Speaker
Description
Fast neutron background poses a significant limitation to the signal-to-noise ratio in many neutron instruments. At the BOA beamline at the Swiss spallation neutron source SINQ, cold neutrons are reflected out of the direct line of sight from the source using a supermirror bender, while fast neutrons (>1 MeV) propagate through the beamline shielding due to their high penetration capability. To attenuate this fast neutron component, we investigated the use of a sapphire single-crystal filter.
The spatial and spectral characteristics of the fast neutron beam were determined using a fast neutron scintillator coupled to a CCD camera and an extended range Bonner sphere spectrometer. In the setup, cold neutrons were shielded using a borated neutron absorber to observe only the fast neutron component of the spectrum. Beam spectra were measured in and out of beam with and without the sapphire filter in place and analyzed with Bayesian parameter estimation. The effect of the sapphire filter on the cold and thermal components were characterized with an unshielded time-of-flight setup. MCNP and McStas simulations were performed for the BOA beamline to compare and proof the filter’s attenuation across the full energy range.
The sapphire filter demonstrated significant attenuation of epithermal and fast neutrons in the beam, while cold neutrons were only mildly attenuated. The measured attenuation in the beam is in good agreement with simulations of the sapphire filter across the energy spectrum from 25 meV to 200 MeV. Implementation of the filter in the beamline bunker led to a measurable decrease in neutron background which agrees with the simulations, improving experimental conditions and signal-to-noise ratio.
These results demonstrate that sapphire filters are a viable option for reduction of fast neutron background, enabling more efficient measurements by an improved signal-to-noise ratios in neutron scattering applications.
