732 ' RADIATION BIOLOGY 



breaks are 0.09, 0.09, and 0.5 for X rays, neutrons, and a particles, 

 respectively. 



The over-all evidence thus indicates that a substantial fraction of 

 originally induced breaks undergo restitution. Only failure of restitution 

 permits new reunion and the two processes of restitution and reunion are 

 thus competing ones in which both space and time factors are important. 



RELATIVE EFFICIENCIES OF VARIOUS RADIATIONS 



Comparisons of the relative efficiencies of various ionizing radiations 

 and of X rays of various wave lengths in producing breaks and aberra- 

 tions have been of considerable importance in the development of the 

 theory of aberration production in Tradescantia. As indicated pre- 

 viously, the greater efficiency of fast neutrons as compared with 160-kv 

 X rays provided part of the initial evidence that single ionizations were 

 insufficient to produce chromosome breaks. The conclusion that several 

 ionizations (or related chemical events, as will be discussed later) are 

 necessary for chromosome breakage arises from the following considera- 

 tion: Ionizations are much more densely spaced along a proton track 

 (derived from neutron radiation) than along an electron track (derived 

 from X radiation) . Hence, when a proton traverses a chromosome, many 

 ionizations will be produced within its volume ; if only one ionization were 

 required for breakage, neutrons should be less rather than more efficient 

 than X rays, since additional ionizations produced within the chromosome 

 would be wasted, contributing to the physical dose but not to the bio- 

 logical effect. 



Utilizing the neutron-X-ray comparisons, Lea and Catcheside (1942) 

 attempted to determine the approximate number of ionizations required 

 to produce a break. Their conclusion was that seventeen ionizations, on 

 the average, are required for breakage to occur. Further, with the usual 

 X-irradiation procedures, in which relatively hard X rays are employed, 

 it appears that breakage arises primarily from the concentrated ionization 

 produced at the end (tail) of the electron path. The conclusion that the 

 tail of the electron path is responsible for break production with X rays 

 was tested experimentally by comparing the efficiencies of X rays of 

 different wave lengths, particularly in the soft X-ray range. For this 

 purpose, the effects on pollen tube chromosomes were examined and the 

 experimental results were found to agree well with the theoretical predic- 

 tions (Catcheside and Lea, 1943). 



The calculations of Lea and Catcheside (1942) indicated that the prob- 

 ability of break production when a proton (from a fast neutron) traverses 

 a chromosome is somewhat less than one. On this basis a particles, 

 because of their much greater ionization density, should be actually less 

 efficient than fast neutrons. The studies of Kotval and Gray (1947), 

 however, indicate that a particles are actually more efficient than neu- 



