15 



KASHA: They are probably vacuum ultraviolet frequencies, and the most 

 unambiguous experiment might be a two-cubicle experiment in which a sapphire 

 window between two compartments would allow high frequency electromagnetic 

 radiation generated in one to pass into the other. A collimated primary irradia- 

 tion beam could be passed through a cell containing one of the liquid scintillation 

 solvents, e.g., benzene. The second cell could contain a scintillation solution, 

 e.g. , one of those given in Kallmann's tables (29). The photomultiplier tube 

 could be collimated to observe only emissions from the second cell. The initial 

 part of Kallmann's scintillation intensity versus concentration curves could be 

 explained on the basis of a reabsorption of such intermediate radiation. 



ALLEN: The feeling is that this distance is too long to be accounted for by 

 migration of excitation? 



o 



KASHA: Yes, that would be at most something like 1000 A, I think. The 

 Kallmann and other experimental results indicate something like 100 A at the 

 most, and Kallmann thinks it is 40 or 50 a . This experiment really seems to 

 indicate distances beyond that. 



BURTON: What is the phenomenon, induced fluorescence or simple quench- 

 ing? 



KASHA: You have to pick up the radiation and, therefore, to quench it. 



PLATZMAN: Do you mean experiments with light? 



KASHA: It is the average separation at the concentration given, and there is 

 some doubt as to whether that means the distance for transmission of the energy. 

 In choosing a scintillation medium, Ageno selected some of Kallmann's solute- 

 solvent pairs. 



BURTON: In the work that Franck spoke about on induced fluorescence, 

 what is the greatest distance that he assumes the energy can travel? 



PLATZMAN: What system? 



BURTON: I don't know. What is the greatest distance for any system you 

 know about? 



o 



KASHA: In solutions it is 40 or 50 A. 



BURTON: Did Ageno work on solutions? 



KASHA: Yes, on liquid hydrocarbon mixtures. I think the thing that is miss- 

 ing from all the work on liquid scintillation counters is the high-energy spec- 

 troscopy. Which pairs work and which don't work are really quite mysterious, 

 and yet when the spectroscopists look at the higher energy states, they certainly 

 vary extremely between the kinds of molecules used. It could very well be that 

 the scintillator which picks up the energy in the system may be capable of ab- 

 sorbing higher energy radiation to which the solvent is always partially trans- 

 parent. 



PLATZMAN: If the solvent is transparent, you should be able to observe 

 the direct light with the pure solvent. Would the absorption spectrum of the 

 solvent fall off in the far ultraviolet? 



o 



KASHA: Yes. For instance, in the case of benzene between 2000 and 500 A, 

 there are regions of some transparency. 



