Isotropic Neutron Flux Measurements With Comptel And The Neutron-Induced Gamma-Ray Background

A substantial part of the instrumental background in the COMPTEL gamma-ray telescope results from interactions of neutrons in its upper (D1) liquid-scintillator detector modules and nearby passive materials. The high-energy neutron flux (>12.8 MeV) has been measured in orbit by using a single D1 module as an isotropic neutron detector. Four measurements of this sort have been carried out, lasting a total of about four hours. These measurements covered nearly the total range of vertical cutoff rigidity and spacecraft geocenter zenith angle encountered by COMPTEL. Two of the measurements were taken near solar maximum in 1991, with one near the bottom of the record Forbush decrease in July of that year. The other two were taken in 1994, near solar minimum, so that the effect of solar modulation can be determined. The strength of the 2.2 MeV line in COMPTEL gamma-ray spectra, produced by the absorption of thermal neutrons in the liquid scintillator, serves as a proxy for the low-energy neutron flux. High- and low-energy neutron fluxes show similar dependencies on rigidity and zenith angle. The flux varies exponentially with rigidity over the observed range of 4 to 16 GV and linearly with zenith angle. At the time of the Forbush decrease a flux contrast of a factor of 4 was seen between rigidities 4.4 and 13.5 GV; a factor of 5 contrast was seen at the time of solar minimum; with a higher overall flux. The flux varies linearly with zenith angle, being nearly a factor of two greater when the spacecraft is pointed at the nadir than when it is pointed at the zenith. This shows that shielding of COMPTEL from atmospheric albedo neutrons by the mass of the spacecraft is much more important than production of secondary neutrons in that mass. When the spacecraft is pointed at the nadir, the neutron flux is consistent with atmospheric neutrons alone. The instrumental gamma-ray background is estimated by combining the neutron flux with results from the calibration of a COMPTEL prototype in a neutron beam at energies 17 to 200 MeV. This instrumental background accounts for most of the difference between COMPTEL's measurement of the cosmic-diffuse gamma-ray flux and earlier measurements at a few MeV, largely eliminating the "bump" in the cosmic-diffuse spectrum.