Calibration of a space thermal/epithermal neutron detector: The Mars Observer Gamma-Ray Spectrometer anticoincidence shield

Procedures used for, and results of, a calibration of the combined anticoincidence shield and neutron detector (A/C-N subsystem) of the Mars Observer Gamma-Ray Spectrometer (MOGRS) are reported. Pulsed energy-dispersed neutrons were generated by directing a chopped deuterium beam from the Los Alamos Van De Graaff accelerator onto a thick Be target enclosed within a specially designed neutron moderating assembly. Absolute energy and angle-dependent detection efficiencies were determined for energies in the range between 0.05 eV and 2 keV and for incident angles between 0° and 150°. A ⁶Li glass detector and a uranium ion chamber were used as calibrated references for the absolute efficiency determination, which was checked at 0.05 eV using neutrons provided by a large graphite neutron pile, also at Los Alamos. The results are found to be close to those calculated using a Monte Carlo Neutral Particle transport code for an isolated plate of BC454 scintillator (the active sensor of the A/C-N subsystem) with two notable exceptions. First, the measured efficiency below about 0.5 eV is less than the simulated one; second, the measured efficiency decreases more slowly with increasing energy than does the simulated efficiency. Although the actual detector is too complicated to be simulated accurately, we speculate that the relative reduction in efficiency below 0.5 eV may be caused in part by (i) reduced neutron transmission through the material of the sensor head assembly outside of the BC454 pyramid, (ii) reduced effective sensor area because of poor light-collection efficiency around the perimeter of each of the trapezoid plates, and (iii) molecular binding effects of hydrogen in both the BC454 scintillator and in its 1-mm carbon-epoxy cover. We also speculate that the increase in efficiency at about 0.5 eV and the general enhancement of detection efficiency at all higher epithermal energies may be due to the multiple scattering of incident neutrons in materials present in the MOGRS sensor head.