Design and characterization of zero magnetic field chambers for high efficiency neutron polarization transport

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Abstract

Several methods of polarized neutron scattering call for a zero magnetic field (ZF) region to reduce magnetic field integral aberrations while preserving the neutron polarization. Though the design for large angle neutron scattering has been presented in various places, the design characterization and tuning has not been discussed before. In this report, the tuning procedure will be discussed with both neutron polarization transport method and utilization of fluxgate magnetometers. As a tuning procedure, polarized neutrons are sensitive to any local field distortions along all trajectories within the beam, but the process is slow. With fluxgates, the entire beam region cannot be accessed simultaneously, but very fast and precise measurements can be made in accessible regions of interest. Consequently, we would like to benchmark the usage of fluxgates as a fast tuning probe compared with polarization measurements made with neutrons. Polarization transport results for tuned ZF chambers, up to 2.25 m in length, are presented.

Introduction

Methods which utilize the precession of the neutron spin in a well designed magnetic field enable the investigations of material structures or dynamics with a resolution beyond the conventional neutron scattering techniques. Mezei proposed the first technique of this type, neutron spin echo (NSE) [1], which was further developed into neutron resonance spin echo (NRSE) [2], modulated intensity with zero effort (MIEZE) [3], spin echo small angle neutron scattering (SESANS) [4] and spin echo modulated small angle neutron scattering (SEMSANS) [5]. NRSE uses radio frequency (RF) flippers to measure the excitations of quasiparticles, for example, phonon or magnon and MIEZE only uses one arm of NRSE to allow more relaxed sample environment. Similar to the relation between NRSE and MIEZE, SESANS and SEMSANS employ magnetic Wollaston prisms to measure the inter particle correlations of materials in a extended length scale.

Since all of these methods label the change in either neutron energy (E) or momentum transfer (Q) into a change in Larmor phase, i.e., the accumulated angle of neutron spin precession, they may be referred as neutron Larmor labeling techniques. To maximize the resolution of a small change in either E or Q, it is critical to maximize the achievable Larmor phase, ΦFI×λ, where FI is the magnetic field integral along the neutron path and λ is the neutron wavelength. From this it follows that maximizing the Larmor phase requires maximizing the magnetic field, path length of neutron, or neutron wavelength. To achieve a high signal to noise ratio for a given setup, the Larmor phase aberrations need to be minimized across the beam. To maximize the resolution while maintaining a high signal to noise ratio, one approach is to introduce the usage of zero magnetic field (ZF) chamber to separate the spin manipulation components far apart from each other, where the neutron polarization vector will be preserved when passing through this region without accumulating additional Larmor phase. This is especially true for the techniques involving resonance radio frequency flippers, such as NRSE or MIEZE, where an effective precession is produced inside the ZF chamber such that the accumulated phase is proportional to the time neutrons spend between and the angular frequency of the flippers. This is also true for methods using static magnetic fields, such as SESANS or SEMSANS, where the gradient of the field integral inside the spin manipulation devices matters for the Larmor phase. It is true that people can also use well designed uniform magnetic guide field to preserve the neutron polarization [6]. But careful shielding of this field is critical at the magnetic field boundary where one does not need it, for example, a π2 spin flipper.

Another situation where a ZF chamber is preferable originates from the fact that a uniform Larmor phase across the sample is essential, especially for large angle scattering on a sample with finite size, i.e., a diffraction setup. Introducing ZF chambers in the regions where spin precession is not desired is one approach to achieve this, though care must be taken to preserve the neutron polarization vector.

One of the recent developments in Larmor labeling methods is the introduction of superconducting magnetic Wollaston prisms [7]. Such devices are composed of pairs of triangular shaped magnetic field regions using superconducting coils. These adjacent regions of field are opposite in direction and both transverse to the beam direction, and by using superconducting films at the interfaces, the magnetic field is very uniform and well defined within the regions. The angle between the interface of the opposite fields and the beam direction introduces a splitting of the spin states at the interface in a beam which is polarized perpendicular to the fields. Wollaston prisms can be applied for large angle neutron scattering, including neutron Larmor diffraction [8], [9] and inelastic neutron scattering spectroscopy [10]. In such applications, a sample with finite size will lead to a variation of the neutron path length introducing Larmor phase aberrations in a uniform magnetic field. A ZF chamber with high efficiency polarization transport is essential in the sample region, i.e., between the precession devices, so that the Larmor phase accumulation is controlled purely by the precession devices. Such prisms have also been used to implement SEMSANS for small angle scattering, in which a long separation between the spin flippers yields longer spin echo lengths and the span should have ZF to prevent additional phase accumulation. In this report, we discuss the design and characterization of the ZF chamber for both large and small angle applications, and discuss the benefits of using magnetometry as a fast method of optimizing compensation coils for ZF chambers, which could also be used for other techniques.

Section snippets

The design of the ZF chamber

The ZF chamber used in MuPAD [11], a spherical neutron polarimeter at the Paul Scherrer Institut (PSI) triple axis spectrometer, and NRSE of FRM-II [12], [13], [14] consists of three components: a cylindrical ZF chamber with a gap in the middle at the sample height; and two rectangular mu metal boxes with beam port open ends to magnetically shield the gaps between the central ZF chamber and the precession devices. As shown in Fig. 1, the design presented here is similar to those used for MuPAD

The ZF chamber for low angle neutron scattering technique

The SEMSANS method employs two magnetic Wollaston prisms separated by a ZF region, as discussed in [18], [19], which generates spatial intensity modulations on the detector, due to the gradient of magnetic field integrals across the devices. The intensity modulation can be used to measure the real-space density autocorrelation function of samples and for dark field imaging. To achieve a high resolution, an intensity modulation with a small period is always favorable, which can be achieved, for

Summary

We have presented the design of ZF chambers and discussed the operation and characterization of such chambers in combination with superconducting Wollaston prisms applied to both large angle and small angle polarized neutron scattering. Such techniques have the potential to be utilized for other polarized neutron scattering methods. We have demonstrated that a well designed ZF chamber is capable of achieving high efficiency polarization transport over long distances, 2.25 m in this work, which

Acknowledgments

This research used resources at the High Flux Isotope Reactor, a DOE Office of Science User Facility operated by the Oak Ridge National Laboratory. We would like to thank Jack Doskow with Indiana University for CAD model contributions, Thomas Keller for supporting the design of the ZF chamber, Matthew Pearson with ORNL for software development and assistance setting up power supplies, and Tianhao Wang with ORNL for providing polarization results for the V-cavity and 3He analyzer performance on

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