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

Authors :: Salvat, D. J.
Adamek, E. R.
Barlow, D.
Broussard, L. J.
Bowman, J. D.
Callahan, N. B.
Clayton, S. M.
Cude-Woods, C.
Currie, S.
Dees, E. B.
Fox, W.
Geltenbort, P.
Hickerson, K. P.
Holley, A. T.
Liu, C. -Y.
Makela, M.
Medina, J.
Morley, D. J.
Morris, C. L.
Penttila, S. I.
Ramsey, J.
Saunders, A.
Seestrom, S. J.
Sjue, S. K. L.
Slaughter, B. A.
Sharapov, E. I.
Vanderwerp, J.
VornDick, B.
Walstrom, P. L.
Wang, Z.
Womack, T. L.
Young, A. R.
Source :: Phys. Rev. C 89, 052501 (2014)
Publication Year :: 2013
Abstract: The 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 provide a background "holding" field to eliminate depolarization-induced UCN loss caused by magnetic field nodes. We present a measurement of the storage time $\tau_{store}$ 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 $\tau_{store}=860\pm19$ s: within $1 \sigma$ of current global averages for $\tau_{n}$. The storage time with the holding field deactiveated is found to be $\tau_{store}=470 \pm 160$ 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.<br />Comment: 6 pages, 5 figures