Neutron pulsed optimized moderation

For more than four decades thermal neutrons have been increasingly employed to probe condensed matter microstructure and dynamics within a variety of disciplines. The advent and foreseen diffusion of pulsed neutron sources has led to propose the present study of an optimal manner to moderate a pulse of fast neutrons aimed at yielding the highest possible neutronic leak at thermal energies, while introducing the lowest time spread of that pulse. For this purpose, experimental studies have been carried out comprising slow neutron population decay, time of flight spectra, and intensity and time resolution analysis through neutron diffraction by means of a diffractometer built by the author. Based on the attained understanding of the multiple scattering processes involved, numerical simulations were later performed by means of a locally developed Monte-Carlo program which employs the Synthetic Model for the generation of incoherent cross sections conceived by J.R.Granada. The validation process of this program through contrast with experimental information is reviewed. As a result, a heterogeneously poisoned novel moderator concept was conceived and exhaustively investigated; it consists of an array of small moderating elements, decoupled at low energy by a neutron absorbing grid which nevertheless exerts no influence at high energy. Experiments reveal that while diffractometer resolution component exhibits the expected independence of the particular moderator chosen, the resolution component due to the neutron source displays a marked 'family' behaviour, which belongs to each grid spacing, and is independent of moderator thickness. This behaviour constitutes a result not foreseeable according to all known experience. This new knowledge has opened the way to reach an optimal moderator thickness which yields maximum intensity while keeping constant the time resolution due to the particular grid spacing chosen. The possible influence of grid absorbing material thickness was explored. Among practical limits of these values, no influence on optimal moderator thickness was observed. Validation of the ad hoc Monte-Carlo program comprised the contrast of a variety of calculated population decay spectra and neutron leak spectra, with the corresponding experimental cases measured locally at 300 K temperature, and others available in the literature for 20 K. The result is satisfactory to the extent of deciding to include predictions calculated for methane at 20 K. (author)