NEUTRON MEASUREMENTS IN NEAR-EARTH ORBIT WITH COMPTEL

The fast neutron flux in near-Earth orbit has been measured with the C OMPTEL instrument on the Compton Gamma Ray Observatory (CGRO). For thi s measurement one of COMPTEL's seven liquid scintillator modules was u sed as an uncollimated neutron detector with threshold of 12.8 MeV. Th e measurements cover a range of 4.8 to 15.5 GV in vertical cutoff rigi dity and 3 degrees to 177 degrees in spacecraft geocenter zenith angle . One of the measurements occurred near the minimum of the deepest For bush decrease ever observed by ground-level neutron monitors. After co rrection for solar modulation, the total flux is well fitted by separa ble functions in rigidity and zenith angle. With the spacecraft pointe d near the nadir the flux is consistent with balloon measurements of t he atmospheric neutron albedo. The flux varies by about a factor of 4 between the extremes of rigidity and a factor of 2 between the extreme s of zenith angle. The effect of the spacecraft mass in shielding the detector from the atmospheric neutron albedo is much more important th an its role as a source of additional secondary neutrons. The neutron spectral hardness varies little with rigidity or zenith angle and lies in the range spanned by earlier atmospheric neutron albedo measuremen ts.