Long-Pulse Spallation Source Neutronic Performance

There are potential gains of about a factor of seven in the time-average neutron brightness for a coupled liquid H2 moderator compared to a decoupled one, and a factor of around five for a H20 moderator. However, these gains come at the expense of putting “tails” on the neutron pulses. The particulars of the neutron pulses from a moderator (e.g., energy-dependent rise times, peak intensities, pulse widths, and decay constant(s) of the tails) are crucial parameters for designing instruments and estimating their performance at an LPSS. The spallation target system designer can alter moderator neutronic performance by: (a) the choice of target material and geometry; (b) proper selection of the moderator material and geometry; (c) varying the target–moderator geometry; (d) the choice of reflector material(s); (e) the presence/absence of decouplers and liners; and (f) the proton pulse width. A 1-MW Long-Pulse Spallation Source (LPSS) has world-class neutronic performance. We show that the calculated performance of a liquid H2 moderator at a 1-MW LPSS is equivalent to one-fourth the calculated performance of the best liquid D2 moderator at the Institut Laue Langevin (ILL) reactor.