The $\mathrm{UCN}\tau$ experiment is designed to measure the lifetime ${\tau }_n$ 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 ${\tau }_{\text{store}}$ 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 ${\tau }_{\text{store}}=860\pm 19$ s, within $1\sigma$ of current global averages for ${\tau }_n$. The storage time with the holding field deactivated is found to be ${\tau }_{\text{store}}=470\pm 160$ 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.

• Received 28 October 2013
• Revised 30 April 2014

DOI:https://doi.org/10.1103/PhysRevC.89.052501