Using B${\mathrm{F}}_3$ proportional counters, containing boron enriched in ${\mathrm{B}}^{10}$, imbedded in a paraffin block 15×15×17 inches, as a moderator of fast neutrons, the intensity of such neutrons generated in the atmosphere by cosmic rays has been measured at a number of altitudes between 5000 and 13,000 feet above sea level. These locations were all along the same geomagnetic latitude of 20° 32′ N in Kashmir. The intensity thus measured was found to increase approximately in an exponential manner with decreasing atmospheric pressure under the conditions of the experiments, in general agreement with observations made by other investigators in the free atmosphere, giving an attenuation length in the atmosphere of 128 g/${\mathrm{cm}}^2$.

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