208 BELL SYSTEM TECHNICAL JOURNAL 



Ability to penetrate matter, inability^ to make showers: these are 

 the complementary aspects of the property which distinguishes this 

 other type of particle, the mesotron. If one wishes to contrive a 

 particle having this property and differing otherwise as little as possible 

 from the electron, how must it be done? The electron has the qualities 

 of charge and mass; also those of spin and magnetic moment, but 

 these are considered (perhaps wrongly) to be little or not at all con- 

 cerned with shower-production. If we imagine the mass to be 

 increased while the charge remains the same, the liability to Brems- 

 strahlung will diminish; for Bremsstrahlung occurs when sudden sharp 

 deflections or decelerations occur, and these are less sharp and sudden 

 the more massive the particle is. Now Bremsstrahlung is the prelude 

 to the entire manifold process of the forming of a shower, and hence a 

 mere increase in the mass of the hypothetical particle leads in the 

 desired direction. The theory indicates that a particle with the 

 electronic charge and a few dozen times the electronic mass will be 

 penetrating enough. We do not need, however, to be contented with 

 such vague intimations, for there is yet another phenomenon in 

 respect of which the mesotron differs from the electron, and from this 

 the mass can be deduced more sharply. 



So far, we have been considering the passages of particles through 

 solids. There, the paths are concealed, the adventures of the particles 

 can only be inferred — from the difference between energy before and 

 energy after traversal, or from the photons and the secondary electrons 

 which are driven out of the solid. Now we are to consider the passages 

 of charged particles through the gas of the Wilson chamber, which, 

 unlike the scriptural way of the eagle through the air, are preserved 

 for our inspection by the droplets. Figure 1 has shown to us a track 

 in which the number of droplets in unit length of path can rather 

 readily be counted. What does this number signify? And is it truly 

 an indication of the mass oT the traveling particle, as I hinted on an 

 early page? 



The latter question might perhaps be sufficiently answered without 

 reference to the former; but for completeness, and for the sake of its 

 own interest, the former ought to be treated more fully than it was in 

 that brief earlier mention. In the voyage recorded in Fig. 1, nothing 

 so drastic happened to the traversing particle as would have been the 

 losing of a large part of its energy in the form of a photon of Brems- 

 strahlung. It lost its energy in driblets, spent in detaching electrons 

 from molecules and giving them a small extra bonus of kinetic energy 



^ It is better to say "relative inability" since occasional showers are attributed to 

 mesotrons, which perhaps operate by making a violent impact on an electron and so 

 giving it the energy needful for starting the process. 



