Neutronic Performance of the Lens Tmr

The Low Energy Neutron Source (LENS) is based on Be(p,xn) reactions at proton energy up to 13 MeV. Its novel design employs a cylindrical light water reflector/moderator surrounding the Be target primary neutron source and a solid CH4 cold neutron moderator downstream from the Be target that is coupled to the reflector. The design of the Target/Moderator/Reflector (TMR) assembly was optimized with MCNP and MCNPX Monte Carlo codes. The 12 x12 cm surface area cold moderator is viewed by three 10x10cm cross sectional area neutron beam tubes at -20, 0, and 20 degrees to the surface normal of the moderator. The optimum thickness of this coupled moderator for cold neutron yield is found to be 1 cm. The solid methane moderator can be cooled to temperatures below 10K with a closed-cycle He refrigerator to which the moderator is linked with a high-purity Al conducting rod. The system is designed with a flexible gas handling system to permit the study of a variety of moderator gas mixtures. The cryogenic moderator has been installed, and the facilty is producing neutrons using a 7 MeV proton beam. The expected neutronic performance of the TMR at full operating power is discussed and measurements of the neutron yield and spectra obtained with the present 7 MeV proton beam are reported.