INFLUENCE OF LINEAR ENERGY TRANSFER 323 



sophila) ; salamanders ; birds (chickens) ; laboratory mammals ; humans ; 

 seed plants. 



In the third column are listed the qualitative effects which were studied 

 and measured. In the fourth are briefly described the various radiations 

 used. For brevity, the notation 7 (" Ra ") is used to designate the 7 radi- 

 ation from a radium or radon source filtered with as much as 0.5 mm of 

 platinum or its approximate ecjuivalent. (The 7 rays are emitted by 

 decay products of radon.) Likewise the notation jS ("Ra") designates 

 hard /3 rays from a thin-walled radium or radon source. Monochromatic 

 X rays are designated by their wave lengths in angstroms. Hetero- 

 chromatic X rays are designated by their effective wave lengths or by 

 the kilovoltage. Fast neutrons are listed as "fn" and slow neutrons as 

 "sn." The nuclear reactions used in generating the fast neutrons are 

 indicated in parentheses, e.g., " (Be + d) " for beryllium bombarded with 

 deuterons. 



In column 5 are listed the values of LET corresponding to the radiations 

 entered in column 4. These figures have been calculated from the data 

 tabulated by Lea (1947a) and by Gray (1947) and are subject to the 

 uncertainties emphasized by these authors. Unless stated otherwise, 

 they are mean values (track-average method). For fast neutrons the 

 mean value of LET obtained by a given nuclear reaction, e.g., Be -f- d, 

 depends on the kinetic energy of the bombarding particles. For X rays 

 generated at a given kilovoltage, the mean LET depends on filtration 

 and other variables. These various factors have been taken into account 

 so far as the authors' data permit. 



In column 6 are entered the relative values of effectiveness of the radia- 

 tions listed in column 4. For each item (column 1) these effectiveness 

 numbers are inversely proportional to the doses required to produce a 

 standard amount of effect, the effectiveness of the radiation of lowest 

 LET (column 5) being set equal to unity (except in item 1). In most 

 cases no correction has been made for variation of energy absorption per 

 roentgen with the type of radiation (Lea, 1947a, Table 2). 



Column 7 contains references to notes which immediately follow Table 

 6-1, and column 8 contains references to literature. 



DISCUSSION OF RESULTS 



By glancing at columns 5 and 7 of Table 6-1 and at Figs. 6-1 and 6-2, 

 it is readily seen that as the linear energy transfer increases, the relative 

 effectiveness of the various radiations may remain unchanged, may 

 increase, or may decrease, depending upon the nature of the irradiated 

 object and upon the effect studied. 



Cases of No Change in Effectiveness. In most investigations in which 

 no effect of LET was observed, the radiations differed in mean LET by a 



