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

Abstract

Ultracold neutron (UCN) storage measurements were made in a trap constructed from 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. A mechanical UCN valve at the bottom of the trap was used for filling and emptying. This valve was covered with Fomblin grease to induce non-specular reflections and was used in combination with a movable polyethylene UCN remover inserted from the top for cleaning of above-threshold UCNs. Loss due to UCN depolarization was suppressed with a minimum 2 mT bias field. Without using the UCN remover, a total storage time constant of (712 ± 19) s was observed; with the remover inserted for 80 s and used at either 80 cm or 65 cm from the bottom of the trap, time constants of (824 ± 32) s and (835 ± 36) s were observed. Combining the latter two values, a neutron lifetime of τn = (887 ± 39) s is extracted after primarily correcting for losses at the UCN valve. The time constants of the UCN population during cleaning were observed and compared to calculations based on UCN kinetic theory as well as Monte-Carlo studies. These calculations are used to predict above-threshold populations of ∼ 5%, ∼ 0.5% and ∼ 10 −12 % remaining after cleaning in the no remover, 80 cm remover and 65 cm remover measurements. Thus, by using a non-specular reflector covering the entire bottom of the trap and a remover at the top of the trap, we have established an effective cleaning procedure for removing a major systematic effect in high-precision τn experiments with magnetically stored UCNs.

Spin flip loss in magnetic confinement of ultracold neutrons for neutron lifetime experiments

“…(1), which does not take into account the deviations induced in the actual designs by the discrete geometry of electric currents [23,28], or by the permanent magnet blocks with constant magnetization within each block in the schemes of Refs. [20,[24][25][26][27]. Equation (1) neglects gravity and assumes a uniform holding field B ζ in the axial direction.…”

“…2 if we use values of holding field close to their minima: 0.08 T/0.1 T for HOPE (vertical configuration with B min = 0.013 T/horizontal with B min = 0.09 T) [26] and 0.1 T (0.6 T) for NIST mark 2 (mark 3) with B min ≈ 0.1 T (0.6 T) [34,35].…”

“…So is the starting time at a TP. Thus, the initial value of the first term in (26), at t 0 ≈ 0, which is ∼ e −iΘ1(t0) ≈ e −iω1(0)t0 , should be averaged over the rapidly varying phase ω 1 (0)t 0 with the result e −iΘ1(t0) t0 = 0. This holds except for the initial few micrometers of the path subsequent to a TP.…”

“…(a) The HOPE octupole magnet [26] has bore radius 4.7 cm and its axis oriented vertically or horizontally. For the vertical configuration we assume activation of only the bottom solenoid with a maximum axial field of 1.4 T while gravity provides the cap.…”

“…[23][24][25][26][27][28], where the trapping fields are generated by Halbach arrays of permanent magnets [24][25][26][27] or, for [23,28] and, earlier [29], by superconducting currents. For these cylindrical configurations we use a 2D field model which enables us to obtain a semi-analytic expression for the ensemble-averaged spin-flip loss and which can be analyzed with no need to involve simulations.…”