316 P. J. LINDOP AND J. ROTBLAT 



effect was established in bacteria by Dewey and Boag (1959). A detailed 

 analysis shows, however, that to explain the observed variation of sensitivity 

 with dose-rate it would be necessary to assume that the vital tissues in the 

 animals are initially at a very low oxygen tension. This conflicts with the 

 finding that by making the mice breathe nitrogen for 25 seconds the LD50 

 can be reduced by nearly a factor of 3 (Lindop and Rotblat, 1960), which 

 means that the tissues must have been initially at a high oxygen tension. 



Furthermore, if the protective action of anaesthesia and dose-rate both 

 resulted from a lowered oxygen tension, then the combination of anaesthetic 

 with high dose-rate should have resulted in a much greater increase in the 

 LD50, or a decrease in the life-shortening effect, particularly at very high 

 dose-rates; the reverse is in fact observed. We must thus conclude that the 

 variation of sensitivity with dose-rate is not due to an oxygen effect and that 

 some other mechanism must be responsible for it. 



REFERENCES 



Corp, M. J., and Neal, F. E. (1959). Int. J. Bad. Biol. 3, 256. 

 Dewey, D. L., and Boag, J. W. (1959). Nature, Lond. 183, 1450. 

 Lawrey, J. M., and Fowler, J. F. (1959). Brit. J. Radiol. 32, 630. 

 LiNDOP, P. J., and Rotblat, J. (1960). Nature, Lond. 185, 593. 

 Lindop, P. J., and Rotblat, J. (1961). Proc. roy. Soc. B154, 332. 

 LmDOP, P. J., and Rotblat, J. (1962). Brit. J. Radiol. 35, 23. 

 Mack, H. P., and Figge, F. H. J. (1952). Science 115, 547. 



DISCUSSION 



lamerton: At these very high dose-rates, I'm wondering if you are approaching some of 

 the effects of high LET radiation? What I would like to ask is whether you noticed any 

 difference in grouping of deaths for the LD50, whether at the very high dose-rates you 

 are getting more mtestmal deaths or more in the first phase than in the second phase? 

 rotblat: I suppose that by high LET you mean that at very high dose-rates the clusters 

 of ions from two independent particles tend to overlap. We have observed such an effect 

 on the yield of chemical reactions but it occurs at much higher dose-rates (over 10^ 

 r/sec) than are used in the present work. We have not noticed any significant difference 

 in the causes of death or in their time distribution in mice exposed at different dose-rates. 

 ALEXANDER: About the dosimetry, how do you compare doses at rates of 100,000 r/mui 

 and at 100 r/min? Can you use the same dose meter for these two? 

 ROTBLAT: The dosimetry at liigh dose-rate isn't very easy. We had to build special ioni- 

 zation chambers with very small air gaps to avoid the loss of efficiency due to recombina- 

 tion. Of course we always cahbrate the ionization chambers by a calorimetric method. 

 ALEXANDER: So you use two different dose meters for the lower and higher rates? 

 rotblat: No, we used the same chambers but to make sure that they were working 

 properly, we always calibrated them by an absolute method. 



ALEXANDER: But you use the same maclime. How do you manage to slow it down, to 

 make the same macliine give you 100,000 r/mm and 100 r/min? 



