PENETRATION EFFECTS OF LOW-ENERGY PARTICLES 171 



ultimate sums for energy loss, must be of the utmost importance in the 

 formulation of interpretations of mechanisms of radiation action. They 

 may even have serious repercussions in dosimetry — for instance, in the 

 conversion of density of energy loss to density of "ionization." This is 

 true for all types of high-energy radiation, including x-rays, and there 

 may exist unknown dependencies on energy or character of radiation. 

 Even the common practice of equating two different types of radiation, 

 or radiations of different energy, on the basis of equivalent total gaseous 

 ionization, is suspect. The possible anomalies of stopping power may 

 enter too, of course (for instance, in the conversion of roentgens to den- 

 sity of energy loss). The possibility of serious error in conventional 

 means of treating these problems based on gas-like models would appear 

 to demand careful study of the problems here posed. 



IX. Penetration Effects of Low-Energy Particles 



Very little can be said at the present time about the energy loss and 

 ionization by particles of velocity of the order of magnitude of Vq or less 

 except in emphasis of the fact that very little indeed is known. To this 

 circumstance has already been attributed the impossibility of any sort 

 of theoretical calculation of the stopping power of water — liquid or 

 vapor — for heavy charged particles of low energy, and the consequent 

 importance of realizing that theoretical ranges such as those in Fig. 2 

 are uncertain by a constant additive amount (probably at most of order 

 of magnitude 2 microns) at higher energies and virtually meaningless at 

 low energies (below about 200 kev for protons). 



The lack of understanding of penetration phenomena of low-energy 

 particles in gases is enhanced in the case of a condensed medium. Spe- 

 cific effects of importance in liquid water are as follows : 



1. Capture-and-loss of electrons will be different in liquid and vapor 

 states, since these phenomena are very sensitive to details of the bind- 

 ing of the valence electrons. 



2. Energy loss by excitation of vibrational and rotational degrees of 

 freedom plays an important role at low velocities. 



3. The polarization of the medium by the penetrating particle (Fermi 

 effect) influences the energy loss at low energies. 



4. The details of electronic binding are of paramount importance for 

 the energy loss to electrons of the medium by slowly moving particles. 

 Thus the Bragg rule will break down completely at suflSciently low ve- 

 locities. This tendency has been discussed in Section IV. For example, 

 it is responsible for the familiar and important fact, already mentioned, 

 that the total ionization of gases by high-energy radiations is strongly 



