;^7() RADIATION HIOLOGY 



lufj; phase. Cytolofiical studies show a hifi;h correspondeiu'e between the 

 iiuniher of ol^servahle nuclear l)odies and the numher of t argots estimated 

 for cells in different f^rowth jihases. The survival curves become expo- 

 nential and the target iniinber ix'comes 1 as growth proceeds through the 

 logarithmic phase and the stationary phase is approached. Stapleton 

 made the further interesting observation that stationary-phase cells sub- 

 jected to 7 rays from a Co^" source exhibit a "two-hit " killing curve as 

 compared to an exponential curve for X rays. 



Lea et al. (193()), in addition to discussing the principal theories of the 

 mechanism of the bactericidal action of ionizing radiations, present 

 important (luantitative data on survival curves of irradiated bacteria. 

 They studied primarily the survival of Bacillus mesentericus spores and, 

 less extensively, the survival of E. colt and S. aureus. The radiations 

 employed were a particles emitted by polonium and /3 rays produced by 

 radon disintegration. Careful energy measurements were made and the 

 geometrical conditions were controlled. The spores and bacteria were 

 irradiated in dried gelatin films of approximately l-yu thickness. The 

 fraction of organisms surviving was found to be a diminishing exponential 

 function of the time of exposure, or dose, for all organisms with both 

 radiations. The target areas and mean lethal doses (MLD) were com- 

 puted for each organism for both a and 13 rays. 



The studies with B. mesentericus spores and E. colt were extended to 

 include radium y rays (Lea et al., 1937), neutrons, hard X rays of an effec- 

 tive wave length of 0.15 A, and soft X rays of L5, 4.1, and 8.3 A (Lea 

 et al., 1941). The organisms were irradiated in aqueous suspensions; 

 suspending the cells in previously irradiated distilled water given com- 

 parable exposures was shown to have no effect. Again, exponential 

 survival curves were observed in all cases. 



Spear (1944), utilizing the same bacterial strains employed by Lea and 

 his associates, presents the most extensive data on the bactericidal effects 

 of neutrons. The neutron source was a beryllium target bombarded by 

 8-Mev deuterons accelerated in a 37-in. cyclotron. Acjueous suspensions 

 were exposed, and the dose was measured with a Victoreen dosimeter 

 calibrated in roentgens. The sur\i\'al cur\'e did not depart systemati- 

 cally from an exponential curve. The ratio of the T-ray dose in roentgen 

 units to the neutron dose in n units refjuired to produce 50 per cent 

 lethality was 3.2 for E. coli and 5.3 for B. mesentericus spores. Because 

 of the lack of absolute dosimetry with neutrons, Spear presents a curve 

 (Fig. 10-2) showing a systematic decrease in the MLD ratio of B. mesen- 

 tericus spores to E. coli cells as the ionization density increases. This 

 curve includes all the data of Lea and his associates and shows that the 

 neutron data fit into the general relation. 



No effect of varying temperature between 2.5° and 3()°C at the time of 

 irradiation was observed by Hercik (1934a) in studies employing a par- 



