17 



into the water, but no spectrum was obtained, even with a wide slit, in one and 

 a half hours. By reducing the energy below 250 kev the Cerenkov radiation was 

 eliminated, but one would expect any recombination radiation to be reduced by 

 a factor of only six. One may calculate an upper limit to the energy radiated as 

 recombination radiation in the following way: 



^s*^~ 40 ^ f* o ^ 



Recombination radiation escaping \C j '- x Cerenkov emission 



lj hours 



The sign Y^ denotes that there are extra factors for (a) wide slit vs. narrow slit 

 and (b) no photograph vs. good photograph. Now the loss of energy of an elec- 

 tron in water due to Cerenkov radiation is of the order of 10 kev per cm. where- 

 as the total rate of energy loss is about 2 Mev per cm. so the Cerenkov emis- 

 sion accounts for only one-half per cent. It seems that any recombination radi- 

 ation in the ultraviolet or visible must be less than one per cent of the Cerenkov 

 and thus less than 10-4 of the total. This applies of course only to the spectral 

 region between 2300 and 5000 A . 



KASHA: You have to check two other things. One is -- are there any elec- 

 tron-excited states in the water which come in the region. That would be cov- 

 eredby your question. The other is -- are there any metastable excited states. 

 And there are none, are there, in water? 



BOAG: An experiment has been reported by Dee and Richards (30) in which 

 alpha particles impinging on a thin film of water gave a soft X radiation which 

 passed through quartz and was detectable by a photomultiplier. 



PLATZMAN: What is the present status of that work? In this country most 

 people are skeptical of its validity. 



BOAG: Richards himself did not support it very strongly at the Faraday 

 Society discussion at Leeds. However, some American workers have recently 

 published apparently confirmatory data, and they even gave a spectrum they had 

 photographed. (31) 



PLATZMAN: All I remember is that it was not very impressive. 



BOAG: I agree. 



BURTON: One could say that the most impressive evidence against it has 

 not been published. Other people have tried to do what Richards has done and 

 obtained negative results. For instance, Miller at Edinburgh has done some 

 experiments. 



ALLEN: Miller controlled the conditions more carefully. 



BURTON: Essentially, the experiment involved a very thin smear of water 

 on a quartz plate, on the other side of which there was a chemically sensitive 

 system (for example, aqueous ferrous sulfate). They found that although the 

 water layer was thick enough to stop the alpha particle, nevertheless the ferrous 

 sulfate solution on the other side of the quartz was oxidized. That is the story. 

 So far as I know, no one else has ever duplicated the result. 



ALLEN: Miller went to great length to show first there is no chemical 

 reaction; second, that there is no radiation transmitted. 



PLATZMAN: We don't know enough to say that there is no radiation. The 



