KEMPNER AND POLLARD 



131 



meter) was found. The statistical analysis is similar. If we assume that a 

 deuteron is able to inactivate as long as it passes through the sensitive region, 

 then the area of the sensitive region is all that matters. On that basis, if D is 

 the number of deuterons per square centimeter and S is the area of the sensi- 

 tive unit concerned with amino acid uptake, we see that the average number 

 of deuterons per target is SD, and once again the probability of escape is e~ SD . 

 Therefore at the value of D for 37 per cent remaining, SD = \. S is referred to 

 as the "cross section." It must be remembered that a deuteron may not be per- 

 fectly efficient in producing inactivation, and in particular, if the ionizations 

 have a finite separation, then a thin target can be "straddled." This effect is 

 apparently present for proline uptake. 



In figure 7, the cross section is plotted against the air equivalent of the foil 

 thickness for the TCA-insoluble fraction containing C 14 proline. The maxi- 

 mum of this curve is near 26 X 10" 12 cm 2 at a value of 5.5 cm air equivalent of 

 foil. From the residual range of the beam, the value of the linear energy trans- 

 fer (LET) at this cross section is found to be 400 electron volts per 100 A. 



30 



25 



1 2 



\- 

 o 



CO 



CO 



to 

 o 

 or 

 o 



10 - 



10 



10 



12 



14 



16 cm. 



ABSORPTION (AIR EQUIVALENT) 



Fig. 7. Measured cross section for proline incorporation into the TCA-insoluble portion 

 of E. coli after deuteron bombardment as a function of added aluminum in the cyclotron 

 beam; 12.5 cm is seen to be the range of the deuteron beam in air. 



Incorporation studies after a-particle bombardment are indicated in figure 8 

 for proline. The survival curves for such studies were reasonably exponential, 

 with a 37 per cent dose for the TCA-insoluble portion labeled with proline 

 of about 2.7 X 10 10 a particles per cm 2 . 



The results of y-ray, deuteron, and a-particle irradiations are combined in 

 the LET plot [5] in figure 9. The two relations, in terms of volume V and 

 area S, reduce to the same expression when the density of ionization, or linear 

 energy transfer, is low. Under such circumstances, if / is the number of pri- 

 mary ionizations per unit length, l — Di, so that we obtain for the probability 

 of escape e~ DiV , or S=iV. Thus the value of S close to the origin can be found 



