sparrow: cytogenetic effects of ionizing radiations 



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CHROMOSOME ABERRATIONS 



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AVERAGE RATE OF ENERGY LOSS PER MICRON OF TRACK 

 IN TISSUE FROM CONGER. RANDOLPH AND JOHNSTON 



Figure 15. — The effect of ionizing radiation on different energies on 

 their relative efficiency in inducing chromosomal aberrations in Tra- 

 descantia microspores. After Sax (137). [See Conger, Randolph, and 

 Johnston (25).] 



ionizations, e.g., if a sensitive target volume requires only x ionizations 

 to produce an effect, then 2x ionizations in the same volume will not 

 increase the detectable effect (Figure 14). Certain effects, such as those 

 produced by single ionizations, are less efficiently produced with 

 increasing LET (Figure 16). It is generally assumed that point muta- 

 tion can be induced by one ion pair or one ion cluster (108, 199). For 

 chromosome breakage, however, a different relationship apparently 

 exists. 



From early work, Lea concluded that chromosome breakage (in 

 Tradescantia) required an estimated minimum of 15 to 20 ionizations 

 to occur within a cross section of a chromatid thread not thicker than 

 0.1 [i (93, page 280). This means that breakage of such chromosomes 

 can only be produced by relatively dense ion clusters. The response 

 of Tradescantia chromosomes to radiations of different ion density 

 has recently been studied in detail by Conger, Randolph, Sheppard, 

 and Luipold (26). By comparing the efficiency of various radiations 

 to that of Co 60 gamma rays, they showed that the RBE climbs to a 



