Performance of the vertical optical filter for the NG-3 30m SANS instrument at the National Institute of Standards and Technology's Center for Neutron Research

Abstract

The straight neutron guide and crystal filter formerly used to supply a cold neutron beam to the NG-3 30m small angle scattering instrument at the National Institute of Standards and Technology Center for Neutron Research has been replaced by a vertically-kinked “optical filter” neutron guide that eliminates direct lines-of-sight between the instrument and the neutron source. Due to pre-existing lateral spatial constraints, the optical filter bend is in a vertical plane requiring a vertical displacement of the sample-detector axis by about 14cm. The optical filter is successful in excluding unwanted fast neutrons and gamma rays from the beam at the sample position without the use of crystal filters. We show that the optical filter provides neutron current density gains at the sample by a factor of about 1.8 at 15Å neutron wavelength with negligible increase in the beam divergence, whilst allowing some measurement capability at wavelengths shorter than 4Å (previously excluded by the beryllium–bismuth crystal filter).