ALPHA PARTICLES 569 



tional to the ionization per unit path, then it would be obvious that the 

 distribution, as well as the number, of the ions was a factor influencing 

 the biological effectiveness. 



The possibility that the distribution as well as the number of ions 

 may influence the biological effectiveness has been recently investigated 

 by Zirkle (28). Polonium was used as source, and the ionization along 

 the path of the average alpha particle varied as shown in Fig. 1. Since 

 the total length of path was 32 m, it was possible to place nuclei of vari- 

 ous lots of fern spores (10 fi in diameter, see above) so that some were 

 traversed by alpha particles at the beginning of their paths where the 

 ionization per unit path was low; some at the ends of the paths, where 

 the ionization per unit path was high; and others in intermediate por- 

 tions of the paths. The nuclei of all lots were turned upward (see above) 

 and, by interposition of a suitable thickness of aluminum, the alpha 

 particles could be retarded sufficiently to cause them to traverse the nuclei 

 at any desired linear portion of the path. Seven portions of the path 

 were investigated. The ionization produced in the nucleus by the 

 average alpha particle under these seven conditions is shown in the 

 second column of Table 1 (arbitrary units). The arbitrary values, it will 

 be noted, range from 1.00 at position 1 (near the end of the path) to 

 0.58 at position 7 (near the beginning of the path). The biological 

 effectiveness at the seven positions was obtained as follows: A complete 

 survivor-dosage curve was obtained for each position. From this curve 

 could be determined the number of alpha particles per nucleus necessary 

 to produce a given degree of biological effect {e.g., inhibition of cell 

 division). The reciprocal of this number was then a measure of biologi- 

 cal effectiveness per alpha particle. The seven measures of biological 

 effectiveness^ thus obtained are shown in the third column of Table 1 

 (arbitrary units). Now, if biological effectiveness of an alpha particle 

 were determined solely by the number of ions it produced in the nucleus, 

 it is plain that the observed biological effectiveness at the seven positions 

 should, within experimental error, be proportional to the ionization at 

 these positions. In other words, the figures in columns 2 and 3 of Table 1 

 should be substantially the same. However, there is a systematic 

 divergence in these two columns. It is hence clear that biological 

 effectiveness is not dependent solely upon number of ions per nucleus. 

 Distribution of ions also must be a factor. For a given amount of nuclear 

 ionization, the maximum effect is obtained by using the fewest possible 

 paths, with of course the highest production of ions per unit path. 



' These values were the same, regardless of the degree of effect chosen as end point; 

 that is, the seven curves had the same shape but different slopes. The same values, 

 within experimental error, were obtained with all the three processes studied (cracking, 

 chlorophyll development, division). The figures in column 3 of Table 1 are averaged 

 from the three corresponding sets of data. 



