Speaker
Description
In spallation neutron sources, pre-moderators surrounding coupled cryogenic moderators helps thermalizing fast and epithermal neutrons before they reach the cold volume, directly influencing the emitted cold-neutron intensity and spectrum of beam to be extracted. At the Spallation Neutron Source (SNS), the coupled hydrogen moderator currently operates with a water pre-moderator, whose scattering kernel is effectively fixed under normal operating conditions. Replacing water with a solid, hydrogen-rich pre-moderator introduces additional design flexibility. Because the thermal scattering law (TSL) of a crystalline or molecular solid is governed by its phonon spectrum and vibrational density of states (VDOS), changing the pre-moderator temperature alters the double-differential scattering cross sections which impacts neutrons thermalization. This work investigates candidate solid pre-moderators and evaluates temperature as an additional design and operational degree of freedom, examining the impact on the neutron spectra emitted by a coupled hydrogen moderator system.
A detailed Monte Carlo model representative of the SNS coupled hydrogen moderator assembly is employed, incorporating the proton target region, all four SNS moderator types, the coupled beryllium reflector, the inner reflector plug (IRP) with general key features of the as-built geometry, and representative instrument viewing geometry. Beginning from the baseline water pre-moderator configuration, a systematic parametric study of candidate solid materials is carried out, quantifying their impact on moderator emission as observed at detector and beamport locations. The study considers both material selection and the underlying scattering physics, along with the operating parameters of temperature and thickness. Candidate pre-moderators like LiH, YH₂, ZrH₂, BeH₂, MgH₂, and liquid NH₃, are selected to cover a range of hydrogen number densities and elastic/inelastic scattering characteristics. For LiH, YH₂, and ZrH₂, evaluated TSL data from ENDF/B-VIII.1 are used, including mixed elastic scattering with both coherent and incoherent contributions. For BeH₂, MgH₂, and NH₃, where evaluated TSL data are limited, new scattering kernels are generated from the VDOS, literature-derived in the case of BeH₂, MgH₂, and computed in-house for NH₃. Each material is assessed at six temperatures (20 K, 77K, 100K, 200 K, 293.6 K, and 400 K) and across multiple pre-moderator thicknesses. Comparison metrics include time-integrated energy-dependent brightness at representative viewing positions, a cold-to-thermal spectral index defined as the ratio of integrated cold to thermal brightnesses, and pulse width at full width at half maximum (FWHM) relevant to time-of-flight instrument performance.
Thermal scattering data processing is carried out with both NCrystal and NJOY/LEAPR, enabling independent generation of thermal ACE libraries through two distinct routes. A complementary Python-based verification framework automates TSL and ACE production, supports cross-checking of ENDF and ACE representations, and provides consistency tests between the two processing routes. Beyond material selection, this study explores the temperature of the solid pre-moderator as a potentially controllable parameter for influencing cold and thermal spectra and instrument-level brightness, a degree of freedom not available when liquid water serves as the pre-moderating medium at standard operating conditions. The results indicate that operating a solid pre-moderator across the 77–400 K temperature range produces expected variations in the spectral output, suggesting a possible pathway for spectral tuning in current and future coupled cold neutron sources without modifying the hydrogen moderator system itself. Practical considerations, including radiation damage, heat removal, and stored-energy accumulation in the solid pre-moderator under spallation-relevant fluences, are also noted as important factors for implementation.