An Alpha-Gamma Counter for Absolute Neutron Flux Measurement

An alpha-gamma counter was used to measure the absolute neutron flux of a monochromatic cold neutron beam to sub-0.1\,\% precision. Simultaneously, the counter was used to calibrate a thin neutron flux monitor based on neutron absorption on ${}^{6}$Li to the same precision. This monitor was used in the most precise beam-based measurement of the neutron lifetime, where the limiting systematic effect was the uncertainty in the neutron counting efficiency (0.3\,\%). The counter uses a thick target of ${}^{10}$B-enriched boron carbide to completely absorb the beam. The rate of absorbed neutrons is determined by counting 478 keV gamma rays from neutron capture on ${}^{10}$B with calibrated high-purity germanium detectors. The calibration results and the implications for the neutron lifetime will be discussed.