A Polarised-Target Neutron Flux Monitor Using a Spin-Polarised Helium-3 Cell

Beamline neutron flux is usually monitored by fission chambers or activation foils, instruments that report a count but carry no spin information. A spin-polarised helium-3 cell monitors flux and polarisation together, because its capture cross section depends on the relative spin orientation of neutron and nucleus. This paper specifies such a monitor: a one-bar, two-centimetre cell at a polarisation of 0.70, with a computed thermal-neutron capture efficiency of 23.2 per cent over a 25-square-centimetre aperture, a count rate of 5.79e+05 per second at a reference flux of 1e9 per square metre per second, and a flux precision from counting statistics and the analysing asymmetry of 4.38e-05 relative in one hour. The usable flux range spans seven decades. SIMULATED, NOT MEASURED: the figures are computed from stated assumptions.