in deaminating a 

 1 per cent and a 20 per cent solution of glycine by the a radiation produced 

 from boron in a neutron flux. The ionic yields were 15.2 and 19.5 per 

 cent, respectively, of those achieved by X radiation. 



