786 RADIATION BIOLOGY 



radiation (about 8.5 ions//* in tissue), the denser ionizations of medium 

 X rays (about 100 ions//z), the still more densely ionizing recoil protons 

 from fast neutron irradiation (up to 1100 ions//x), to a particles with 

 densities of 3700-4500 ions/ M (Gray, 1946, 1947). Knowledge of the 

 relative efficiencies of the different radiations in interfering with mitosis 

 might give us a clue to the means by which radiation energy is trans- 

 formed into observable effects on cell morphology and behavior. Since 

 such determinations depend directly on the accuracy with which the dose 

 absorbed by the cells is measured, conclusions reached from comparative 

 studies of this type are valid only insofar as the energy measurements are 

 accurate. As might be expected, only quantitative, not qualitative, 

 differences in the effects produced by different radiations have been 

 demonstrated. 



Fast neutrons have been found to be more effective than y rays in 

 reducing the mitotic activity of a number of different kinds of cells. 

 Zirkle, Aebersold, and Dempster (1937), who compared the efficiencies of 

 neutrons and X rays by irradiating fern spores, germinating them, and 

 determining the proportion that had undergone their first cell division at 

 the end of 10 days (normally this division occurs on the sixth day of 

 germination), found that neutrons were 2.5 times as effective as X rays in 

 reducing division by 50 per cent. Small doses of neutrons were found by 

 Gray et at. (1940) to be 2.1 times as effective as 7 rays in reducing the 

 number of mitoses in broad bean root tips 3 hours after the mid-point 

 of the treatment period. Similar experiments carried out with chick 

 fibroblasts in culture also showed neutrons to be more effective than 

 7 rays, as determined by the reduction in mitotic activity 80 minutes 

 after irradiation. When the percentage of normal mitoses was plotted 

 against dose, the neutron curves were exponential and the 7-ray curves 

 sigmoid, the two intersecting at some dose higher than any used in the 

 study. They postulated that the neutron effect may result from the 

 passage of a single recoil proton within a certain limited volume of radius 

 2 or 3 (j. in the cell, and that the 7 radiation effect would require some two 

 hundred secondary /3 particles. 



The evidence with regard to the effectiveness of a particles in inhibiting 

 mitosis, however, is conflicting. Gray and Read (1950) found a rays to 

 be less effective than either neutrons or 7 rays in lowering the mitotic 

 count 3 hours after the mid-point of treatment in the broad bean root tip 

 (Fig. 11-8). The relative efficiencies of 7 rays, neutrons, and a rays were 

 about 1:2.1:0.6. They point out that these relative efficiencies of neu- 

 trons and a rays are in good agreement with the ion density ratios of 

 recoil protons to a rays, if reasonable assumptions be made for the 5-ray 

 ionization. On the other hand, in the frog tadpole treated with a radia- 

 tion by immersion of the animal in water containing radon and subse- 

 quently sectioned and stained at intervals for study, a particles proved to 



