Storage of ultracold neutrons in the UCN$τ$ magneto-gravitational trap

Abstract

The UCN$τ$ experiment is designed to measure the lifetime $τ_{n}$ 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 provide a background "holding" field to eliminate depolarization-induced UCN loss caused by magnetic field nodes. We present a measurement of the storage time $τ_{store}$ 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 $τ_{store}=860\pm19$ s: within $1 σ$ of current global averages for $τ_{n}$. The storage time with the holding field deactiveated is found to be $τ_{store}=470 \pm 160$ 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.

6 pages, 5 figures