A Long-Pulse Spallation Source at Los Alamos: Facility Description and Preliminary Neutronic Performance for Clod Neutrons

摘要

The los Alamos National Laboratory has discussedinstalling a new 1-NW spallation neutron target station in anexisting building at the end of its 800-MeV proton linearaccelerator. Because the accelerator provides pulses of protonseach about 1 msec in duration, the new source would be a Long-Pulse Spallation Source (LPSS). The facility would employvertical extraction of moderators and reflectors, and horizontalextraction of the spallation target. An LPSS uses coupledmoderators rather than decoupled ones. There are potential gainsof about a factor of 6 to 7 in the time-averaged neutron brightnessfor cold-neutron production from a coupled liquid H_2 moderatorcompared to a decoupled one. However, these gains come at theexpense of putting "tails" on the neutron pulses. The particulars ofthe neutron pulses from a moderator (e.g., energy-dependent risetimes, peak intensities, pulse widths, and decay constant(s) of thetails) are crucial parameters for designing instruments andestimating their performance at an LPSS. Tungsten is our referencetarget material. Inconel-718 is the reference target canister andproton beam window material, with Al-6061 being the choice forthe liquid H_2 moderator canister and vacuum container. A 1-MWLPSS would have world-class neutronic performance. We describethe proposed Los Alamos LPSS facility, and show that, for coldneutrons, the calculated time-averaged neutronic performance of aliquid H_2 moderator at the 1-MW LPSS is equivalent to about1/4th the calculated neutronic performance of the best liquid D_2moderator at the Institut Laue -Langevin reactor. We show thatthe time-averaged moderator neutronic brightness increases as thesize of the moderator gets smaller.