INFLUENCE OF LINEAR ENERGY TRANSFER 325 



neutron dosimetry, the small effects tabulated may be nonexistent. 

 There does appear, however, to be a small differential effect of LET on 

 recessive as compared to dominant lethals (items 32, 33). More clear- 

 cut effects of LET are observed in inhibition of hatching of "eggs" (lethal 

 action) (item 37), and reduplication of organs in flies developed from 

 irradiated larvae (item 42) is produced only by radiation of high LET. 



In actions on mammals (items 48-68) the effectiveness increases with 

 LET to varying degrees. In early lethal action of total-body irradiation 

 the effect is moderate when a single dose is given in about an hour (items 

 48-50, 52) but is much greater when delivered in a day or two (item 51). 

 Also, the influence of LET is greater in late than in early lethal action 

 (items 53 and 54 vs. 49), whether the dose be single or periodic. Effects 

 on gonads, blood, and skin are only moderately influenced, but cataract 

 production appears to be greatly so (item 59). A very striking depend- 

 ence on LET is shown by regression of mouse tumors irradiated in vivo 

 (item 68). Effects on the same tumor in vitro (item 69) are not so 

 highly although still impressively influenced (cf. also items 70-72). 



Deleterious actions on roots (items 75-80) increase with LET, some- 

 times quite impressively (items 77, 80). The same is true of chromosome 

 aberrations of all types investigated, whether in roots (items 82, 83) or in 

 pollen formation or germination (items 84-86) , the extent of the influence 

 of LET depending on the type of aberration. 



Different species of yeast appear to respond differently to LET. In 

 Saccharomyces ellipsoideus (item 21) inhibition of the second cell division 

 is favored by relatively low LET, whereas, in S. cerevisiae the same effect 

 is favored by high LET (items 22, 23). 



Cases of Effectiveness Passing through a Maximum. For some actions 

 the effectiveness clearly passes through a maximum as LET increases. 

 The clearest example is inhibition of mitosis in the broad bean root (Fig. 

 6-2), although the maximum cannot be exactly located because of the 

 small number of radiations used. Another case, not so certain and 

 striking, is inhibition of hatching of Drosophila eggs (item 37). The 

 apparent maximum in lethal action on mice (item 49) may be real.^ 



^ This item is complicated by the fact that the 7-ray and fission-neutron experiments 

 were carried out at Oak Ridge and the X-ray and cyclotron-neutron experiments at 

 Chicago. However, mice of the same CF-1 stock were obtained from the same 

 breeder and were maintained under as nearly identical conditions as monthly consulta- 

 tions could promote. Close liaison was also maintained between investigators. 

 Neutron dosimetry is of course always suspect, but the Victoreen chambers used at 

 the two sites were calibrated against the same arbitrary standard in the neutron 

 beam from the cyclotron. The sensitivity of Victoreen chambers probably is depend- 

 ent on neutron energy, but one would expect it to be less for the relatively low-energy 

 fission neutrons than for cyclotron (Be -t- d) neutrons, whereas the values of effective- 

 ness indicate the opposite, if one ascribes their difference to dosimetric uncertainty. 

 Only with caution can the effectiveness value of the a rays be compared with those of 



