3He spin filter based polarized neutron capability at the NIST Center for Neutron Research

A large beam high efficiency radio frequency neutron spin flipper

A design for a radio frequency (RF) neutron spin flipper obtained from magneto-static and neutron spin transport simulations is presented. The RF flipper constructed from this design provides a flipping probability of 0.999 or better for a beam size 6 cm wide and 15 cm high and a wavelength band between 0.4 and 0.6 nm. Three permanent magnet guide field sections with air gaps provide a linear field gradient along the beam propagation direction over a large cross-sectional area. An RF oscillator based on coupling the resonant coil of a Hartley oscillator to the excitation coil was developed, which provides a higher current and, thereby, a larger RF amplitude, as compared to a conventional RF power amplifier. Two opaque He3 neutron spin filters were employed to measure the flipping probability of the flipper with very high precision. A spatially uniform flipping probability of 0.9995(2) or higher was measured over the large cross-sectional area neutron guide. This RF neutron spin flipper will be employed in a polychromatic beam reflectometer at the National Institute of Standards and Technology Center for Neutron Research. This design can be applied to other polarized neutron instruments or applications requiring a very high continuous flipping probability of the neutron spin for a large cross-sectional area beam.

Optimizing magnetically shielded solenoids

An important consideration when designing a magnetostatic cavity for various applications is to maximize the ratio of the volume of field homogeneity to the overall size of the cavity. We report a design of a magnetically shielded solenoid that significantly improves the transverse field gradient averaged over a volume of 1000 cm³ by placing compensation coils around the holes in the mu-metal end caps rather than the conventional design in which the compensation coils are placed on the main solenoid. Our application is polarized ³He-based neutron spin filters, and our goal was to minimize the volume-averaged transverse field gradient, thereby the gradient induced relaxation time, over a ³He cell. For solenoids with end cap holes of different sizes, additional improvements in the field gradient were accomplished by introducing non-identical compensation coils centered around the non-identical holes in the end caps. The improved designs have yielded an overall factor of 7 decrease in the gradient in the solenoid, hence a factor of 50 increase in the gradient induced relaxation time of the ³He polarization. The results from both simulation and experiments for the development of several such solenoids are presented. Whereas our focus is on the development of magnetically shielded solenoids for ³He neutron spin filters, the approach can be applied for other applications demanding a high level of field homogeneity over a large volume.

Sensitive neutron transverse polarization analysis using a 3He spin filter

We report an experimental implementation for neutron transverse polarization analysis that is capable of detecting a small angular change (≪10-3 rad) in neutron spin orientation. This approach is demonstrated for monochromatic beams, and we show that it could be extended to polychromatic neutron beams. Our approach employs a 3He spin filter inside a solenoid with an analyzing direction perpendicular to the incident neutron polarization direction. The method was tested with polarized neutron beams and a spin rotator placed inside a μ-metal shield just upstream of the analyzer. No cryogenic superconducting shields or additional neutron spin manipulations are needed. With a counting detector, we experimentally show that the angular resolution δθ=1/(PnAN) rad is only determined by the counting statistics for the total counts N and the product of the neutron polarization Pn and the analyzing power A. With a high-flux neutron beam, 10-6 rad angular sensitivity is feasible within a day. This simple, classical-quantum-limited transverse polarization analysis scheme may reduce the overall complexity of experimental implementation for applications requiring sensitive neutron polarimetry and improve the precision in fundamental science studies and polarized neutron imaging.

Generation and detection of spin-orbit coupled neutron beams

Spin-orbit coupling of light has come to the fore in nanooptics and plasmonics, and is a key ingredient of topological photonics and chiral quantum optics. We demonstrate a basic tool for incorporating analogous effects into neutron optics: the generation and detection of neutron beams with coupled spin and orbital angular momentum. The 3He neutron spin filters are used in conjunction with specifically oriented triangular coils to prepare neutron beams with lattices of spin-orbit correlations, as demonstrated by their spin-dependent intensity profiles. These correlations can be tailored to particular applications, such as neutron studies of topological materials.

Small-angle neutron polarimetry apparatus (SANPA): Development at the NIST Center for Neutron Research

Spherical neutron polarimetry directly measures the rotation of the neutron polarization after scattering from materials with magnetic structure. It is an under utilized measurement technique that is capable of measuring all nine elements of the polarization tensor of a material. In this article, we describe our new cryogen-free small-angle neutron polarimetry apparatus and infrastructure at the NIST Center for Neutron Research. The resulting apparatus is capable of continuous operation and is designed for measurements at low temperatures (4-8 K) using niobium Meissner shielding and mu-metal shielding to produce a zero-field (≤1 μT) cooling sample environment.

Conceptual design of CHESS, a new direct-geometry inelastic neutron spectrometer dedicated to studying small samples

CHESS is a new direct-geometry inelastic spectrometer, which is planned for the Second Target Station (STS) at the Spallation Neutron Source (SNS) in Oak Ridge. It will take full advantage of the increased peak brilliance of the high-brightness STS coupled moderators and of recent advances in instrument design and technology to achieve unprecedented performance for inelastic scattering in the cold energy range. This paper presents a conceptual design that addresses key requirements and technical solutions which are derived directly from the science case and anticipated use of the instrument.

Optically polarized 3He

This article reviews the physics and technology of producing large quantities of highly spin-polarized 3He nuclei using spin-exchange (SEOP) and metastability-exchange (MEOP) optical pumping. Both technical developments and deeper understanding of the physical processes involved have led to substantial improvements in the capabilities of both methods. For SEOP, the use of spectrally narrowed lasers and K-Rb mixtures has substantially increased the achievable polarization and polarizing rate. For MEOP nearly lossless compression allows for rapid production of polarized 3He and operation in high magnetic fields has likewise significantly increased the pressure at which this method can be performed, and revealed new phenomena. Both methods have benefitted from development of storage methods that allow for spin-relaxation times of hundreds of hours, and specialized precision methods for polarimetry. SEOP and MEOP are now widely applied for spin-polarized targets, neutron spin filters, magnetic resonance imaging, and precision measurements.

New generation high performance in situ polarized 3He system for time-of-flight beam at spallation sources

Modern spallation neutron sources generate high intensity neutron beams with a broad wavelength band applied to exploring new nano- and meso-scale materials from a few atomic monolayers thick to complicated prototype device-like systems with multiple buried interfaces. The availability of high performance neutron polarizers and analyzers in neutron scattering experiments is vital for understanding magnetism in systems with novel functionalities. We report the development of a new generation of the in situ polarized ³He neutron polarization analyzer for the Magnetism Reflectometer at the Spallation Neutron Source at Oak Ridge National Laboratory. With a new optical layout and laser system, the ³He polarization reached and maintained 84% as compared to 76% in the first-generation system. The polarization improvement allows achieving the transmission function varying from 50% to 15% for the polarized neutron beam with the wavelength band of 2-9 Angstroms. This achievement brings a new class of experiments with optimal performance in sensitivity to very small magnetic moments in nano systems and opens up the horizon for its applications.

Non-magnetic flexible heaters for spin-exchange optical pumping of 3He and other applications

Spin polarized ³He gas is currently widely used in various scientific fields and in medical diagnosis applications. The spin polarization of ³He nuclei can be achieved by spin-exchange optical pumping (SEOP). In SEOP, the ³He gas is enclosed in a glass cell together with alkali metals and is then heated to maintain the alkali metal vapor pressures at the appropriate levels. However, polarized ³He gas is highly sensitive to any inhomogeneity in its magnetic field, and any small field gradients caused by the heaters may cause degradation of the ³He polarization. To overcome this conflict between the heating process and the magnetic field, we have developed electrical heaters that essentially cause no magnetic fields. These heaters are thin and are flexible enough to be bent to within a radius of a few centimeters. These carefully designed heater elements and a double layer structure effectively eliminate magnetic field generation. The heaters were originally developed for SEOP applications, but can also be applied to other processes that need to avoid unwanted magnetic fields.

Polarization analysis in neutron small-angle scattering with a novel triplet dynamic nuclear polarization spin filter

A novel neutron spin filter whose principle is based on the strong spin dependence of the neutron scattering on protons has been developed. The dimensions of this filter are small, and it works very efficiently and is stable even in inhomogeneous fields. The protons in the naphthalene spin filter crystal are polarized by a recently developed method of dynamic nuclear polarization using photoexcited triplet states. This technique allows the design of a very compact apparatus that can be placed at a close distance to the sample under investigation. The application of this filter as a polarization analyzer is demonstrated in a magnetic small-angle neutron scattering experiment with the measurement of the spin-dependent scattering signals of a CuFeNi alloy. This sample has a pronounced textured structure factor of ferromagnetic precipitates in a paramagnetic matrix. The performance of the spin filter as an analyzer is illustrated by the excellent agreement of the experimental data with simulations based on a model of homogeneously magnetized spherical particles which are ordered in a simple cubic paracrystalline lattice.