89 



LINSCHITZ: There should be some maximum, yes. 



POLLARD: It has not been observed. For example, it has been observed, 

 in the inactivation of catalase by ultraviolet, that it goes very definitely down at 

 low temperatures to a minimum which is not zero and rises sharply at high 

 temperatures. 



LINSCHi'TZ: What is the quantum yield? 



POLLARD: One, or something in that magnitude. 



LINSCHITZ: I have just one more point to make in this matter of what can 

 be learned from scintillation counters regarding biochemical effects, and that is 

 the change in the efficiency of the scintillation counters as one goes from gamma 

 to alpha irradiation. It is found, again by Kallmann, that alpha irradiation gives 

 an efficiency of light conversion of about 10 percent of the gamma energy. It is 

 difficult to say just what the mechanism is of such enhanced quenching in the al- 

 pha case. 



BURTON: Yesterday I was told that for sodium iodide the efficiency is ap- 

 proximately the same for gamma and alpha. 



FANO: For ionorganic materials it is. For the organic it isn't. 



ALLEN: There is a theory on this by J. B. Birks (17). The idea is that 

 along the track of the alpha rays many damaged molecules are produced which 

 are capable of quenching the excitation. They are apparently formed in a short- 

 er time than is required for the excitation to get this far, and since there is a 

 high concentration of them, the light emission is quenched. 



BURTON: These molecules happen to be the kind that are not very badly 

 damaged by radiation; that is, the G_ value for the decomposition is very low. 



PLATZMAN: I agree -- and for this, and certain other reasons, I have been 

 skeptical of their part of Birks' theory. 



KASHA: There is one other difference here which I am reminded of from a 

 conversation with Dr. Boag. That is the difference in the Cerenkov radiation. 

 If it were active in being absorbed in the vacuum ultraviolet range, it would give 

 quite a difference. It would, infact, make gamma radiationmore efficient, as you 

 say it tends to be. 



LINSCHITZ: A factor of ten? 



ALLEN: The intensity of the Cerenkov radiation is known to be relatively 

 low. 



LINSCHITZ: I have only one more point to cover. That is the possible role 

 of triplet states in all of this. Ideally, if we want to get a complete theory for 

 the function of electronic energy in radiobiology, we should like to know such 

 things as the actual energies which are excited, the yields of each level and the 

 lifetime of each level. The latter is especially relevent. 



Some years ago triplet states, especially of complex aromatic or unsaturated 

 molecules, came into prominence through the work of G.N. Lewis and his group 

 on the spectroscopy of organic molecules in rigid solvents. The rigid medium 

 impedes radiationless quenching to the ground state and thus stabilizes the mole- 



