The local neutron flux at low Earth-orbiting altitudes

The COMPTEL instrument onboard the Compton Gamma Ray Observatory (CGRO) has been used to determine the dependence of the atmospheric neutron flux below 5 MeV as a function of vertical cutoff rigidity and spacecraft orientation at an altitude of 450 km. The instrumental 2.2 MeV line, arising from thermal neutron capture on hydrogen, was used in the measurement. For the COMPTEL orbit the vertical cutoff rigidity ranges from 4.5 GV to 15.5 GV, while the spacecraft geocentre zenith and the geocentre azimuth range from 0° to 180° and from 0° to 360°, respectively. The dependence of the 2.2 MeV rate on rigidity and geocentre zenith can be described by an analytic function: the line rate varies linearly with geocentre zenith, and exponentially (or power law) with the vertical cutoff rigidity. The flux varies on average by about a factor of 3.7 between the extremes in rigidity, and by a factor of 1.7 between the extremes of spacecraft orientation with respect to the Earth. No dependence of the low-energy neutron flux on the spacecraft geocentre azimuth could be found. We believe that mass shielding is more important in attenuating the atmospheric albedo than as a source of secondary neutrons. Therefore the COMPTEL instrument is well suited for a longtime study of the dependence of the low-energy neutron flux on the vertical cutoff rigidity and phase of the solar cycle.