New para-hydrogen cold neutron source at the Budapest Research Reactor: Monte Carlo simulations

The planned upgrade of the neutron guide system at the Budapest Research Reactor (BRR) provides an opportunity to enhance the performance of its cold neutron source. In this work, two replacement concepts for the existing liquid hydrogen disk moderator are investigated: (i) a box-shaped para-hydrogen (pH₂) moderator and (ii) assemblies of low-dimensional (flat) pH₂ moderators. Both designs are optimized using the curve of full and optimal sample illumination method and through Monte Carlo simulations tailored to the BRR geometry and the thermal neutron distribution. The box pH₂ moderator is found to increase the cold neutron brightness by up to 2.2 times in the 3-7 Å wavelength range. Flat moderator assemblies, as either staircases or chessboards, provide an additional gain of about 20% at 2 Å, although their advantage diminishes at longer wavelengths due to the non-uniform thermal illumination of the reactor channel and the wavelength dependence of the optimal moderator length. These results demonstrate that pH₂ moderators, particularly in low-dimensional geometries, can substantially enhance the performance of the upgraded BRR cold neutron source, with the achievable gains largely limited by the thermal neutron distribution within the existing moderator channel.