imately, follows the Stern- Volmer relationship (23), as it should if the afore- 

 mentioned mechanism is operative. 



I might mention another interesting result of Jesse's studies. Although the 

 concentration of impurity required to reduce W to approximately its lowest value 

 varies somewhat from one impurity to another (corresponding to different cross 

 sections for the ionization process), the minimum value of W is more or less 

 independent of the nature of the impurity, even for polyatomic impurities. This 

 is in accord with the theoretical expectation that, in the impact of a metastable 

 excited atom and a molecule, molecular processes such as dissociation of the 

 latter are much less probable than ionization, because ionization, if it is ener- 

 getically possible, is a case of "exact resonance". 



The effect of impurities on the total ionization should be observable not only 

 in helium, but also in other gases. Jesse has actually found an effect in admix- 

 tures of CO^ with argon. This suggests that normal excited molecules -- that is 

 excited molecules of normal lifetime (in this case, argon) -- can contribute to 

 the ionization of the proper impurity (CO2) is present in the proper concentra- 

 tion. This is because the ionization potential of CO2 is greater than the excita- 

 tion energy of argon metastables. Of course, in the case of normal excited 

 molecules the concentration of impurity, at normal pressures, must be of the 

 order of magnitude of a per cent. Further, if the (directly) excited molecules 

 can dissociate, this process will be competitive with the collisional ionization. 

 However, in the case of metastables, because of their longer lifetime, only 

 hundredths of a per cent are required to change the value of W. 



All this has a strong moral in radiobiology, and it is one which even the 

 physicist has often taken too lightly -- namely, that one really knows virtually 

 nothing about the value of W to apply for tissue. Even with a wholesale amount 

 of grains of salt, it seems highly doubtful that this quantity has any present 

 significance in radiobiology. 



FANO: Thus far I know that the rare gases can be changed a lot. Has any 

 other substance been changed by a comparable amount? 



PLATZMAN: The necessary experiments have not yet been undertaken. 

 For a large effect it will be necessary to find a gas in which metastables are 

 produced abundantly by the ionizing radiation, and for which the excitation en- 

 ergy of the metastable state exceeds the ionization potential of some convenient 

 impurity. 



BURTON: Are there any substantial changes in energies per ion pair for 

 organic compounds? You know, we usually assume that they are somewhere 

 around 25 to 28 ev. 



PLATZMAN: Such measurements have not yet been made. Of course the 

 situation which I have been describing is ripe for detailed theoretical interpre- 

 tation. The basic explanation is that which I have mentioned, namely, ionization 

 of impurities by metastable excited atoms (and perhaps by ordinary excited 

 atoms or molecules in some cases). Thus far, air has not been studied, in 

 part because air forms negative ions which interfere with the measurements. 

 Incidentally, additional suspicion of the conventionally adopted value of W for 

 air is aroused by some recent measurements in Japan (24) of the Bragg curve 

 of alpha particles in air, which indicate that previous investigators often did not 

 achieve the necessary saturation of current. This was demonstrated by working 

 at very great electric field strengths, at which modification of the Bragg curve 

 was noted. 



