Preliminary Implementation of a Movable Miniature UCN Detector as a UCN Energy Spectrometer*

The UCN$\tau$ experiment at the Los Alamos Neutron Science Center measures the mean lifetime of free neutrons by loading ultracold neutrons (UCN) into a magneto-gravitational trap and counting remaining UCN after waiting various storage times. The energy of a UCN determines the maximum height it can reach, thus the energy spectrum of the UCN affects the number of UCN loaded into the trap as well as the relative counts on the monitor detectors that are placed at different heights.

To extract the energy spectrum from an experimental observable, we mounted a 1cm by 1cm $^{10}$B coated ZnS:Ag multi-layer UCN detector on a linear translation stage with 1.2 m dynamic range, and we installed the detector assembly on top of a buffer volume along the UCN$\tau$ beamline. The relative flux of the UCN as a function of height in the buffer volume was measured via this apparatus. Measured flux profile has been reproduced in a Monte Carlo simulation of the geometry, and this model provides the ability to extract a set of energy spectra and to characterize the impact of detector design parameters on the extracted spectrum. In this talk, we will present the instrumentation of the movable miniature UCN detector, the measured UCN flux profile, and a set of reconstructed UCN energy spectra.