Title:              The local neutron flux at low Earth-orbiting
                     altitudes.
 Authors:            WEIDENSPOINTNER,G.; BENNETT,K.; VAN DIJK,R.;
                     KAPPADATH,S.C.; LOCKWOOD,J.; MORRIS,D.; RYAN,J.;
                     SCHOENFELDER,V.; VARENDORFF,M.
 Journal:            Astronomy and Astrophysics Supplement, v.120,
                     p.631-634
 Publication Date:   12/1996
 Origin:             A&A via CDS
 A&A Keywords:       ATMOSPHERIC EFFECTS, INSTRUMENTATION: DETECTORS
 Abstract Copyright: (c) 1996: Astronomy & Astrophysics
 Bibliographic Code: 1996A&AS..120C.631W

                                  Abstract

The COMPTEL instrument onboard the Compton Gamma Ray Observatory (CGRO) has
been used to determine the dependence of the atmospheric neutron flux below
5MeV as a function of vertical cutoff rigidity and spacecraft orientation at
an altitude of 450km. The instrumental 2.2MeV line, arising from thermal
neutron capture on hydrogen, was used in the measurement. For the COMPTEL
orbit the vertical cutoff rigidity ranges from 4.5GV to 15.5GV, while the
spacecraft geocentre zenith and the geocentre azimuth range from 0{deg} to
180{deg} and from 0{deg} to 360{deg}, respectively. The dependence of the
2.2MeV 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
long-time study of the dependence of the low-energy neutron flux on the
vertical cutoff rigidity and phase of the solar cycle.