Konstantin Batkov | Searcharxiv

arXiv2023

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.

physics.ins-det↗

arXiv2014

Voluminous D2 source for intense cold neutron beam production at the ESS

The development of the flat moderator concept at ESS recently opened up the possibility that a single flat moderator above the target could serve all the scattering instruments, that rely on high brightness. This would allow for the introduction of a fundamentally different moderator below the target for the complementary needs of certain fundamental physics experiments. To facilitate experiments depending on the total number of neutrons in a sizable beam, the option of a voluminous D2 moderator, in a large cross-section extraction guide is discussed and its neutronic performance is assessed.

physics.ins-det↗

arXiv2013

Low dimensional neutron moderators for enhanced source brightness

In a recent numerical optimization study we have found that liquid para-hydrogen coupled cold neutron moderators deliver 3 - 5 times higher cold neutron brightness at a spallation neutron source if they take the form of a flat, quasi 2-dimensional disc, in contrast to the conventional more voluminous shapes used by now. In the present paper we describe a simple theoretical explanation of this unexpected behavior, which is based on the large difference in para-hydrogen between the values of the scattering mean free path for thermal neutrons (in the range of 1 cm) and its much larger equivalent for cold neutrons. This model leads to the conclusions that the optimal shape for high brightness para-hydrogen neutron moderators is the quasi 1-dimensional tube and these low dimensional moderators can also deliver much enhanced cold neutron brightness in fission reactor neutron sources, compared to much more voluminous liquid D2 or H2 moderators currently used. Neutronic simulation calculations confirm both of these theoretical conclusions.

physics.ins-det↗