PRINCIPLES OF RADIOLOGICAL PHYSICS 135 



throughout each treatment and to vary it m a series of experiments in 

 inverse proportion to the duration of the treatment. Alternately, each 

 treatment may be fractionated into two, or possibly more, short high 

 intensity bursts and the interval between the bursts varied in a series of 

 experiments ; this method appears to offer considerable advantages (Fano 

 and MarinelU, 1943). 



No influence of the time factor is expected if the separate photons or 

 particles of a radiation contribute independently to the eventual observ- 

 able effect. Therefore the interpretation of the exponential dose-effect 

 curves in Sect. 5-2a implies that these curves do not depend on the rate of 

 delivery of the radiation. This expectation obtains in practice in a sub- 

 stantial number of instances (provided, of course, that the physiological 

 properties of the treated organisms remain constant even during pro- 

 tracted exposures to radiation). For example, the sperm cells of a fruit 

 fly may remain under constant conditions for a period of a month ; equal 

 doses of radiation variously distributed during this period produce equal 

 genetic effects (see Muller, 1940; Kaufmann, 1941). 



On the other hand, the time factor may well be expected to influence 

 sigmoid dose-effect curves. In a few instances the relationship of the 

 time factor to the mechanism of origin of the biological effect seems fairly 

 clear. One such example concerns the occurrence of two separate breaks 

 in the chromosomes. Here it appears plausible that the two breaks may 

 not combine to yield the final rearrangement if they arise at too distant 

 instants of time, e.g., more than a few minutes apart (see Sax, 1940; Lea 

 and Catcheside, 1942). Therefore a given dose becomes less effective if 

 spread over a long interval of time. 



The process of degradation and distribution of radiation energy up to 

 the production of chemical activations takes place very rapidly and 

 involves an exceedingly small fraction of the atoms of a material. There- 

 fore the proximity in time of the arrival of different particles or photons 

 should not influence their immediate physical action vip to the point at 

 which activations are produced. It should then be expected that experi- 

 mental data on the influence of the time factor would supply evidence on 

 the stages of the action of radiation which follow the production of 

 activations. Unfortunately, so little is known in general about these 

 stages that it is difficult to draw any conclusions from the data on the time 

 factor. 



5-5. COMPARATIVE EFFECTIVENESS OF DIFFERENT RADIATIONS 



Two different elements influence the comparative effectiveness of radia- 

 tions, namely, their initial potency and the distribution of the activations 

 which they produce within a material. 



As explained in Sect. 3, all ionizing radiations have a potency well in 

 excess of that required to produce any chemical activation, and the 



