The neutron performance of a pre-moderated beryllium reflector-filter hydrogen moderator system for the Manuel Jr. Lujan Neutron Science Center

Demonstration of a single-crystal reflector-filter for enhancing slow neutron beams

“…While the cold neutron emission increase at wavelengths above the Bragg edge is significant (as much as a factor two in the right geometry, such as at the Lujan Center [1,7,8]) the major problem with a polycrystalline reflector-filter is the suppression of neutrons with wavelengths below the Bragg edge at 4 Å but still within the slow neutron range useful for neutron scattering. These 1-4 Å neutrons are essential to many instruments, even those primarily considered to be "cold neutron instruments.…”

“…As such, the reflector-filters tested there did not distinguish between the gains arising from a (partial) reflector assembly, such as is now common [5], and a reflector-filter blocking only the outgoing beamline within the neutron reflector. Studies in Japan [6] and at the Lujan Center [7,8] on the beryllium (polycrystalline) reflector-filter reinforce this distinction. The optimum thickness for a polycrystalline beryllium reflector in the unreflected Kurchatov measurement is 20-40 mm, and gives a factor of two low-energy intensity gain [3].…”

“…An even thicker beryllium reflector filter (50 mm) was tested in Japan and found to provide no low-energy gain [6], which those authors attribute to the fact that their test assembly was already fully reflected. At the Lujan Center, the successful deployment of a beryllium reflectorfilter in a fully reflected system, which did achieve that factor two gain at low energies, required a much thicker reflector-filter of 120 mm [7,8]. This study describes a set of experiments carried out at the Low Energy Neutron Source [9][10][11] (LENS) between 21 April 2014 and 11 May 2014.…”

Neutronic design for ESS-Bilbao neutron source

“…Studies carried out at the source at Los Alamos National Laboratory [27] showed that a significant increase of brightness of flux-trap moderators would result if a beryllium filter was set close to the moderator surface. The reason for such a rise stems from the very low scattering cross-section for neutrons below 5.6 meV of Be below 80 K, whereas at room-temperature its cross-section increases up to a considerable value.…”

“…Therefore, considering only the production of cold neutrons, the introduction of the beryllium filter represents a significant improvement. The price to pay results in a more complex layout as well as the need of a somewhat more complicated cryogenic system [27].…”