20 



BURTON: You just don't have a sufficiently complicated molecule in that 

 case. The hydrogen atoms are easily expelled so that the persistent highly ex- 

 cited state does not exist in this case. Thus the picture that Dr. Kasha suggest- 

 ed appears highly plausible in the case of organic molecules. 



ALLEN: In regard to the difference between the effects of ultraviolet and 

 ionizing radiations, the talk has been about high-energy fluorescence as an ex- 

 planation. I think we should not forget the possible geometric effects. In the 

 high energy radiation you have excitations occurring to groups very close to- 

 gether, and these may very well interact with each other, whereas the isolated 

 ones you get with the ultraviolet will not. 



POLLARD: That is why it is so important to know where they are. 



ALLEN: Exactly. 



TOBIAS: I wish to make a brief remark on the status of investigations with 

 multiply-charged ion beams at the sixty-inch cyclotron of the University of Cali- 

 fornia in Berkeley (2). Miller, Putnam and Rossi, working with Hamilton, have 

 succeeded in obtaining a fairly intense internal carbon (6 + ) beam and a moderate 

 external beam. Miller (33) has recently completed a Ph.D. thesis, working with 

 McMillan on reactions of fast carbon nuclei in photographic emulsions. He has 

 considered electron exchange, and he came to the conclusion that the Bragg 

 curve is not very much influenced by this effect. Further, Barkas (34) has made 

 a study of a number of light ions, such as lithium, beryllium and boron, which 

 may be obtained from nuclear transmutations. The range-energy relationship of 

 these nuclei was evaluated near the end of their range, yielding important data 

 for electron exchange. 



PLATZMAN: What energy do the recoils have? 



TOBIAS: A few Mev per nucleon. The protons producing the interaction 

 may have as much as 340 Mev. The ions themselves are products from inter- 

 actions on various elements, e.g. , carbon or beryllium. They usually recoil 

 at lower energies than the initial proton energy and are selected for momentum 

 in a magnetic field combined with a slit system. Barkas determined the range 

 correction in photographic emulsions due to the part of the track where electron 

 exchange is important, and found this proportional to Z-*. Returning now to the 

 carbon ions, perhaps it may be of interest to you that the 120-Mev carbon ions 

 are now in use for radiobiological research. We have had experiments in which 

 the dose rate from this beam was as high as 3000 r.e. p. per minute. 



FANO: What sort of beam current do you have? 



TOBIAS: The external beam current is about 3x10" amperes; the inter- 

 nal beam, which may be used in transmutation experiments, is more than 1000 

 times greater. The energy is about 120 Mev. Actually the particles are not 

 monoenergetic, and if you wish to select a monoenergetic beam, you have to 

 sacrifice intensity. The range of these particles in tissue is about 50 microns, 

 so that the particles are suitable for studies on microorganisms, liquid systems, 

 and surface layers of tissues. 



FAILLA: Do you have any idea of the RBE of the carbon ions? 



TOBIAS: The only organism for which sufficiently complete data are now 

 available is the haploid yeast cell. The RBE for this organism increases with 

 specific ionization (35), and for polonium alpha particles it is about 2.5 (when 

 compared to X rays). In our group Ann Birge and Joseph Sayeg (36) performed 



