Instrumental design and performance of a new pulsed-neutron reflectometer (ARISA) at KENS for studying free surfaces

A new neutron reflectometer (ARISA) for studying free surfaces was installed at one of the thermal neutron ports viewing an ambient-temperature water moderator at the KENS pulsed spallation neutron source. ARISA is quite a unique reflectometer; it uses thermal neutrons at the pulsed neutron source, and it is the first one in Japan with a vertical scattering-plane geometry. The neutron beam is guided downward onto the sample surface at angles of 0–0.47° and 1.4° to the horizontal. Through several test measurements using different types of samples, it was clarified that specular reflection can be currently measured down to 10-6 in reflectivity and up to 3 nm-1 in neutron momentum transfer along the vertical direction using white neutrons with a wavelength of 0.1–0.4 (0.6) nm. This performance could be further improved by future reinforcement of the shield for background reduction.