LOCALIZATION OF RADIATION EFFECTS 261 



initial specific P^" content (specific radioactivity in curies per gram); 

 and S, the surviving phage titer per miUiUter. Thus, a, the efl^iciency 

 of inactivation, can be determined, knowing the rate of decay of the 

 phage population. Setting S = Sq and t = when the initial specific 

 activity is Aq, it follows that 



log 5 - log>So = 1.48 X 10-V4oA^(l - e"'') 



Extensive experimental data have been obtained which in all cases 

 agree with this relation in presenting a straight line when the logarithm 

 of the phage surviving is plotted against the function (1 — e~^'). The 

 intercept at f = agrees with the value log Sq, and the slope is propor- 

 tional to the initial specific radioactivity Aq. The values of aN derived 

 from these plots vary from 41,000 to 58,000, with a mean of 43,000 for 

 the phage tested (phage type T'* and three closely related strains were 

 examined). Since N is known to be 5.0 X 10^ E, where E is the efficiency 

 of the counting method (plaque count), the value for a is 0.086/E' per 

 atomic disintegration. E is believed to be very close to 100 per cent, so 

 that on the average, under the experimental conditions used, a phage 

 particle is inactivated at least once in 11.6 disintegrations. 



Now what can be done about interpreting this figure? First it is 

 necessary to examine the efficiency of inactivation to be expected, on 

 the basis that the inactivation results from the passage of beta particles 

 resulting from the P^^ decay. The average energy of the P^^ beta par- 

 ticles is 7.0 X 10^ ev. The average initial specific ionization is 7.1 ion 

 pairs per mm air. The air-path equivalent of such beta particles in a 

 phage molecule would average 0.04 mm, so that 0.28 ion pair would be 

 produced per phage particle. For the highly energetic P^^ beta particles 

 this probably represents a maximum, because clustering of ion pairs 

 would be expected to occur rather frequently. Using the value for a 

 derived experimentally, this means 11.6 X 0.28, or 3.3, ionizations pro- 

 duced by the beta particles inside a phage for inactivation. Thus, the 

 efficiency is 0.3 per ionization. 



Actual measurements of inactivation of unlabeled phage placed in 

 labeled phosphate solutions show an efficiency of only 0.009 per ioniza- 

 tion. Comparable efficiencies found from perusal of x-ray data in the 

 literature are somewhat higher but still far from the expected figure of 

 0.3. Hence it is highly improbable that we are dealing with inactivation 

 resulting from ionization by beta particles. It can be concluded that 

 the overwhelming majority of inactivations occur as a consequence of 

 the events following transformation of the P atom itself, and that these 

 experiments bear on the question of the relative radiosensitivity of the 

 — P bonds in the nucleoprotein. 



