Neutronic performance of rectangular and cylindrical coupled hydrogen moderators in wide-angle beam extraction of low-energy neutrons

The JSNS design criteria for the coupled hydrogen moderator requires a large number of beam lines. We therefore performed neutronic calculations to maximize time-integrated and pulse-peak intensities of low-energy neutrons for 100% para-hydrogen coupled moderators. It was found that a larger number of beam lines increased the total intensity due to the increase of the angular coverage although the average intensity of each beam line decreased a bit due to a finite reflector missing caused by the beam extraction holes in the reflector. In the case of a rectangular moderator, the spatial distribution of the vector flux of low-energy neutrons becomes undesirable at a large beam extraction angle (25∘). As an alternative, we propose a cylindrical moderator to substantially improve the spatial distribution. Time-integrated and pulse-peak intensities, pulse widths, and pulse decay characteristics were calculated as a function of the moderator diameter. The results indicated that the optimal diameter is about 140 mm. Compared to the rectangular moderator, the cylindrical design gives higher pulse-peak intensities with narrower pulse widths, without penalizing the time-integrated intensities, thus providing a more uniform angle dependence. Some explanations are given for the differences in pulse characteristics between the two moderator shapes.