A Quantitative Measure of the Cleaning of Quasi-bound Neutrons in UCNtau

The UCNtau experiment, at Los Alamos National Lab, uses permanent magnets and gravity to trap polarized free ultracold neutrons (UCN's) for measuring the lifetime. This value with neutron decay correlation coefficients can be used to probe beyond Standard Model physics and with the increased precisions now available can begin to be used to evaluate different theoretical approaches to beta decay corrections. During part of the 2020 run cycle a vertical G10 copper coated plate was inserted in a buffer volume upstream of the trap to block all but the higher energy neutrons normally populating the experiment. Data was taken with and without the usual cleaning of the highest energy quasi-bound neutrons which may kinematically escape the trap before decaying. Comparing the resulting trap lifetimes gives a measure of cleaning efficacy by allowing an empirical bound to be placed on its efficiency. Analysis and results will be presented.