Neutron Spin Filter Research Articles

In-situ 3He neutron spin filters at JCNS, status and updates

The JCNS has been developing and using in-situ polarized neutron spin filters for many applications. The system used for analysis on MARIA and polarization for TOPAS were completed about 10 years ago with the MARIA system in standard operation for users and the TOPAS system employed for a long measurement on the POLI instrument. In the meantime we are progressing on several new in-situ polarizers based on these first two but with additional innovations. The KWS-1 analyzer device which was recently used in tests at TU Delft and ISIS is essentially a 50%-sized copy of the MARIA device. The two devices in construction for polarization and analysis on POLI for hot neutrons feature magic-boxes with angled plates on both the entrance and exit sides to minimize overal length and the polarizer device will employ an additional passive magnetic shield of soft iron so that it can operate inside the stray field area of a 8-T vertical (compensated) sample magnet. We will summarize the current status of our 3He neutron spin filters and provide extra focus on the technical aspects and measured performance characteristics of the new devices for KWS-1 and POLI in particular.

Micro in situ 3He Polarizer for Dimensional Extreme Magnetic Neutron Diffractometer at the High Flux Isotope Reactor

A new micro in situ polarized 3He neutron spin filter has been custom built for use as a neutron polarizer on the Dimensional Extreme Magnetic Neutron Diffractometer (DEMAND) at the High Flux Isotope Reactor (HFIR). The 3He is polarized via spin-exchange optical pumping with a high-power fiber-coupled laser. The 3He neutron spin filter polarizer is designed to be compact in size with the length along the neutron beam only 30 cm because of the limited space available on the beamline. The polarizer is used for the shorter wavelength neutrons (1.005 Å and 1.546 Å) at DEMAND. A 3 bar 3He cell was filled to provide enough opacity so that good neutron polarization can be attained. At DEMAND the polarizer is mounted on an Automated Polarizer Changeover system (APCS) enabling quick switching between polarized and unpolarized neutron diffraction.

Polarisation Development at the European Spallation Source

To meet the ever-increasing user demand, eleven of the fifteen European Spallation Source (ESS) instruments under construction aim to offer polarised neutrons for user experiments. They include an imaging instrument, a SANS instruments, two reflectometers, three diffractometers, and four spectrometers. In conjunction with in-kind contributions and instrumentation grants, the ESS Polarisation Project will support the incorporation of polarisation analysis on eight of the eleven instruments. The project aims to deliver polarised neutrons for first-science experiments as instruments enter operation. Different polariser and polarisation analyser techniques will be available to accommodate the specifics of experiments on a given instrument. Polarised 3He neutron spin filter using either Metastable Optical Pumping (MEOP) or Spin-Exchange Optical Pumping (SEOP) techniques will provide shared-use equipment among many instruments, with SEOP’s main application being in situ beam-polarisation. Several instruments will also use polarising-supermirror devices. To provide wide-bandwidth spin-flipping capability to the time-of-flight instruments, Adiabatic Fast Passage (AFP) neutron spin flippers, also known as gradient-field radiofrequency spin flippers will be the main method of choice. Devices based on the same AFP principle will also be used to flip 3He nuclear spins. We are constructing our first 3He polariser setup, including field coils to produce highly uniform magnetic field. Monte Carlo simulations are being done for the supermirror polarisers. To ensure science-focused development, we are working with university partners in doing scientific experiments with polarised neutrons. These are some of the activities developing polarisation analysis for ESS instruments in our project.

Development of a Spin-Exchange Optical Pumping-Based Polarized 3He System at the China Spallation Neutron Source (CSNS)Supported by the National Key Research and Development Program of China (Grant No. 2020YFA0406000), the National Natural Science Foundation of China (Grant No. 11875265), the Scientific Instrument Developing Project of the Chinese Academy of Sciences (Grant No. 284(2018)), Guangdong Basic and Applied Basic Research Foundation

Polarized 3He neutron spin filters (NSFs) can be used as a vital tool for neutron polarization production and analysis. The China Spallation Neutron Source (CSNS), as one of the major neutron facilities in China, has committed resources to the development of a polarized 3He NSF program to support its growing polarized neutron research. A spin-exchange optical pumping (SEOP)-based polarized 3He system and other necessary hardware for NSF transport has been recently developed. The performance of the system is benchmarked using an in-house developed cell named “Trident”. Neutron beam measurements yield a 3He polarization of 77% with over 200 h of on-beam relaxation time. Combining this newly developed SEOP system with the recently reported cell fabrication station, CSNS is now capable of the fully self-sustained production of 3He NSFs that shall support its future neutron polarization research.

New design of a magnetic device for wide-angle XYZ polarization analysis PASTIS-3, from the concept to first tests with thermal neutrons

We present the new concept and neutron test results of the third generation PASTIS device for XYZ wide-angle polarization analysis for thermal neutrons. PASTIS-3 uses two polarized 3He Neutron Spin filter Cells (NSF) for polarizing and analyzing neutrons spins, flipping the incident neutron beam polarization by reversing the 3He spin state of the polarizing entrance cell. We detail a new coil design for high spatial homogeneity of the magnetic field in any field direction, and the calculation method employed to optimize the geometry and to characterize the field gradients. Various components of the apparatus are described with particular attention to their performance and impact on the relaxation time of the NSF cells. Finally, we present test results of the new device tested on the thermal triple-axis spectrometer IN20 at the ILL.

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 6cm wide and 15cm high and a wavelength band between 0.4 and 0.6nm. 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.

Development of a 3He Gas Filling Station at the China Spallation Neutron SourceSupported by the National Key Research and Development Program of China (Grant No. 2020YFA0406000), the Scientific Instrument Development Project of the Chinese Academy of Sciences (Grant No. 284(2018)), and the National Natural Science Foundation of China (Grant No. 11875265).

At the China Spallation Neutron Source (CSNS), we have developed a custom gas-filling station, a glassblowing workshop, and a spin-exchange optical pumping (SEOP) system for producing high-quality 3He-based neutron spin filter (NSF) cells. The gas-filling station is capable of routinely filling 3He cells made from GE180 glass of various dimensions, to be used as neutron polarizers and analyzers on beamlines at the CSNS. Performance tests on cells fabricated at our gas-filling station are conducted via neutron transmission and nuclear-magnetic-resonance measurements, revealing nominal filling pressures, and a saturated 3He polarization in the region of 80%, with a lifetime of approximately 240 hours. These results demonstrate our ability to produce competitive NSF cells to meet the ever-increasing research needs of the polarized neutron research community.

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.

Development and application of a [formula omitted]He Neutron Spin Filter at J-PARC

We are developing a neutron polarizer with polarized 3He gas, referred to as a 3He spin filter, based on the Spin Exchange Optical Pumping (SEOP) for polarized neutron scattering experiments at Materials and Life Science Experimental Facility (MLF) of Japan Proton Accelerator Research Complex (J-PARC). A 3He gas-filling station was constructed at J-PARC, and several 3He cells with long spin relaxation times have been fabricated using the gas-filling station. A laboratory has been prepared in the MLF beam hall for polarizing 3He cells, and compact pumping systems with laser powers of 30 W and 110 W, which can be installed onto a neutron beamline, have been developed. A 3He polarization of 85% was achieved at a neutron beamline by using the pumping system with the 110 W laser. Recently, the first user experiment utilizing the 3He spin filter was conducted, and there have been several more since then. The development and utilization of 3He spin filters at MLF of J-PARC are reported.

A modular apparatus for use in high-precision measurements of parity violation in polarized eV neutron transmission

We describe a modular apparatus for use in parity-violation measurements in epithermal neutron–nucleus resonances with high instantaneous neutron fluxes at the Manuel Lujan Jr. Neutron Scattering Center at Los Alamos National Laboratory. This apparatus is designed to conduct high-precision measurements of the parity-odd transmission asymmetry of longitudinally polarized neutrons through targets containing nuclei with p-wave neutron–nucleus resonances in the 0.1–10 eV energy regime and to accommodate a future search for time reversal violation in polarized neutron transmission through polarized nuclear targets. The apparatus consists of an adjustable neutron and gamma collimation system, a 3He-4He ion chamber neutron flux monitor, two identical cryostats for target cooling, an adiabatic eV-neutron spin flipper, a near-unit efficiency 6Li-7Li scintillation detector operated in current mode, a flexible CAEN data acquisition system, and a neutron spin filter based on spin-exchange optical pumping of 3He gas. We describe the features of the apparatus design devoted to the suppression of systematic errors in parity-odd asymmetry measurements. We describe the configuration of the apparatus used to conduct a precision measurement of parity violation at the 0.7 eV p-wave resonance in 139La which employs two identical 139La targets, one to polarize the beam on the p-wave resonance using the weak interaction and one to analyze the polarization.

Optimization of a magnetostatic cavity for a 3He spin analyzer on the CANDOR polychromatic reflectometer

A new instrument, the CANDOR (Chromatic Analysis Neutron Diffractometer or Reflectometer) polychromatic beam reflectometer is being built at the National Institute of Standards and Technology Center for Neutron Research. CANDOR will have a capability of polarization analysis for both specular and off-specular scattering, and allow for measuring the magnetic structures of magnetic thin films with a significantly shortened measurement time as compared to a conventional monochromatic beam reflectometer at a continuous neutron source. A critical component of the polarization analysis capability is to develop a 3He neutron spin filter (NSF) for polarization analysis for both specular and off-specular scattering for a continuous wavelength range between 4 Å and 6 Å and a large solid angle coverage for up to 20 energy dispersive detector channels. For ex-situ spin-exchange optical pumping (SEOP) operation, a magnetically shielded solenoid (MSS) will be employed to provide a volume averaged field gradient, better than 5×10−4 cm−1 to maintain 3He polarization relaxation times of at least a few hundred hours on the beam line even when there are significant stray fields from a magnet operating at a field up to 3 T at the sample position. We describe a procedure to design, construct, and optimize a MSS with non-identical compensation coil dimensions on the ends to match the scattered neutron beam rays. The volume averaged field gradient is quantified and optimized over a cell for a MSS using the finite-element software package RADIA and compared with that from the experiment.

Polarization analysis in small-angle neutron scattering with a transportable neutron spin filter based on polarized protons

We present the status of development of a novel neutron spin filter based on the strong spin dependence of the neutron scattering on protons. Using photo-excited triplet states for the dynamic nuclear polarization (DNP) process, proton polarization values of 80% can now be achieved in pentacene doped single crystals of naphthalene at a field of 0.36 T in a simple helium flow cryostat. Careful sample preparation lead to extremely long polarization decay times under moderate conditions that allowed us to develop a transportable device. The filter is polarized in the lab under well controlled conditions and is then transferred to the neutron beam line where it can be operated during several days with almost constant polarization while requiring only a minimum of equipment. We describe briefly the main features of the spin filter and its use as a spin analyzer in a small-angle neutron scattering (SANS) experiment probing the magnetic structure of a nanocrystalline soft magnetic material. Furthermore the procedure for performing the background and spin leakage corrections of the neutron data is outlined in detail.

In-situ 3He SEOP polarizer for Thermal neutrons

In this article, we describe a neutron polarizer for a new time-of-flight neutron spectrometer with the polarization analysis TOPAS. This instrument will use the convergent focused incoming beam of thermal neutrons with energies up to 150 meV (wavelength down to 0.74Å). The polarizer employs a high opacity 3He neutron spin filter with continuous 3He polarization provided by two laser array bars frequency narrowed by an ultra-compact volume Bragg grating. The system was successfully prototyped, and tested. In the course of first experimental test at the polarized hot neutron diffractometer POLI at MLZ, neutron polarization of 97.6% was achieved for 0.895Å neutrons and kept constant during 30 days.

Experimental test of 3He neutron-spin filter in MIEZE spectrometer

A modulated intensity by zero effort (MIEZE) spectrometer is a type of neutron-resonance spin-echo spectrometer and can be used to analyze the slow dynamics of condensed matter by measuring the intermediate scattering function I(Q,τ). A crucial feature of a MIEZE spectrometer is it leaves completely free the space between the sample and the detector because the sample is placed after the neutron-spin analyzer. Measuring the magnetic dynamics with a conventional neutron-spin-echo spectrometer is difficult, but with a MIEZE spectrometer such a measurement may be done simply by introducing a second analyzer after the sample. Measuring the magnetic dynamics with high precision requires the background to be reduced to a minimum and is a job for which a 3He neutron-spin filter (3He-NSF) is a promising candidate as a second analyzer. However, one difficulty with this approach is the diffusion of 3He gas from the 3He-NSF, which may be measured and interpreted as dynamics. Therefore, we investigate herein whether the dynamics of gas diffusion is detected when using a 3He-NSF as second analyzer in a MIEZE spectrometer. Toward this end, we experimented with the MIEZE spectrometer installed at beamline 06 (BL06) at the Materials and Life Science Experimental Facility at Japan Proton Accelerator Research Complex (J-PARC MLF). The results confirm that the gas-diffusion dynamics of the 3He-NSF are not detected, and that the 3He-NSF, therefore, can be used as a second analyzer in this MIEZE spectrometer.

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.

Development of a polarized 3He neutron spin filter based on spin exchange optical pumping at China Mianyang Research Reactor

Development of a polarized 3He neutron spin filter based on spin exchange optical pumping at China Mianyang Research Reactor

A novel broad-band neutron spin filter based on dynamically polarized protons using photo-excited triplet states

The use of polarized protons as a broad-band neutron spin filter is an attractive alternative to the well-established neutron polarization techniques, namely polarized 3He gas and super mirrors, since the spin-dependent neutron proton scattering cross-section is large in a broad wavelength range. We have developed a novel neutron spin filter where we create the necessary large proton polarization in a solid with a recent method of dynamic nuclear polarization (DNP) that uses photo-excited triplet states. This requires only moderate experimental means and allows a compact design. In order to quantify the efficiency of the spin filter, we have measured the relevant spin-dependent and spin-independent terms of the neutron scattering cross-section of a naphthalene single crystal. The data allows to estimate the triplet spin filter performance over a broad wavelength range. With the recently achieved proton polarization of 80% the triplet filter compares well with a state of the art 3He filter.

A transportable neutron spin filter

We report a transportable neutron spin filter based on the strong spin dependence of neutron proton scattering. The necessary high proton polarization is created by dynamic nuclear polarization (DNP) that uses short-lived optically excited triplet states instead of stable radicals, which eliminates the main path of nuclear spin lattice relaxation. The polarization can be preserved for many days under moderate experimental conditions and allows to realize a transportable spin filter which is conveniently polarized in the laboratory and then operated on a neutron beam line with practically frozen polarization maintained with a minimum of equipment. Using triplet DNP bulk proton spins in a pentacene doped single crystal of naphthalene have been polarized to a record value of 80% at a field of 0.36T using a simple helium flow cryostat for cooling. This highly polarized sample can be transported in the compact cryostat without loss of polarization to a neutron beam line where it serves as a spin filter for polarization analysis in a neutron scattering experiment. Relaxation times of T1∼800h are achieved at a field of 20mT and the polarization can be reversed by adiabatic fast passage with an efficiency of above 99%.

The high-intensity reflectometer of the Jülich Centre for Neutron Science: MARIA

MARIA (magnetism reflectometer with high incident angle) is a world class vertical sample reflectometer dedicated to the investigation of thin films in the fields of magnetism, soft matter and biology. The elliptical vertically focusing guide allows one to measure small samples with a typical size of 1 × 1 cm very efficiently. The double-bounce polarizer and the in situ pumped 3He SEOP (spin-exchange optical pumping) neutron spin filter cell for analysing the polarization of the reflected neutron beam can be moved into the beam in seconds. The polarized flux of MARIA amounts to 5 × 107 n (s cm2)-1 at the sample position with a horizontally collimated beam of 3 mrad, a wavelength of λ = 4.5 Å and a wavelength resolution of Δλ/λ = 10%. In the non-polarized mode a flux of 1.2 × 108 n (s cm2)-1 is achieved in this configuration. MARIA is also capable of grazing-incidence small-angle neutron scattering measurements, using a pinhole collimation with two four-segment slits and an absorber that prevents the focusing of the elliptical guide in the vertical direction.

A new magnetic field system for 3He polarization

The nuclear spin-polarized 3He gas has been in depth studied and widely used in various scientific experiments. The polarized 3He gas can be used as a polarized neutron target to study the reaction of neutrons with charged particles or photon beams. On the other hand, spin polarized 3He gas is a good probe for detecting the new interactions in the supernormal model, and has many other applications as follows:the spin-dependent interaction can be studied quantitatively by measuring the NMR frequency shift but the spin-dependent interaction can also be studied by measuring the relaxation time of polarized 3He gas; the polarized 3He gas can be applied to magnetometers and magnetic resonance imaging (MRI); the highly polarized 3He gas can be used as a neutron spin filter for neutron polarization and polarization analysis because of the high correlation between the absorption cross section of the neutron in polarized 3He nucleus and the spin orientation. At present, the three major domestic sources of neutron, CMRR, CARR, and CSNS, are used to study the neutron polarization and polarization analysis techniques based on spin polarized 3He gas. The longitudinal (or spin-lattice) relaxation time (i.e., T1) of 3He is a key parameter that limits the polarizability of 3He gas. In order to reduce the effect of magnetic field gradient on the longitudinal relaxation time of polarized 3He gas, large-sized Helmholtz coils are usually constructed to provide the main magnetic field where the uniformity in the magnetic field central region reaches 10-4 cm-1. To obtain enough magnetic field uniformity, some magnetic field systems even exceed 1.5 m in size. However, it is expected to have a small magnetic field configuration from the view of practicality and convenience. For the common size (3He cells, Merritt coil and Saddle coil can effectively reduce the size of the magnetic field apparatus. However, for electron scattering experiments of 3He cells, the chamber length can be 40 cm. The system length exceeds 1 m even by using the Merritt coil. In this work, a new six-coil system for 3He polarization is obtained. Within the coils, the magnetic field gradient satisfies the requirement that √|▽Bx|2+|▽By|2/B0 -4 cm-1 in more than 30% area, which is better than all the existing coils used in polarized 3He experiments and can be applied to the future 3He instruments. For other experiments that require magnetic field to have a large uniform area, the new six-coil system is also a good option.