Information and Inactivation of Biological Material 



279 



then L must be at least 



{MBf 



H is then given by 



H = //max — X logo MB + l0g2 x\ 



(3) 



It was argued above from equation (I) that there are about 3 bits per atom, 

 so that //,nax is a number of the order of 3A/; thus if x is small in comparison 

 to M, a structure requiring multiple hits will still have an information content 

 close to //max* i^ cascs where no energy transfer is assumed. 



200 600 1000 2000 



LET IN EV/ 100 A 



3000 



4000 



Fig. 1 



There are cases in which x may be an appreciable fraction of M. If one 

 plots cross-section as a function of linear energy transfer (LET), the shape 

 of the curve gives an indication of target thickness and number of ionizations 

 required. If it can be shown that x ionizations are required in a distance A 

 for an inactivation, we can divide the structure into sub-structures of volume 

 A^ in which case H is given by 



H = ZA^m - 1022 ^^^' (4) 



'- x\ 



where m is the number of atoms per unit volume. If x is an appreciable fraction 

 of A^m the substructure may have an information content smaller than the 

 maximum value. The information content of the entire structure, which is 

 the sum of that for the substructures, will be correspondingly smaller than 

 in the case of complete specification. 



Let us now consider the specific case of the irradiation of spores of Bacillus 

 subtilis with fast charged particles. At all values of LET studied the inactivation 

 curves are exponential functions of the dose. Fig. 1 shows the curve obtained 



