Modernization of the small-angle neutron scattering spectrometer SANS-U by upgrade to a focusing SANS spectrometer

The small-angle neutron scattering spectrometer SANS-U at the research reactor (JRR-3) of the Japan Atomic Energy Agency, Tokai, Japan, has been successfully upgraded. This major upgrade was undertaken in order to install a high-resolution position-sensitive detector consisting of a cross-wired position-sensitive photomultiplier tube combined with a ZnS/⁶LiF scintillator on the SANS-U spectrometer. Without changing the total length of the spectrometer, the aim was to extend the accessible low-Q limit (Q is the magnitude of the scattering vector) and to shorten the measurement time by employing focusing small-angle neutron scattering (FSANS). By using both spherical MgF₂ biconcave lenses and the new high-resolution position-sensitive detector, the accessible low-Q limit was extended from 2.5 × 10^-3 to 3.8 × 10^-4 Å^-1. As a result, SANS-U can continuously cover a wide Q range from 3.8 × 10^-4 to 0.35 Å^-1 with a wavelength of 7 Å. FSANS can be utilized not only to improve the accessible low-Q limit but also to increase the intensity of incident neutrons passing through the sample in the conventional Q range from 2.5 × 10^-3 to 0.35 Å^-1. The use of `high-intensity' FSANS also allowed a reduction of the measuring time by approximately 1/3.16 by increasing the incident neutron intensity.