Storage of ultracold neutrons in the UCN$\ddot{I}„$ magneto-gravitational trap

Abstract

The UCN$\ddot{I}„$ experiment is designed to measure the lifetime $\ddot{I}{„}_n$ 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 $\ddot{I}{„}_{store}$ 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 $\ddot{I}{„}_{store}=860\pm 19$ s: within $1\ddot{I}ƒ$ of current global averages for $\ddot{I}{„}_n$. The storage time with the holding field deactiveated is found to be $\ddot{I}{„}_{store}=470\pm 160$ 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