Long-Pulse Spallation Source at Los Alamos: facility description and preliminary neutronic performance for cold neutrons

The Los Alamos National Laboratory has discussed installing a new 1-MW spallation neutron target station in an existing building at the end of its 800-MeV proton linear accelerator. Because the accelerator provides pulses of protons each about 1 msec in duration, the new source would be a Long-Pulse Spallation Source (LPSS). The facility would employ vertical extraction of moderators and reflectors, and horizontal extraction of the spallation target. An LPSS uses coupled moderators rather than decoupled ones. There are potential gains of about a factor of 6 to 7 in the time-averaged neutron brightness for cold-neutron production from a coupled liquid H₂ moderator compared to a decoupled one. 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. Tungsten is our reference target material. Inconel-718 is the reference target canister and proton beam window material, with Al-6061 being the choice for the liquid H₂ moderator canister and vacuum container. A 1-MW LPSS would have world-class neutronic performance. We describe the proposed Los Alamos LPSS facility, and show that, for cold neutrons, the calculated time-averaged neutronic performance of a liquid H₂ moderator at the 1-MW LPSS is equivalent to about 1/4 th the calculated neutronic performance of the best liquid D₂ moderator at the Institut Laue-Langevin reactor. We show that the time-averaged moderator neutronic brightness increases as the size of the moderator gets smaller.