12 



PLATZMAN: Dr. Fano mentioned a remarkable precision. Was any great 

 advance made since the experiments of Mather? 



TOBIAS: I think the chief experimental advance was in the technique of 

 ionization measurements. The problem is to measure the ratio of ionizations 

 at two different points in the Bragg curve. This is done with two ionization 

 chambers. A number of combinations of absorbers are placed between the two 

 ion chambers. Now it is difficult to obtain the two ionization measurements 

 since the proton beam shows considerable intensity fluctuations. Thompson 

 built a ratio meter, an instrument which measures the ratio of ionizations in 

 two ion chambers instead of the actual values. The ratio is independent of beam 

 fluctuations. He could also make measurements with several combinations of 

 absorbers without shutting the beam off. 



BOAG: Thompson claims accuracy of the order of 0. 1 per cent, and demon- 

 strates that the stopping power of compound molecules can be calculated with 

 an error much less than one per cent from the atomic stopping powers of the 

 constituent atoms, provided different values are assigned to the stopping power 

 of H in saturated and unsaturated compounds, and some similar allowances 

 made for the type of chemical binding of other light elements. 



PLATZMAN: The percentage deviation from the Bragg rule is smaller at 

 high energy, of course. 



FANO: Yes. 



PLATZMAN: The point is that if there were a deviation from the Bragg 

 rule of one per cent at very high energy, it would be considerably greater at 

 low energy. 



TOBIAS: A small difference detected with 340 Mev protons should become 

 greater with say 1 Mev protons. In the latter case the experimental technique 

 is much more difficult, however. 



PLATZMAN: Did he measure the stopping power of liquid water? 



BOAG: He used liquid oxygen and liquid hydrogen as well as liquid water? 



TOBIAS: Yes, and also liquid nitrogen. 



PLATZMAN: At least for gases, information on lower energy particles is 

 provided by the Cal. Tech. work which I mentioned. Has Thompson compared 

 his results with these? 



TOBIAS: He did not make a comparative study. The values from Thomp- 

 son's high energy work given in Table II may be of interest. 



TABLE II 



RELATIVE STOPPING POWER AND MEAN IONIZATION POTENTIAL 

 OF VARIOUS ATOMS AS AFFECTED BY CHEMICAL BINDING 



Type of 

 Element Compound S I 



Hydrogen Saturated 0.04797+0.00007 15.5 ev 



Unsaturated 0.04879+0.00010 13.0 



