LXIII. Nuclear transmutations produced by cosmic-ray particles of great energy.—<i>Part V</i>. The neutral mesons

Summary The spectrum of the γ-radiation in the atmosphere at 70,000 ft. has been determined by observations on the scattering of pairs of fast electrons recorded in photographic emulsions exposed in high-flying balloons. The detailed form of the spectrum is consistent with the assumption that the γ-rays originate by the decay of neutral mesons. It is found that the mass of the neutral mesons is 295±20 m e , and that they are created in nuclear explosions with an “energy spectrum” similar to that of the charged π-particles. A method is described for determining the lifetime of the neutral mesons and their frequency of occurence compared with charged π-particles. The lifetime, τ π 0, is less than 5×10−14sec. It may be possible to determine the lifetime, by observations of greater statistical weight, if it is longer than 2×10−14sec. The ratio of the number of neutral mesons to charged mesons, produced in nulcear explosions of great energy, is equal to 0·45±0·10.