A measurement of the Flight Path 12 cold H2 moderator brightness at LANSCE

We have measured the brightness of the Flight Path 12 upstream/back-scattering partially coupled cold hydrogen moderator at LANSCE. This measurement was performed in the neutron energy range of 0.8–76.9 meV by using a time-of-flight technique in conjunction with a two-pinhole collimator system. Cold neutrons were observed with a redundant 6Li-loaded glass scintillation detector having an efficiency of close to unity. The detector viewed an area of 0.93 cm2 on the center of the 12×12 cm2 moderator surface. A maximum brightness of 1.25×108 neutrons s−1 sr−1 cm−2 meV−1 μA−1 was measured at 3.3-meV neutron energy. Statistical and systematic errors of the measurement are 3% and 7%, respectively. The measured brightness is compared to the result of a model calculation and there is a significant disagreement, by a factor of 1.5 at the peak. The performance of the m=3 supermirror neutron guide system on Flight Path 12 was also studied with the two pinhole-detector system.