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Physical Review C

Spin-flip loss in magnetic traps for ultracold neutrons

A. Steyerl, E. Gutsmiedl

Published Oct 13, 2020• 1 citation
Publisher

Abstract

A particle with spin and magnetic-dipole moment can be trapped in certain suitable magnetic-field configurations only as long as its spin remains aligned with the field direction as it moves through the trap. Here, we extend earlier analyses of spin-flip probabilities, which lead to storage loss for ultracold neutrons, and present arguments indicating that some of the earlier results should be multiplied by a factor of two.

DOI
10.1103/physrevc.102.045203

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A particle with spin and magnetic-dipole moment can be trapped in certain suitable magnetic-field configurations only as long as its spin remains aligned with the field direction as it moves through the trap. Here, we extend earlier analyses of spin-flip probabilities, which lead to storage loss for ultracold neutrons, and present arguments indicating that some of the earlier results should be multiplied by a factor of two.
Abstract In this chapter the storage of neutrons in magnetic traps will be considered, as well as issues of UCN polarization and methods of analysing it, and also methods of UCN spin reversal. Magnetic traps have the advantage that no absorption or inelastic scattering exists in them within the range of thermal energies. There are, however, certain peculiarities that must be taken into account in estimating the UCN storage time in these traps. The magnetic field in a trap is extremely inhomogeneous and can confine neutrons within a restricted region of space only in the case of a certain neutron spin projection on to the field direction. When the spin direction changes, and in an inhomogeneous field it is sure to undergo changes, the magnetic ‘walls’ win stop being walls and a neutron will leave the trap with ease. It is noteworthy that a spin flip is most probable when the neutron is as far as possible from the ‘magnetic wall’, and not when it impinges upon the ‘wall’.
We have constructed a permanent-magnet trap that provides a tightly confining potential useful for producing dense ultracold atomic vapors.1 This trap will be used to investigate quantum-statistical effects of 6Li and 7Li. Permanent magnets can produce field strengths and gradients comparable to those of superconducting-magnet traps but with reduced complexity and greater optical access. Our trap uses an arrangement of magnets in an Ioffe configuration,2 which features a nonzero magnetic-field minimum. Atoms are trapped about this minimum through the interaction of their total magnetic moment with the trap's magnetic field. The nonzero minimum prevents trap- loss-producing spin-flip (Majorana) transitions.
Further improvement in the accuracy of any neutron lifetime experiment by means of ultracold neutrons (UCN) in material bottles is limited due to unavoidable systematic effects when the UCN are reflected from the walls. However, such effects can be excluded in principle if magnetic trapping of UCN is used. The storage of UCN in a small magnetic trap made of permanent magnets was demonstrated for the first time ever. The measured storage time in this feasibility study was (882 ± 16) s. At this level of accuracy no depolarization was observed.
We analyze the accumulation of ultracold neutrons (UCNs) in a superfluid-helium converter vessel surrounded by a magnetic multipole reflector. We solved the spin-dependent rate equation, employing formulas valid for adiabatic spin transport of trapped UCNs in mechanical equilibrium. Results for saturation UCN densities are obtained in dependence of order and strength of the multipolar field. The addition of magnetic storage to neutron optical potentials can increase the density and energy of the low-field-seeking UCNs produced and serves to mitigate the effects of wall losses on the source performance. It also can provide a highly polarized sample of UCNs without need to polarize the neutron beam incident on the converter. This work was performed in preparation of the UCN source project SuperSUN at the Institut Laue--Langevin.