Cloud-Chamber Study of Electrons from Meson Decay

which have penetrated a considerable layer of absorbing material in the laboratory. If such particles w'ere produced by an exchange process, their detection would depend on the life of the neutral particle and the frequency of occurrence of the exchange. Recently de Vos and du Toit~ reported an effect which they ascribed to neutral particles in incident cosmic rays detected by means of parafFIn placed either above or below the first tube of a coincidence counter. In the present experiment measurements were made to see if the ionizing rays which have already passed through the first tube of a coincidence-anticoincidence arrangement would produce an effect in iron or lead which would then be sensitive to the presence of a light substance such as paraffin. Measurements with iron were of interest because of the observations that negative mesons absorbed in iron do not give evidence of ionizing decay products. Four Geiger-Mueller tubes 18X2~ inches were used in a vertical array. The second tube from the top was connected in anticoincidence. Layers of iron, lead, and parafFin were used in thicknesses of 10, 7, and 3.7 centimeters, respectively. Above the lowest tube 7 centimeters of lead served to insure that the rays which were counted after passing through the absorbing material were penetrating rays, presumably mesons. Kith lead or iron above the anticoincidence tube and parafFIn below it, as many as 20 percent more counts was registered than when the position of the heavy absorber and paraffin were reversed. Comparison measurements with layers of one material in different positions, however, indicated that these large differences were mostly due to scattering and absorption depending on the position and nature of the material. The differences in counts which were obtained were always in the direction to be expected if a neutral ray from iron or lead passed through the anticoincidence tube and produced an ionizing ray in the parafFin, but it is obvious that the probability of such a twofold occurrence would be small unless the separate probabilities were fairly large. Because of low counting rate, in spite of large tubes, the measurements were made over a period of several months and the total counts for each of a considerable number of combinations was of the order of 600. Upon subtracting the corrections for scattering and absorption a small effect remained which was of the order of magnitude of the statistical error. However, in repetitions of the measurement this difference was always in the same direction and could be explained by the presence of neutral mesons. Since the heavier meson is now thought more probably to interact with nuclei, the effect may not be measurable with the lighter meson. It may be concluded from the measurements that exchange, if present, is small but not completely ruled out. On account of the size of the tubes and their arrangement, shielding from side showers and accidental counts was impractical. A more exact check of the possibility of the above exchange phenomena might be made if adequate shielding could be provided.