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Journal of Research of the National Institute of Standards and Technology

First ever storage of ultracold neutrons in a magnetic trap made of permanent magnets

В. Ф. Ежов, A. Z. Andreev, A.A. Glushkov, A. G. Glushkov, M.N. Groshev, V. A. Knyaz’kov, G. B. Krygin, V. L. Ryabov, Anatolii P. Serebrov, B. A. Bazarov, Peter W. Geltenbort, F.J. Hartman, Stephan Martin Paul, R. Picker, O. Zimmer, Коврижных Николай Александрович

Published Jul 1, 2005• 23 citations

Abstract

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.

DOI: 10.6028/jres.110.051

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•Jul 1, 1983•3 citations

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.

Ultracold neutrons (UCN) with energy up to the Fermi potential were stored for the first time in a volume coated with diamondlike carbon. As a function of the UCN energy and wall temperature of the storage volume, we measured the wall loss parameter \ensuremath{\eta} and obtained $\ensuremath{\eta}=(3.1\ifmmode\pm\else\textpm\fi{}0.9)\ifmmode\cdot\else\textperiodcentered\fi{}{10}^{\ensuremath{-}4}$ at

T=290

K and $\ensuremath{\eta}=(1.8\ifmmode\pm\else\textpm\fi{}0.2)\ifmmode\cdot\else\textperiodcentered\fi{}{10}^{\ensuremath{-}4}$ at

T=115

K.

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’.

A surprising new escape channel for ultracold neutrons (UCNs) in traps was reported recently. It could be relevant to the long-standing puzzle of the “too high” loss rate of UCNs from traps, which has yet to be completely understood and eliminated. In the present work we positively identify the new phenomenon and investigate it in detail. The escape of UCNs from traps is due to rare events in which their energy undergoes a small increase (∼10 −7 eV). The reason for such an energy gain and its impact on the physics of UCN storage is still to be investigated.

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.