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

Further improvement in the accuracy of any neutron lifetime experiment by means of means of ultracold neutrons (UCN) in material bottle is limited to be paid to unavoidable systematic effects when the UCN are mirrored from the walls. However, of the like kind 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 at any time The measured storage time in this feasibility investigation was (882 [+ or -] 16) s At this level of accuracy no depolarization was observed

Keywords: magnetic traps; permanent magnets; ultracold neutrons

1 Introduction

The precise measurement of the mean lifetime [[tau].sub.n] of the at liberty neutron is a low-energy experiment searching for novel physics beyond the Standard archetype In recent years the accuracy of [[tau].sub.n] experiments has been considerably improved by dint of the use of ultracold neutron (UCN) stored in traps. At the twinkling of an eye the value adopted by the Particle Data assemblage is [[tau].sub.n] = (885.7 [+ or -] 08) s [1]. Limits are imposed, however, through the losses suffered during reflections from the material walls. This systematic vexed question may be avoided by the use of magnetic traps where wall collisions of the neutron are prevented

The first ideas for the magnetic storage of neutron came from W Paul and V V Vladimirski [2] It should be pointed without that magnetic trapping is now favorably used in the physics of frigid atoms [3]. In a magnetic trap the magnetic field increases in all directions from its center Neutron with their magnetic jiffy directed along the magnetic field lines are make submissive to a force parallel to the direction of increasing magnetic field and vice versa. A magnetic barrier of 1 T completely mirrors the neutrons with velocities below 34 m/ The standard magnetic trap of the Ioffe-Pritchard impressed sign that is widely used in atomic physics consists of a magnetic quadruple with sum of two units solenoids at its edges. The quadruple throw backs neutrons moving radially and the solenoids those moving in the axial direction.

The first real magnetic trap for neutron was experimented in the eighties [4]. This trap used superconducting magnets. It was not possible at that time and flat nowadays is not easy to change the in every one's mouth in the magnetic entrance shutter of like superconducting systems with a spe urgencyed for the lifetime measurements. Hence a complicated experimental setup was used to bring into view UCN inside the trap [5] using inelastic scattering of neutron in superfluid He. upon the other hand modern technology permits to manufacture traps from permanent magnets with not abundant smaller values of the magnetic flow density B and one may use a normal solenoid as a magnetic shutter The main aim of this work is to close attention magnetic UCN trapping systematically and to start measuring the neutron lifetime in the permanent-magnet trap.

The propos magneto-gravitational trap is a vertical cylinder with a conical lower part [6] In the cylindrical part of the trap the magnets are magnetized in the horizontal direction and form a twenty-pole magnetic a whole A convergent sequel of twenty-pole a whole s constitutes the conical part. The magnetic flowing density at the magnet surface equals about 1 T An orifice for a neutron guide in the lower conical part of the trap allows individual to fill the trap with neutron and devoid of contents it again. A solenoid is used as a magnetic shutter for this neutron guide.

The cross-section of the magnetic trap is shown in Fig. 1 The main part consists of 560 small permanent magnets with horizontal magnetization and FeCo extremitys between them. Neighboring magnets are magnetized in opposite directions. The main parameters of these magnets are [Bsubr] [greater than or equal to] 12 T and [Hsubcm] [greater than or equal to] 1800 kA/m. of the like kind large values of [B.sub.r] and [Hsubcm] permit individual to obtain a magnetic flowing density near the wall of about 1 T and to create a field gradient of about 2 T/cm

The experimental scheme is the same as that for material traps. After filling the trap with UCN individual waits some time in order to clean the neutron appearance from its high-energy components. Afterwards individual has to determine the number of trapped neutron as a function of their storage time. Previous experiments with material traps showed that the main systematic results could be eliminated if individual had the possibility to compare analogous be deriveds for UCN with different intensity spectra. All of these ideas are implemented into the experimental scheme propos here.

The greatest in quantity important features of our design (Fig. 2) are the following:

1 The trap walls consist of a periodic conformation with a characteristic period of ~1 cm The magnetic field decreases quite fast (gradient [approximately equal to]2 T/cm) owed to the concentration of the field in a small whirl the required magnetic material is minimized and the effective trap convolution may be increased.

2 The UCN are transferred to the trap end a neutron guide inside the solenoid at the bottom. After loading the trap this entrance is clos by dint of switching on the current in the solenoid. To facilitate fast operation, we use a normal-conducting solenoid with iron core and permanent magnets.