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RADIATION BIOLOGY 



when the same dose is extended over a long period of time, restitution 

 intervenes to reduce the number of breaks which can participate in 

 reunion. On this view, absence of an intensity effect with chromatid 

 and isochromatid breaks is expected, since these are one-hit aberration 



types. 



The data from neutron experiments (Giles, 1943) indicate that, con- 

 trary to the X-ray results, there is no intensity effect with either exchanges 



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DURATION OF EXPOSURE, minutes 



• CHROMATID BREAKS (XRAYS, 150/)- CATCHESIDE et a/., 1946 

 A ISOCHROMATID BREAKS (X RAYS,'l50 r)- CATCHESIDE eta/., 1946 

 A ISOCHROMATID BREAKS (X RAYS, 130 r)- GILES, 1943 

 O ISOCHROMATID BREAKS (FAST NEUTRONS, 10 nj-GlLES, 1943 



Fig. 10-6. Effect of intensity of radiation dose on yield of chromatid and isochromatid 

 breaks induced by X rays and neutrons. 



or simple break types (Figs. 10-5, 10-6). The absence of an intensity 

 effect with exchanges affords strong support for the previously outlined 

 mechanism of aberration production, since these aberrations have been 

 shown by dosage data to be one-hit types, both breaks being produced by 

 a single proton path; consequently, an intensity effect is not anticipated. 

 Lea and Catcheside (1942), Lea (1946), and Catcheside, Lea, and 

 Thoday (1946b) have used the experimental data on the intensity effect 

 for exchanges to calculate the average restitution time (t), i.e., the 

 average time elapsing between breakage and restitution. Using the 

 original data of Sax (1939, 1940), the value of r is approximately 4 



