the penetration of high- energy radiation through matter has usually been under- 

 taken as a by-product to more pressing problems, or, at any rate, problems 

 that the physicists considered more pressing, we have had to look for progress 

 in all sorts of strange and unexpected ways. One of the advantages we have 

 recently injoyed is a good deal of study of the penetration of many sorts of 

 radiation, of low, intermediate, and high energy, through certain types of or- 

 ganic compounds useful as solid or liquid scintillant materials. This, I think, 

 has already contributed a good deal to our understanding of the penetration of 

 high-energy radiation through condensed materials, and will without doubt con- 

 tribute much more in the future. 



Similar to this one is a second category, which I have already mentioned, 

 namely, the penetration of radiation through photographic emulsions . This 

 presents many vexing problems, and only modest progress has thus far been 

 made. 



I have saved for the last in this group of miscellaneous topics the work 

 which I consider to be the second of the two experimental projects most impor- 

 tant for radiobiology that have been performed in the past few years. This is 

 the work by Jesse, at the Argonne National Laboratory, on the inonization of 

 gases. Jesse has devoted his meticulous experimental skill to this problem 

 for a number of years, and the results have been outstanding. 



A few physicists -- not by any means a majority of them -- have been 

 suspicious for years of the validity of many of the experimental results on the 

 average energy required by high-energy radiation to form ion pairs in gases. 

 Some take every experimental result literally, including the third, fourth, or 

 fifth decimal place; others are often skeptical of the second. Jesse has made 

 a study of ionization in a variety of gases by single alpha particles; there is, 

 of course, no reason to doubt that his results have completely general validity, 

 in that the physical mechanisms they reveal are applicable to any sort of high- 

 energy radiation. He has found that many of the values in the literature are 

 wrong, not in the, fourth or fifth decimal place, but in the first or second. For 

 example, he finds that the total ionization in the rare gases is enormously sen- 

 sitive to very slight traces of impurities. To take the most striking case, 

 helium, which has been variously reported to have a value for W, the mean 

 energy per ion pair, of some 26 or 27 electron volts (ev), turns out, if it is 

 pure, to have a value of about 43 ev (21). 



I might mention in passing that this is interesting confirmation of Dr. 

 Fano's approximate theory of W. When he attacked this problem a number of 

 years ago (22), his theory gave 38 ev, whereas the experimental value at that 

 time was about 26 ev. Now it is 43 ev, which shows, at least, that his theory 

 was a little better than experiment at that time. We still do not have an accu- 

 rate theory for W in any substance. 



Jesse has studied ionization, not only in the rare gases, but in a number of 

 other inorgajuc and organic gases as well. 



Generally speaking, the sensitivity of the ionization to impurities arises 

 from the formation of excited atoms or molecules which can produce an ion 

 pair in an impact with an impurity molecule having a sufficiently low ionization 

 potential. This effect has been studied for a variety of gases, impurities, and 

 relative concentrations of the two. In the case of helium, for example, the 

 mechanism is the formation of metastable excited helium atoms, which are 

 able to ionize any impurity with great probability. Indeed, the value of W for 

 any mixture is a function of concentration of the impurity and, at least approx- 



