High-Altitude Cosmic-Ray Neutron Intensity Variations

Three groups of balloon flights carrying unshielded boron trifluoride counters were made from geomagnetic latitudes 10.1°N, 55.1°N, and 88.6°N. From the data obtained, curves of slow-neutron intensity versus atmospheric depth for depths less than 700 millibars are plotted. The mean absorption lengths for neutrons in the equilibrium portion of the atmosphere were found to be $L\left(10.1\mathrm{}°\right)=212\mathrm{}$ g/${\mathrm{cm}}^2$, $L\left(55.1\mathrm{}°\right)=164\mathrm{}$ g/${\mathrm{cm}}^2$, and $L\left(88.6\mathrm{}°\right)=164\mathrm{}$ g/${\mathrm{cm}}^2$. The depths of the neutron intensity maxima were found to be ${\theta }_{max}\left(10.1\mathrm{}°\right)=120\mathrm{}\pm 5$ mb, ${\theta }_{max}\left(55.1\mathrm{}°\right)=100\mathrm{}\pm 5$ mb, ${\theta }_{max}\left(88.6\mathrm{}°\right)=75\mathrm{}\pm 5$ mb. From these results and those of other investigators, the variation of the mean absorption coefficient ($\mu =\frac{1}{L}$) and the depth of the neutron intensity maximum are plotted as functions of the geomagnetic latitude. A family of curves of neutron intensity versus atmospheric depth is drawn for geomagnetic latitudes at 10-degree intervals between 0° and 90°N, and from this the low-energy neutron capture per square centimeter per second by the ${\mathrm{N}}^{14}(n, p)$ reaction in the atmosphere is calculated and plotted as a function of geomagnetic latitude. It is found that the observed neutron intensity varies by about 420% from 0° to 90°N. A value of 5.8×${10}^{18}$ ${\sec }^{-1\mathrm{}}$ is obtained for the total number of low-energy neutrons captured in the atmosphere.