A concept of a para-hydrogen-based cold neutron source for simultaneous high flux and high brightness
A novel cold neutron source (CNS) employing chessboard or staircase assemblies of high-aspect ratio rectangular (HARR) para-hydrogen (p-H2) moderators is proposed. It is demonstrated that this design can generate neutron beams with higher intensity and brightness, up to 2.5 times more than any p-H2 based CNS with an equal cold neutron beam cross-section made of a single moderator (flat or voluminous). Two limiting factors for this gain are identified: the limited volume of the high-density thermal neutron region surrounding the reactor core or spallation target imposing constraints on the total length of the moderator assembly and the finite width of moderator walls. Additional factors affecting gain can be revealed by dedicated Monte Carlo simulations of the moderator-reflector assembly, which can only be conducted for a particular neutron source and are beyond the scope of this general study. Moreover, the relatively large length of moderator assemblies makes their application for short pulse neutron sources very problematic. Analytic approach for calculating the brightness of p-H2 moderators is introduced. Because brightness gain originates from a near-the-surface effect resulting from the prevailing single collision process, HARR CMs offer a significant increase, up to a factor of 10, in cold neutron brightness. The obtained results are in excellent agreement with MCNP calculations. The concept of 'low-dimensionality' (low-d) in moderators is explored: achieving a substantial increase in brightness necessitates CMs to be low-d both geometrically (HARR) and physically, requiring the moderator's smallest dimension to be smaller than the mean free path for thermal neutrons. This explains why additional compression of the CM along the longest direction, effectively giving it a tube-like shape, does not leads to a significant brightness increase comparable to the flattening of CM.