Speaker
Description
The planned upgrade of the neutron guide system at the Budapest Research Reactor (BRR) creates an opportunity to replace the existing hydrogen disk moderator with a new design. Using the Monte Carlo code PHITS we investigated two options: box-shaped para-hydrogen moderator and assemblies of flat para-hydrogen moderators. The simulations indicate that the optimal solution for the phase-space requirements of the BRR instruments is 4x8x16 cm³ box, which provides a brightness gain of 2 - 2.4 in the wavelength range of 3 - 7 Å. Adding a 4 mm-thick water shell yields an additional 10% increase in brightness. The assemblies of flat para-hydrogen moderators outperform box geometry by about 20% at 2 Å, while being less efficient for colder neutrons. We identified the key limitations that prevent the assemblies from providing higher brightness.