Neutronic Aspects and Recent Experimental Results with Methane Moderators at IUCF Low Energy Neutron Source (LENS)

Christopher M. Lavelle IUCF, Indiana University

The Indiana University Cyclotron Facility (IUCF) Low Energy Neutron Source (LENS) is a long pulsed source of cold neutrons based on Be(p,xn) reactions at 7 and 13 MeV. LENS employs a variable length long pulse to generate high fluxes in a light water reflector strongly coupled to a 1 cm thick methane moderator. The MCNP simulations described demonstrate coupling a thin moderator to the thermal neutrons produced in the reflector maintains a high brilliance of cold neutrons while keeping radiation heating of the cryogenics low enough to achieve very low (~4 K) moderator temperatures and also maintaining adequate Time of Flight energy resolution. Recent experiments with a 4 K methane moderator are described, and are in reasonable agreement with MCNP predictions for fast neutron yields, thermal flux at sample positions, cold neutron energy spectra, and emission time distributions. One surprising result has been the limiting behavior of the neutron spectral temperature as the moderator is cooled, which may be due to the Phase I-II transition in methane. The spin state of the methane may be playing a role in this regime, and methods of investigating this spin effect will be presented.

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