PETER ALEXANDER AND ARTHUR CHARLESBY 



example an enzyme, varies with concentration in solution (i.e. if the 

 inactivation is inversely proportional to concentration, the action is indirect). 

 This test cannot, however, be readily applied at the cellular level since it is 

 usually not possible to vary at will the concenti^ation of constituents within 

 the cells. 



Based on the concepts that direct action is not influenced by the condition 

 of the environment of the material, and that once sufficient energy has been 

 absorbed (or a given number of ionizations have occurred) the subsequent 

 damage to a molecule is unalterable, three other tests are used to distinguish 

 between the two processes. The action of radiations of low specific ioniza- 

 tion {e.g. hard X-rays or gamma rays*) is said to be indirect (a) if the presence 



Figure 2. — The ultraviolet absorption of poly - 

 isobutene showing the effect of radiation in 

 the presence and abserwe of oxygen {dose approx. 

 1 • 5 X 1 0^ r) (0-13 per cent solution in 

 n-hexane) 



^00 



250 



300 

 mu. 



of oxygen in the external medium enhances the biological effect ; {b) if the 

 action of the radiation is decreased on freezing all the water in the system 

 thereby making it impossible for free radicals to diflTuse ; (c) if the presence 

 of other substances can reduce [i.e. protect) the irradiated system. 



Since protection^* and an effect of oxygen^^ is almost invariably found 

 when living systems are irradiated with X-rays or gamma rays it has been 

 concluded that indirect action plays an important part in bringing about 

 the observed effects. 



Our work on the irradiation of dry polymers where the action must be 

 direct shows that the last test is not decisive for indirect action, since 



* The oxygen effect and protection cannot be used to determine whether the action of 

 densely ionizing radiations (e.g. neutrons or alpha rays) is direct or indirect, since they pro- 

 duce radicals in water at ver^' high local concentration and their reacti\ity cannot therefore 

 be influenced by the presence of oxygen or protective agents in the water. 



57 



