262 LOCALIZATION OF RADIATION EFFECTS 



We must now examine mechanisms available for inactivation follow- 

 ing transformation of P atoms. These may be of two types. One in- 

 volves bond rupture, the other bond excitation. 



Many uncertainties arise in attempting to assess the fraction of the 

 phosphate bonds ruptured per disintegration. The bond energies prob- 

 ably range between 10 and 20 ev. If one assumes the nuclear recoil to 

 be taken up by the resultant S atom and one or two of the atoms, the 

 mass available for the recoil momentum ranges from ^^30 to ^^60. 

 Still more uncertainty attends this estimate because the angular dis- 

 tribution of the neutrinos emitted in the beta process is not known. If 

 the neutrino and beta particle fly off in the same direction, the residual 

 nucleus can experience its maximum recoil (~100 ev) while showing a 

 continuous spectrum of recoil energies ranging down to some value be- 

 low 10-20 ev. Other distributions are a consequence of assumptions in 

 which the average angle between neutrino and beta particle is taken as 

 180° or, what appears to be the most popular value, 135°. 



A very rough calculation indicates that for the most part there is 

 sufficient energy to effect a bond rupture for at least 50 per cent of the 

 disintegrations. 



Another effect, namely, the replacement of P by S in the nucleoprotein 

 at the site of the disintegration, might be expected to alter radically the 

 functionality of the particular bond involved. This alteration would 

 not be expected to be so much a consequence of the mere replacement of 

 P by S as of the rearrangement of electrons resulting during the trans- 

 formation. Thus, the overall change would involve only departure of one 

 proton along with the nuclear electron. Little strain on the bridging 

 bonds between the phosphate and the nucleotide moieties would re- 

 sult because the S— and P— O bond distances are not very different. 



It can be appreciated that much uncertainty attaches to the estimate 

 of percentage bond rupture following radioactive decay. It seems rea- 

 sonable, however, to conclude that any of the processes involved would 

 be quite as effective in immobilizing a portion of the nucleoprotein and, 

 taken in the aggregate, would be more than sufficient to explain the high 

 efficiency of assimilated P^^ for inactivation as compared to external 

 p32 or x-rays. We must suspend, for the time being, the interesting 

 query whether bond rupture is more effective than bond excitation in 

 inactivation of nucleoprotein. Incidentally it would be of interest to 

 see whether properties other than mere inactivation are affected by a 

 few radioactive events. 



Although the experimental exploitation of this direct procedure for 

 probing variation in radiosensitivity is still very much in its infancy, it 

 is possible to distinguish certain features of the data which indicate that 



