Monte Carlo optimization of a low enrichment uranium neutron multiplier for the Los Alamos ultracold neutron source

The recent upgrade of the Los Alamos spallation driven ultracold neutron source drove the improved the statistical sensitivity of a precision measurement of the neutron lifetime and provided a path toward a new electrical dipole moment search at the $3.0 \times 10^{-27}$ e$\cdot$cm level. The output of the source can be further improve by borrowing a concept from reactor design, the fission foil neutron multiplier. The neutron flux through the cryogenic insert can be increased by replacing some of the passive room-temperature moderator components with low enrichment uranium. Simulations using MCNP6 demonstrate that the source's performance is increased by a factor of $>100$, however the heat load on the source and uranium itself will be the limiting factor. We will present the results of a simulation study to optimize a fission foil geometry that will lead to an increase of a factor of $4-10$ in the ultracold neutron production under manageable heat loads.