Optimization of the SNS neutron beam current

The proposed Spallation Neutron Source (SNS) facility will consist of two parts: (a) a high-energy (1-GeV) and high-powered (1-MW) proton accelerator (linac) and accumulator ring and (b) a target station that converts the protons to low-energy (<2 eV) neutrons and delivers them to the neutron-scattering instruments. It will be a 60-Hz facility delivering 6 x 1015 protons each second in 60-micros pulses with a linac length of 490 m and an accumulator ring circumference of 220 m. Work is now under way to optimize the neutron beam current. Both the intensity and the time pulse characteristics are important with different requirements for different instruments. The effect of varying the reflector material, the moderator poison location, and the moderator dimensions has been considered. In addition, the impact on the current when a premoderator or a composite moderator is used has been assessed. Two reflector materials were picked to be studied in detail: beryllium and lead. A beryllium reflector gave a greater thermal neutron current but a wider pulse width, with the peak currents about the same. A simple figure of merit (FOM) (intensity/pulse width squared) favored the lead reflector