Solar Minimum Measurements of Fast Neutrons at High Altitude

Several rocket and balloon measurements of neutron intensity above Fort Churchill, Canada, were made during 1964 and 1965. The detector in each case was an anthracene–plastic "phoswich" using pulse-shape discrimination to distinguish between neutrons, gamma rays, and charged particles. Neutrons were recorded in the energy range 2–11 MeV. Measurements indicate that the albedo neutron flux is somewhat lower than theoretical predictions but is in agreement with reasonable extrapolations to zero atmospheric depth of measurements within the atmosphere made by other workers. The exponent of the differential energy spectrum appears to change from −1.4 at large atmospheric depths to −0.8 outside the atmosphere.