Cosmic-Ray Measurements with a Small Ionization Chamber I. Variation with Altitude and Latitude of the Total Ionization for Various Shields

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Abstract
Ionization data have been obtained with a small ionization chamber (230 cc volume) at three corresponding altitudes (76, 51, and 45 cm Hg) in northern and equatorial latitudes, for shielding thicknesses up to 19.4 cm Pb. It is found that within limits of error the latitude ionization ratios, northern: equatorial, are independent of shield at each elevation studied. The values of the latitude ratios are: 1.16 at 76 cm Hg, 1.27 at 51 cm Hg, and 1.30 at 45 cm Hg. An analysis of the lead absorption curves has been made in terms of a penetrating ionizing radiation and a softer secondary radiation. By comparing the ionization data with counter data on the absorption of vertical rays and on showers, available at two altitudes (76 and 51 cm Hg), an estimate has been made of the individual contributions of the penetrating and secondary rays to the measured ionizations. The analysis shows that the ionization due to secondaries associated with the soft component is to be identified with the shower radiation. The contribution of secondaries is important, even under thick shields. For 19.4 cm Pb it amounts to 50 percent of the total ionization at 51 cm Hg and to 20 percent at 76 cm Hg. The curves of intensity of secondary rays vs. thickness of lead indicate a second maximum for thick shields.