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Ultracold neutron (UCN) storage measurements were made in a trap constructedfrom a 1.3 T Halbach Octupole PErmanent (HOPE) magnet array aligned vertically,using the TES-port of the PF2 source at the Institut Laue-Langevin. Amechanical UCN valve at the bottom of the trap was used for filling andemptying. This valve was covered with Fomblin grease to induce non-specularreflections and was used in combination with a movable polyethylene UCN removerinserted from the top for cleaning of above-threshold UCNs. Loss due to UCNdepolarization was suppressed with a minimum 2 mT bias field. Without using theUCN remover, a total storage time constant of $(712 \pm 19)$ s was observed;with the remover inserted for 80 s and used at either 80 cm or 65 cm from thebottom of the trap, time constants of $(824 \pm 32)$ s and $(835 \pm 36)$ swere observed. Combining the latter two values, a neutron lifetime of$\tau_{\rm n} = (887 \pm 39)$ s is extracted after primarily correcting forlosses at the UCN valve. The time constants of the UCN population duringcleaning were observed and compared to calculations based on UCN kinetic theoryas well as Monte-Carlo studies. These calculations are used to predictabove-threshold populations of $\sim 5\%$, $\sim 0.5\%$ and $\sim 10^{-12}\%$remaining after cleaning in the no remover, 80~cm remover and 65~cm removermeasurements. Thus, by using a non-specular reflector covering the entirebottom of the trap and a remover at the top of the trap, we have established aneffective cleaning procedure for removing a major systematic effect inhigh-precision $\tau_{\rm n}$ experiments with magnetically stored UCNs.

physical review. c

Neutron lifetime measurements and effective spectral cleaning with an  ultracold neutron trap using a vertical Halbach octupole permanent magnet  array