RADIATION-INDUCED LIFE-SHORTENING 317 



rotblat: The output from the linear accelerator can be varied over a very wide range by 

 changing the current tlirough the gun filament or the r.f. power. 

 ALEXANDER: And it won't change the quality of the radiation? 



rotblat: Only very slightly. To make sure we have a magnet with which we can check 

 the energy of the electrons. 



ALEXANDER: The tiling seems difficult. I would not lilie to compare X-ray machines with 

 that order of accuracy. 



rotblat: It is not reaUy difficult, although it is not a job just for a technician. 

 mole: The results are very interesting indeed. We tried to get up to 1,000 r/min and 

 didn't get over the hump, but I think the quantitative change between say 50 and 1,000 r 

 a minute is extraorduaarily close to what you have observed. But I would like to ask if 

 you really can't stick to the oxygen idea. Supposing that normally there is a nearly zero 

 oxygen tension at the vital target and oxygen is continually being used up and diffusing 

 in, then if you produce anaesthesia and reduce the concentration difference, you will 

 reduce the rate of diffusion, so you may get some degree of protection. Similarly, the 

 shorter the time in which the dose is delivered the less time there is for diffusion. Could 

 you not use the oxygen effect as a basis? 



rotblat: The only difficulty about this explanation is that in this case, if we give the 

 exposure in nitrogen we shouldn't get a much larger protective effect because we already 

 started off with a very low oxygen tension. 

 mole: But you haven't tried nitrogen at 10^ r/min. 



rotblat: Yes we have tried and we get a very large protective factor, approaching 3. 

 bacq: I think that mice are not very suitable for this kind of work. You should use 

 cliickens, which show remarkable dose-rate effects, very remarkable from 10 to 100 r/min. 

 Now if you have some effect of anaesthesia with tliis animals which is so much more 

 sensitive to the dose-rate in the narrow range then you might confirm your conclusion. 

 ROTBLAT: I quite agree, but in this work we did not set out to study the effect of anaes- 

 thesia in general, but in relation to the effects of liigh dose-rates and anoxia. 

 brinkman: I want to say something about what you call oxygen tension, because that of 

 course is very different in the various tissues of the body. If a mouse dies of anoxia it has 

 still half its amount of oxygen and the only reason it dies is that the brain cannot get 

 enough, although there may be very much oxygen still in other organs. That depends 

 mainly on two factors. One factor is the presence of carbon dioxide; everything depends, 

 not on the oxygen the animal is breathuig but on the oxygen dissociation of the haemo- 

 globin and that is much influenced by carbon dioxide. If you have asphyxia, where 

 carbon dioxide rises and oxygen tension goes down, then you will have very much higher 

 tension than if you had the animal in a low mixture and no asphyxia. That makes a great 

 difference, it might make a crucial difference. For mstance if one wants to liibernate 

 animals you have to put them not in a low oxygen mixture but in a closed chamber so 

 that the carbon dioxode goes up when the oxygen goes down and they can survive very 

 much longer with very low oxygen contents because of this'carbon dioxide effect which 

 sliifts the oxygen dissociation curve' very much to the right.^^The other factor is capillari- 

 zation of the tissues which is very much different. For instance 1 cm' of brain tissue has 

 2 km of capillary, but 1 cm' of cardiac tissue has 12 km, so when you use oxygen tension 

 it is too vague. You should say a bit more about it if you can. 



ROTBLAT: If I understood it correctly it would appear that if you give the animal anaes- 

 thetic then the process causmg protection would be different than if you make the animal 

 breathe nitrogen. This may explain why we get differences in the protection factors 

 against acute and long-term effects under various conditions. For example, we found 

 that for the animals breathmg nitrogen the long-term effects are less protected than the 



