Application of a Randomly Operated Large Wilson Cloud Chamber for the Determination of the Mass of the Meson

Abstract

DURING recent years, increasing interest has been shown in the accurate determination of the mass of mesons, which have been supposed to constitute the penetrating component of cosmic rays. The most direct evidence that such particles have an intermediate mass lying between that of an electron and a proton comes from cloud chamber observations. The momenta of the particles and ionization they produce are directly available from the measurements on the cloud chamber tracks, and are found to be compatible with theory only when the particles are assumed to possess a mass roughly equal to 200 times the mass of an electron. An accurate determination of the mass is possible only when the particle can be photographed near the end of its path through space, and this occurs rarely. Slow mesons seem to be particularly rare in counter-controlled photographs with a thick lead plate inserted inside the chamber, apparently because (1) they have extremely short range in heavy material, (2) they have a large probability of decay, nuclear absorption and transformation into neutrettos¹. Hence they fail to trip the lower counter of the coincidence system.