Abstract
The UCNτ experiment is designed to measure the lifetime τn of the free neutron by trapping ultracold
neutrons (UCN) in a magneto-gravitational trap. An asymmetric bowl-shaped NdFeB magnet Halbach array
confines low-field-seeking UCN within the apparatus, and a set of electromagnetic coils in a toroidal geometry
provides a background “holding” field to eliminate depolarization-induced UCN loss caused by magnetic field
nodes. We present a measurement of the storage time τstore of the trap by storing UCN for various times and
counting the survivors. The data are consistent with a single exponential decay, and we find τstore = 860 ± 19 s,
within 1σ of current global averages for τn. The storage time with the holding field deactivated is found to be
τstore = 470 ± 160 s; this decreased storage time is due to the loss of UCN, which undergo Majorana spin flips
while being stored. We discuss plans to increase the statistical sensitivity of the measurement and investigate
potential systematic effects.
