Latitude survey of the frequency of multiple events in an airborne neutron monitor

To obtain information concerning the energy dependence of the average number of evaporation neutrons produced by a nucleon incident on a neutron monitor, measurements were obtained with a multiplicity detector operated during the epoch of solar minimum aboard the U.S. Naval Oceanographic Office worldwide survey aircraft (Project MAGNET). The latitude and altitude variations of events with different multiplicities were determined. The data indicate, for example, that over the range of threshold rigidity 3–13 GV, the latitude effect for threefold events is 1.7 ± 0.1. This is intermediate between the corresponding results obtained with the conventional neutron monitor and a semicubical meson telescope that was also carried aboard the aircraft. The atmospheric attenuation length for high-multiplicity events is less than for events in which a single neutron is detected, consistent with a slightly increasing inelastic nucleon–nucleus cross section as the nucleon energy increases toward 1 GeV.