XV. ELECTRONS, NEUTRONS, AND ALPHA PARTICLES 503 



found most informative to study some of the very beautiful original 

 pictures taken by Wilson and others (22). 



Characteristically, electron tracks are thin and tortuous, indicat- 

 ing that the ions are widely spaced and that the electron is easily de- 

 flected from its path, while proton and a-particle tracks are thick and 

 straight. The difference between the degrees to which the light and 

 heavy particles are deflected from a straight path is an intrinsic prop- 

 erty associated with their mass but is not usually important, since in 

 all but the slowest electrons large changes in direction are infrequent 

 on the scale of cellular structure. The remaining difference between 

 the light and heavy particles — the spacing of the ions along the 

 track—is not an intrinsic property of the particles at all but a func- 

 tion only of their speed and of the total charge of either sign that they 

 carry. Physically both primary ionization and excitation produced 

 in air by a proton and electron of the same speed are practically 

 indistinguishable. The reason for this is well understood in terms 

 of the forces of attraction and repulsion that exist between the charged 

 particles and the orbital electrons of the atoms through which they 

 pass, and justifies the treatment of these two particles as entirely 

 equivalent from the radiobiological standpoint. The a particle, 

 which carries two positive charges, produces four times as many ions 

 per micron of track as a proton of the same speed. Fission fragments 

 that are multiply charged are more densely ionizing still. The aver- 

 age energy lost by fission fragments per ion pair formed in air, how- 

 ever, is practically the same as for protons and a particles (23). It 

 appears, therefore, that even in the case of these very densely ionizing 

 particles the proportions of ionization and excitation remain un- 

 (!hanged, so that from a biological standpoint we have only to take 

 account of the compression of much more ionization and excitation 

 into a given length of track. 



Evidently, these considerations introduce an immense simplihca- 

 tion into the analysis of radiobiological problems for, from the stand- 

 point of the physical and chemical processes taking place within the 

 cell, we can forget the multiplicit}- of radiation sources and fix atten- 

 tion on a single parameter — the mean linear ion density of the ioniz- 

 ing particles — which is a measure of the spacing of the ionization and 

 excitation along the track of the particles that traverse the cell. This 

 parameter varies with the speed of the particle. It is least for very 

 fast particles and greatest for particles traveling slowly and nearing 

 the end of their range. The lower cui'\'e of Figure la shows this varia- 



