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In situ SEOP polarised 3He neutron spin filter for incident beam polarisation and polarisation analysis on neutron scattering instruments
Abstract
We discuss the development and characterisation of a new in situ spin exchange optical pumping (SEOP) based 3He neutron spin filter polarisation device. We present results from a recent test of the prototype system developed with the Institut Laue-Langevin. The polariser was installed on the polarised reflectometer CRISP at ISIS in the analyser position. The 3He was pumped continuously in situ on the beamline. The system also integrated a 3He adiabatic fast passage spin flipper that allowed reversal of the 3He and therefore neutron polarisation state, allowing for measurement of all four polarisation cross-sections. The system was run for a number of days reaching a 3He polarisation of 63%.
... The RF coil used is dependent on the size and shape of the magnetostatic cavity, but is generally a rectangular end-corrected solenoid, tuned with a capacitor in series to have a resonant frequency equal to the Larmor frequency for 3 He nuclei. An example of a typical coil is shown in Ref. [6]. The amplitude modulated frequency sweep will rotate the bulk magnetisation even if the RF field is highly inhomogeneous, because of the short time spent in B eff [7]. ...
... Previously tested methods of performing AFP on 3 He NSFs have shown relative polarization losses per flip of approximately 1 Â 10 À 4 [6]. The green curve in Fig. 2 shows the effect that continuous flipping with 1 Â 10 À 4 losses would have on the lifetime on an NSF. ...
We describe here a method of performing adiabatic fast passage (AFP) spin flipping of polarized 3He used as a neutron spin filter (NSF) to polarize neutron beams. By reversing the spin states of the 3He nuclei the polarization of a neutron beam can be efficiently reversed allowing for the transmission of a neutron beam polarized in either spin state. Using an amplitude modulated frequency sweep lasting 500ms we can spin flip a polarized 3He neutron spin filter with only 1.8×10−5 loss in 3He polarization. The small magnetic fields (10–15G) used to house neutron spin filters mean the 3He resonant frequencies are low enough to be generated using a computer with a digital I/O card. The versatility of this systems allows AFP to be performed on any beamline or in any laboratory using 3He neutron spin filters and polarization losses can be minimised by adjusting sweep parameters.
... During the course of this work effects of high neutron capture flux densities on the SEOP process were characterized. Performance of this polarizer in the absence of high flux effects is described in [24]. This proceeding will briefly describe the polarizer and the results of the high flux studies and discuss their impact on proposed applications of such 3 He polarizer devices. ...
... The polarizer is described in part in [24]. Here we will focus on some of the technical details of the design. ...
Polarized 3He has shown its unique characteristics in many areas of polarized neutron scattering, its ability to polarize neutrons at short wavelengths, accept wide-angle and divergent beams and low backgrounds enable new classes of experiments. While polarized 3He is not a steady state solution as commonly applied, the benefits have been shown to offset the drawbacks of polarizing and refreshing the polarization in the neutron spin filter cells. As an extension of this work, in-situ polarization using the spin-exchange optical pumping (SEOP) method was explored as a means to construct a system which could be used to polarize 3He in the state used for an effective neutron spin filter to constant polarization while on the neutron beam. An in-situ SEOP polarizer was constructed. This device utilized many devices and principles developed for neutron spin filters which are polarized off the beam line using either SEOP or metastability exchange optical pumping (MEOP) under the same research program. As a collimation of this work effects of extremely high neutron capture flux density incident on the in-situ polarizer were explored.
... Unlike neutron guides or neutron benders, which change the neutron beam alignment and/or neutron spatial beam uniformity owing to their characteristic multiple reflections, polarized 3 He neutron polarizers are free from such beam misalignment and beam interference [1]. In addition, a 3 He polarizer has a wide acceptance angle and creates a smaller amount of background gamma radiation owing to its low neutron activation [2]. Such merits enable the 3 He polarizer to be widely used for neutron experiments in fundamental particle physics or material researches involving neutron radiography, neutron diffraction, neutron reflectometry, neutron interferometry, and small-angle neutron scattering [3,4]. ...
A polarized 3He neutron polarizer for in-situ neutron beam line operation was developed based on a compact magneto-static cavity with a dimension of 280×270×300 mm3 and a fiber-coupled VBG (Volume Bragg Grating) diode laser with a narrow spectral bandwidth of 25 GHz. Built-in NMR coils of the neutron spin polarizer designed for NMR signal measurements were described in detail and their performances were tested for monitoring the progress of in-situ 3He polarization.
... This is critical in our application as the cell cannot be side pumped as in similar online pumping experiments. 26 The windows are 8 mm thick and the cell body is 80 mm long with an inner diameter of 60 mm. The cell pressure measured by neutron transmission at NIST is 0.9 Bar. ...
We present a new instrument for spin echo small angle neutron scattering (SESANS) developed at the Low Energy Neutron Source at Indiana University. A description of the various instrument components is given along with the performance of these components. At the heart of the instrument are a series of resistive coils to encode the neutron trajectory into the neutron polarisation. These are shown to work well over a broad range of neutron wavelengths. Neutron polarisation analysis is accomplished using a continuously operating neutron spin filter polarised by Rb spin-exchange optical pumping of 3He. We describe the performance of the analyser along with a study of the 3He polarisation stability and its implications for SESANS measurements.
Scattering from silica Stöber particles is investigated and agrees with samples run on similar instruments.
... The prototype device was measured on the SESAME beam line at LENS. We employed a continuously pumped nuclear spin polarised 3 He analyser [21], instead of the supermirror device used to test the flipper. The device was cooled to below T c of the YBCO films inside a mu-metal box to prevent flux trapping. ...
The Meissner effect in a thin-film superconductor can be used to create a sharp boundary between regions of different magnetic field and hence can be used as a component of neutron spin manipulation devices. We have developed two cryogenic neutron spin manipulation devices using single-crystal, high-Tc, YBCO films, which can be cooled without using liquid cryogens and eliminate small angle scattering associated with polycrystalline films.
The devices are a spin flipper and a spin precession device both of which use 350-nm-thick YBCO films covered with gold on a 0.5 mm thick sapphire substrate. The spin flipper consists of one such film mounted on an oxygen-free copper frame and connected to a closed-cycle He refrigerator. The flipper is capable of working with a maximum neutron beam size of 42 x 42 mm2 and can be used with both vertical and horizontal guide fields.
The spin precession device was constructed by mounting two of the YBCO films parallel to one another with an H-magnet between them. By changing the current through the H – magnet, the precession of the neutron polarisation between the films can be controlled. Tests at the Low Energy Neutron Source (LENS) show that this device is capable of generating controlled spin precession for a neutron beam up to 20 x 20 mm2 in cross section.
... The 3 He NSF is able to overcome this problem. Moreover, the 3 He NSF enables a large solid angle coverage and polarization of neutron beams with shorter wavelengths by controlling the pressure length of the 3 He gas [5][6][7][8][9][10][11][12]. An in-situ 3 He NSF has been developed for the analyzer on SHARAKU. ...
A new neutron reflectometer, SHARAKU, with a vertical sample-plane geometry was installed at beam line 17 at J-PARC Materials and Life Science Facility. Although a polarizing supermirror was previously installed as a neutron spin analyzer on SHARAKU, a 3He spin filter is advantageous because it can cover a large solid angle. An in-situ SEOP 3He spin filter system using a new compact laser unit has been developed for the analyzer. In this paper, we report a successful off-specular measurement with the new compact in-situ SEOP analyzer at SHARAKU.
... The aim of this proceedings is to add to the existing observations of these e↵ects. In the case of continuously pumped 3 He neutron spin filters [11, 12, 13], provided that the optical pumping rate is much greater than the relaxation rate ( He = 1/T 1 ) of the cell, the cell should reach and maintain a steady-state polarization. However, we report on recent measurements of the 3 He polarization stability of continuously pumped cells with high (T 1 <10 hours) levels of relaxation. ...
The successful use of ³He cells in polarized neutron applications demands that once polarized the gas has both a long relaxation time and high polarization. In spin-exchange optical pumping the cell characteristics determine the maximum polarization as well as the lifetime.
We report on our experience of neutron spin filter cells, showing results on the orientation dependence of such cells. We observe a previously observed lifetime hysterisis effect in ³He cell relaxation lifetime and include new measurements of the angular dependence of the relaxation lifetime, that are characteristic of a dipolar effect which diminishes upon repeated pumping/cooling cycles. We present demonstrations of magnetization effects in ³He and ¹²⁹Xe cells and show the applicability of using sol-gel coatings on reducing the magnetization effect in ¹²⁹Xe cells.
We report recent results of the stability of the polarization in new SEOP cells, which was evident for 2 states, one with high energy spin state, low T1 orientation and the second with low energy spin state, high T1 orientation. These results are suggestive of an abnormal relaxation and have only been observed for cells with short (T1 <10 Hours) lifetimes.
... Because of the current limitation, the laser diode was driven at 47 W. The linear polarized laser beam emitted from the laser diode passes through a half-wave (λ/2) plate to rotate the direction of polarization and to change the ratio of pwave to s-wave. Here we control the laser power of the p-wave component to 14 W. A polarizing beam splitter (PBS) separates the laser beam into the p-wave and the s-wave components [11]. Then the p-wave and s-wave components are sent to an external cavity and beam transform optics, respectively. ...
We launched the polarized 3He neutron spin filters (NSF) project in order to provide neutron polarization for the pulsed neutron beams in Japan. We adopted the in-situ spin exchange optical pumping (SEOP) technique to polarize the nuclear spin of 3He atoms because it has some advantages for our applications. The overall system size is compact and it avoids the problem of the time decay of nuclear spin of 3He thus suppressing the costs of maintenance and providing other advantages [1, 2] with respect to data analysis and quality. In this paper, we performed pulsed neutron beam tests of our compact in-situ SEOP NSF system at the BL10 beamline in the Materials and Life Science Experimental Facility of the Japan Proton Accelerator Research Complex (J-PARC). The polarization of the 3He gas reached was 73 % and a pump-up time constant of 9.5 h was observed. This paper is a status report about the development of in-situ SEOP NSF system for the pulsed beam at J-PARC.
... Therefore TOPAS will use a downsized version of the continuously optically pumped analyzer being prototyped and tested for those applications. It will have a very similar format to the polarizer of [3,4] with somewhat smaller dimensions. A scheme of the polarizer concept is shown in Fig. 4. However the high energy of the TOPAS beam will require the cells to contain 20-30 bar cm of 3 He leading to a cell of 10 to 15 cm long at 2 bar (absolute) pressure. ...
We present the design for the polarization analysis of the future thermal time-of-flight spectrometer at the Juelich Centre for Neutron Science (JCNS) at the FRM II. TOPAS is a time-of-flight spectrometer covering a range of incident energies 20 meV < Ei < 160 meV and an angular range −3° < 2θ < 150°. A set of Fermi choppers selects the incoming energy Ei with a resolution up to 3 %. The instrument is optimized for a high flux on small samples using an elliptical neutron guide. The special feature of TOPAS is the polarization analysis. The incident polarization will be realized by means of a 3He continuously pumped polarizer, which is a downscaled version of the device developed for small angle applications at JCNS. The polarization analysis over a wide angular range demands either short distances between the sample and the analyzer or a large volume of polarized 3He. Here we propose the latter alternative to allow the study of magnetic samples and modest magnetic fields at the sample position.
... Absolute measurements of the 3 He polarisation were performed on the neutron beamlines POLTAX and CRISP [13,20] whilst simultaneously measuring the polarisation via NMR. Typical NMR data for two different coils is shown in figure 4. As the neutron transmission measurement is an absorption measurement it is not effected by radiation damping effects and hence is an accurate measurement of the gas polarisation. ...
Simple methods for manipulating 3He spins at low magnetic fields using low cost digital I/O cards are presented. A brief overview of a digital NMR spectrometer is given along with practical information on the implementation and an assessment of the losses per pulse. Through the use of coils with different quality factors the effects of radiation damping are investigated and assessed in the context of the sample polarisation using neutron transmission measurements. The hardware is further used to synthesize an adiabatic swept pulse which is used to reverse the 3He spin direction, allowing the selection of a particular neutron spin.
... However the ultimate solution of the problem is to preserve the 3 He polarization at a high and constant level. This can be achieved using continuous optical pumping of the 3 He, i.e. matching the pumping rate with the rate of 3 He polarization losses such that the rate balance results in a high polarization at constant level [22]. The so called spin-exchange optical pumping method (SEOP) in principle allows one to keep a high polarization for an essentially unlimited time [23]. ...
Polarization analysis is an important technique for polarized neutron scattering as it allows one to obtain the full information about the vector magnetization in the sample that is critically important for detailed understanding of physical properties of molecular magnets, new superconductors, spin electronic and magnetic nanostructures, as well as the self-organization of magnetic nanostructures. In the simplified 1-dimensional version polarization analysis allows for the separation of coherent and incoherent scattering, making it a potentially important technique for studies of non-deuterated biological objects that themselves produce unavoidable background. We compare some of the major considerations between two different methods for the polarization analysis – supermirror based analyzers and polarized 3He neutron spin filters and point out when the latter is beneficial from the point of view of our neutron experiments and instrumentation. We will also discuss some specific requirements to such neutron spin filters and summarize the classes of instrumentation where they will be applied at the JCNS. Finally we will describe a successful application for small-angle neutron scattering from a biological sample.
The modulated intensity by zero effort small-angle neutron scattering (MI-SANS) technique is used to measure scattering with a high energy resolution on samples normally ill-suited for neutron resonance spin echo. The self-diffusion constant of water is measured over a q – t range of 0.01–0.2 Å ⁻¹ and 70–500 ps. In addition to demonstrating the methodology of using time-of-flight MI-SANS instruments to observe diffusion in liquids, the results support previous measurements on water performed with different methods. This polarized neutron technique simultaneously measures the intermediate scattering function for a wide range of time and length scales. Two radio frequency flippers were used in a spin-echo setup with a 100 kHz frequency difference in order to create a high-resolution time measurement. The results are compared with self-diffusion measurements made by other techniques and the general applicability of MI-SANS at a pulsed source is assessed.
The initial formation stages of surfactant-templated silica thin films which grow at the air-water interface were studied using combined Spin-Echo Modulated Small-Angle Neutron Scattering (SEMSANS) and Small Angle Neutron Scattering (SANS). The films are formed from either a cationic surfactant (CTAB) or non-ionic surfactant (C16EO8) in a dilute acidic solution, by addition of TMOS. Previous work has suggested a two stage formation mechanism with mesostructured particle formation in the bulk solution driving film formation at the solution surface. From the SEMSANS data, it is possible to pinpoint accurately the time associated with the formation of large particles in solution, that go on to form the film, and to show their emergence is concomitant with the appearance of Bragg peaks in the SANS pattern, associated with the 2D-hexagonal order. The combination of SANS and SEMSANS allows a complete depiction of the steps of the synthesis that occur in the subphase.
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 ³ He neutron spin filter with continuous ³ He 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.
We have been developing a ³He neutron spin filter (NSF) for efficient utilization of pulsed neutrons, since it can polarize neutrons effectively in a wide energy range. In order to apply the ³He-NSF to experiments at an intense pulsed neutron experimental facility such as the J-PARC, it is important to make the system stable and easy to setup and operate, because the system is located inside a radiation shield for high energy gamma ray and neutrons. In this study, we have developed compact laser optics with a volume holographic grating (VHG) element for a spin-exchange optical pumping (SEOP) system, and composed an in-situ SEOP ³He-NSF. Current status in the development and application of the in-situ SEOP ³He-NSF at J-PARC is reported.
Over the past several years the JCNS has been developing in-house applications for neutron polarization analysis (PA). These methods include PA for separation of incoherent from coherent scattering in soft matter studies (SANS), and online polarization for analysis for neutron reflectometry, SANS, GISANS and eventually spectroscopy. This paper will present an overview of the user activities at the JCNS at the MLZ and gives an overview of the polarization 3He methods and devices used. Additionally we will summarise current projects which will further support the user activities using polarised 3He spin filters.
A 3He neutron spin filter (NSF) has been designed for a new polarized neutron chopper spectrometer called the Polarization Analysis Neutron Spectrometer with Correlation Method (POLANO) at the Materials and Life Science Experimental Facility of the Japan Proton Accelerator Research Complex. It is designed to fit in a limited space on the spectrometer as an initial neutron beam polarizer and is polarized in situ by spin exchange optical pumping. This will be the first generation 3He NSF on POLANO, and a polarized neutron beam up to 100 meV with a diameter of 50 mm will be available for research on magnetism, hydrogen materials, and strongly correlated electron systems.
We have obtained improvement in the 3He polarization achievable by spin-exchange optical pumping (SEOP). These results were primarily obtained in large neutron spin filter cells using diode bar lasers spectrally narrowed with chirped volume holographic gratings. As compared to our past results with lasers narrowed with diffraction gratings, we have observed between 5% and 11% fractional increase in the 3He polarization PHe. We also report a comparable improvement in PHe for two small cells, for which we would not have expected an increase from improved laser performance. In particular, prior extensive studies had indicated that the alkali-metal polarization was within 3% of unity in one of these cells. These results have impact on understanding the maximum PHe achievable by SEOP, whether the origin of the improvement is from increased alkali-metal polarization or decreased temperature-dependent relaxation. We conclude that the most likely explanation for the improvement in PHe is increased alkali-metal polarization. We have observed PHe of between 0.80 and 0.85 in several large cells, which marks a new precedent for the polarization achievable by SEOP.
For use as a neutron spin polarizer or analyzer in the neutron beam lines of the HANARO (High-flux Advanced Neutron Application ReactOr) nuclear research reactor, a 3He polarizer was designed based on both a compact solenoid coil and a VBG (volume Bragg grating) diode laser with a narrow spectral linewidth of 25 GHz. The nuclear magnetic resonance (NMR) signal was measured and analyzed using both a built-in cosine radio-frequency (RF) coil and a pick-up coil. Using a neutron transmission measurement, we estimated the polarization ratio of the 3He cell as 18% for an optical pumping time of 8 hours.
We measured the neutron beam polarization of the BL05/NOP (Neutron Optics and Physics) beamline at J-PARC with an accuracy of less than 10−3 using polarized 3He gas as a neutron spin analyzer. Precise polarimetry of the neutron beam is necessary to understand the beamline optics as well as for the asymmetry measurements of the neutron beta decay, which are planned in this beamline.
Using a transportable 3He spin filter neutron polariser, we have constructed a polarised thermal neutron diffraction (PND) instrument on a powder neutron diffractometer. To minimise the inhomogeneity of the magnetic fields around the 3He spin filter, an optimised compact non-adiabatic two-coil spin flipper for the PND instrument was developed. Magnetic field criteria were discussed for the design of the flipper. Using the PND instrument with the flipper, we succeeded in observing a difference in the magnetic field dependence of the deposited ferromagnetic phases in nanocomposite materials.
We developed AFP NMR in an aluminum container for polarized noble gas nuclei. The radio frequency magnetic field inside the aluminum container was designed from computer simulations. The polarization loss by the AFP spin flip of 3He was measured to be as low as 3.8×10−4. With this technique, a compact in-situ polarizing 3He neutron spin filter with AFP NMR is demonstrated.
We have begun the development of an in-situ spin-exchange optical pumping (SEOP) system aiming to use it as a neutron spin filter for incident beam polarization at the Japan Proton Accelerator Research Complex (J-PARC). To use it, it is recommended that the optics be adjusted easily, have high stability, and have a small size. In this paper we improved our previous SEOP system aiming to use it in J-PARC and performed a neutron beam test at the JRR-3 NOP beamline to see the performance of the neutron spin filter (NSF). The polarization of the 3He gas reached 73%. This paper gives the present status of the development of in-situ SEOP system in J-PARC.
Maintaining high levels of 3He polarization over long periods of time is important for many areas of fundamental and particle beam physics. Long measurement times are often required in such experiments, and the data quality is a function of the 3He polarization. This is the case for neutron scattering, where the 3He can be used to analyze the spin of a scattered neutron beam. For neutron scattering, the relatively small fluxes of polarized neutrons lead to experiment times longer than several days. Consequently, the Jülich Centre for Neutron Science (JCNS) is developing spin-exchange optical pumping (SEOP) systems capable of polarizing the 3He gas in place on a typical neutron instrument. With the polarizer we have constructed, a very high level of 3He polarization of 80.4%±1.5% was obtained and maintained with good time stability. Having such high levels of polarization that are stable over time will reduce the measurement times for such experiments and eliminate time-dependent data corrections.
The Hybrid Spectrometer (HYSPEC) is a new direct geometry spectrometer at the Spallation Neutron Source at the Oak Ridge National Laboratory. This instrument is equipped with polarization analysis capability with 60° horizontal and 15° vertical detector coverages. In order to provide wide angle polarization analysis for this instrument, we have designed and built a novel polarized (3)He filling station based on the spin exchange optical pumping method. It is designed to supply polarized (3)He gas to HYSPEC as a neutron polarization analyzer. In addition, the station can optimize the (3)He pressure with respect to the scattered neutron energies. The depolarized (3)He gas in the analyzer can be transferred back to the station to be repolarized. We have constructed the prototype filling station. Preliminary tests have been carried out demonstrating the feasibility of the filling station. Here, we report on the design, construction, and the preliminary results of the prototype filling station.
Polarized H3e produced by spin-exchange optical pumping (SEOP) has potential as a neutron spin filter for polarization and polarization analysis in many neutron-scattering and neutron particle physics applications. The advantage of the SEOP method is its suitability for providing continuous stable polarization over the course of long experiments. However, we have discovered that exposure to high neutron flux leads to additional strong relaxation mechanisms in the optically polarized alkali-metal vapor used to polarize the H3e . At a neutron flux density of 4.7×109cm-2s-1 , the alkali-metal relaxation rate increased from 100 to 1000s-1 leading to reduced alkali-metal polarization. Other effects such as time dependence and gas composition dependence were explored to help understand the processes. In this paper we discuss our observations and present possible solutions for practical use of SEOP as a neutron spin filter for high-flux density applications.
Over a period of well over a decade, a large number and variety of polarized 3He cells for neutron applications have been developed and tested at the U.S. National Institute of Standards and Technology (NIST). These cells have primarily been employed for spin-exchange optical pumping (SEOP), and applied to neutron scattering and fundamental neutron physics. We describe the procedures we have employed for producing these cells, as well as their characteristics and applications. Whereas our best results are for cells blown from boron-free aluminosilicate glass, we summarize results with a range of other methods. We discuss our recent work on SEOP cells for wide angle neutron polarization analysis.
The Jülich Centre for Neutron Science is developing spin-exchange optical pumping systems capable of polarizing the 3He gas for use as neutron spin filters with developments focused on individually optimized solutions for each application. When possible steady polarization achieved with in-situ optical pumping will provide high time averaged neutron performance and time-stability, however some applications are foreseen to use a "local filling station" approach. In-situ polarizers are being developed for polarization analysis on reflectometry and small angle neutron scattering instruments. This overview of the current status of our SEOP program describes the motivation of in-situ polarization, the design and first results from the polarizer for analysis on reflectometry, and discusses the designs for a SANS analyzer and plans for wide angle analysis.
Here we report on the development of polarization analysis (PA) techniques to be employed at the ISIS pulsed neutron source second target station. Both spin exchange optical pumping and metastability exchange optical pumping techniques are being developed at ISIS to produce polarized neutron spin filters for use as neutron polarizers and analysers. We focus on the developments of a polarization solution on the LET spectrometer, including the updated design of the PASTIS XYZ coil set and single crystal silicon analyser cell. We also report on the construction of a combined polarizer/analyser solution for the WISH diffractometer.Research Highlights► New PASTIS XYZ coil set provides homogeneous magnetic field for 3He cells. ► New PASTIS XYZ coil set reduces detector dead angles on LET spectrometer. ► Zoolander will provide incident beam and z only polarization analysis for WISH. ► Piston compressor based MEOP filling station for polarization of 3He. ► Over 75% 3He polarization at rate of >1 bar l/h.
Polarized neutron spin filters are being developed based on spin-exchange optical pumping. In the present study a high-power diode laser (85 W) was used to excite Rb atoms and the laser linewidth was narrowed using an external cavity. The optics in the external cavity were designed by ray tracing. The ray-trace calculations demonstrated that a doublet lens in front of the laser eliminates aberrations. The maximum spectral peak height in the doublet optics was found to be 25% higher than for a singlet lens.Research highlights► The optics in the external cavity of a high-power laser diode for SEOP were investigated by ray tracing. ► The ray-trace calculations demonstrated that a doublet lens in front of the laser eliminates aberrations. ► The maximum spectral peak height in the doublet optics was found to be 25% higher than for a singlet lens.
A high power narrow line width (38 W, 0.09 nm full width at half maximum) external cavity diode laser is investigated for rubidium spin exchange optical pumping of <sup>129</sup> X e . This tunable photon source has a constant line width, independent of operating power or wavelength within a 1 nm tuning range. When using this laser, an increase in the <sup>129</sup> X e nuclear polarization is observed when optically pumping at a lower wavelength than the measured Rb electron D <sub>1</sub> absorption. The exact detuning from D <sub>1</sub> for the highest polarization is dependent upon the gas density. Furthermore, at high power and/or high Rb density, a reduction in the polarization occurs at the optimum wavelength as previously reported in spin exchange optical pumping studies of <sup>3</sup> H e which is consistent with high absorption close to the cell front face. These results are encouraging for moderate high throughput polarization of <sup>129</sup> X e in the midpressure range of (0.5–2.0 amagat).
We present measurements of the optical absorption of K vapor at 795 nm due to
the presence of high pressure He gas. The results set a limit on the
polarization attainable in hybrid spin-exchange optical pumping of He-3.
The skew light propagation in optically thick optical pumping cells was investigated. Under conditions common to spin-exchange optical pumping experiments, the circularly polarized light propagating at an angle to the external magnetic field did not optically pump atoms to full transparency. The efficiency of the optical pumping process was reduced even for angles of few degrees due to the excess absorption of the pumping light. The phenomena is expected to greatly increase the amount of light required to pump a given sample of alkali atoms to high polarizations under optically thick conditions.
We have studied the efficiency of spin-exchange optical pumping (SEOP) in cells that contain either pure rubidium, mixtures of rubidium and potassium, or pure potassium. Optical pumping was performed with spectrally narrowed diode array lasers operating at either the wavelength of the Rb 5s1∕2-5p1∕2 transition (795 nm) or the K 4s1∕2-4p1∕2 transition (770 nm). Calculations of the expected efficiency are shown for practical conditions and compared to measurements. We have measured the vapor density ratio in Rb-K mixture cells using absorption of white light. Our focus is on the conditions that are relevant for neutron spin filters, but the results are relevant to all applications of SEOP. We find that both Rb-K and pure K cells yield improvements in the spin-exchange efficiency, and each approach has its own advantages. Overall, the best results for high polarizing rate with high polarization were obtained with mixture cells with K-Rb vapor density ratios between 2 and 6.
Spin-exchange rate coefficients for Rb-3He are measured using three different methods. Two methods, using Rb repolarization and rate balance, require no assumption about the nature of the 3He wall relaxation and agree well; we find spin-exchange rate coefficients of 6.7×10-20cm3/s and 6.8×10-20cm3/s. These results agree with a recent repolarization measurement of Baranga and co-workers [Phys. Rev. Lett. 80, 2801 (1998)]. The third method uses the temperature dependence of the 3He relaxation and is 30% larger than the other two, implying a temperature-dependent wall or an unreasonably large value of the anisotropic spin-exchange interaction for Rb-3He. In the course of these measurements we developed and tested a new method for Rb density measurements under high power pumping conditions, and a new variant of Rb EPR polarimetry, both of which are described here.
We describe here a simple low frequency (<200 kHz) pulsed digital NMR spectrometer suitable for a number of applications where the monitoring of nuclear spin-polarized (hyperpolarized) gases is required. The device is implemented using a digital I/O card and passive duplexer and details of the necessary hardware are presented along with a description of the excitation pulse and digital filtering techniques used. Furthermore we discuss the use of a modified function to describe the free induction decay of the gas that is used to account for the change in lineshape due to magnetic field gradients found across the polarized gas. We present data obtained using the spectrometer including measurements of polarization, destruction per pulse and spin up and relaxation time constants and have shown a low destruction per pulse of only 0.014%, resulting in only a 1 part in 10 000 correction for our system.
We describe the beam characteristics of the first ballistic supermirror
neutron guide H113 that feeds the neutron user facility for particle physics
PF1B of the Institute Laue-Langevin, Grenoble (ILL). At present, the neutron
capture flux density of H113 at its 20x6cm2 exit window is 1.35x10^10/cm^2/s,
and will soon be raised to above 2x10^10/cm^2/s. Beam divergence is no larger
than beam divergence from a conventional Ni coated guide. A model is developed
that permits rapid calculation of beam profiles and absolute event rates from
such a beam. We propose a procedure that permits inter-comparability of the
main features of beams emitted from ballistic or conventional neutron guides.
We have observed depolarization effects when high intensity cold neutron beams are incident on alkali-metal spin-exchange-polarized 3He cells used as neutron spin filters. This was first observed as a reduction of the maximum attainable 3He polarization and was attributed to a decrease of alkali-metal polarization, which led us to directly measure alkali-metal polarization and spin relaxation over a range of neutron fluxes at Los Alamos Neutron Science Center and Institute Laue-Langevin. The data reveal a new alkali-metal spin-relaxation mechanism that approximately scales as sqrt[phi_{n}], where phi_{n} is the neutron capture-flux density incident on the cell. This is consistent with an effect proportional to the concentration of electron-ion pairs but is much larger than expected from earlier work.
We have developed a polarized 3He target for polarized electron scattering measurements. The target has a 3He density of 1.1×1020 nuclei cm-3, a total length of 7.5 cm and thin glass windows of 120-180 mum thickness. 3He polarization as large as 40% was produced, limited only by the available laser power. The 3He polarization was maintained during bombardment with up to 22 muA of 578 MeV electrons. A measurement of elastic scattering of polarized electrons from polarized 3He confirmed that the nuclei in the electron beam had the expected polarization. The polarization is produced by spin exchange with laser optically pumped Rb vapor, and the target design incorporates two separated volumes, one for optical pumping and the other for the electron bombardment. Extensions of this design are practical for fixed target electron scattering from polarized 3He planned for SLAC and CEBAF.
We describe a method to frequency narrow multielement high-power diode bars. Using a commercial 60-W, 49-element, 1-cm-long diode array bar at 795 nm running at 45 W, we narrow the linewidth from 1000 to 64 GHz with only a loss of 33% in output power. The resulting laser light is well suited for spin-exchange optical pumping of noble gas nuclei.
Spin filters based on the large spin dependence of the neutron absorption cross-section by 3He are currently being applied in neutron scattering. We report here the construction and test of a 3He neutron spin filter that incorporates (1) in situ continuous optical pumping to maximize the time-averaged polarization and maintain a stable 3He polarization during experiments, and (2) low-loss adiabatic-fast-passage inversion of the 3He polarization to eliminate the need for a neutron spin flipper. The device was successfully tested at the single-crystal diffractometer at the Intense-Pulsed Neutron Source, Argonne National Laboratory. This device can be used in measurements of static magnetic-materials as well as magnetic-relaxation phenomena with long relaxation times.
We present temperature-dependent measurements of the EPR frequency shifts for Na, K, and Rb interacting with polarized 3He. K and Na frequency shifts were measured via comparison with Rb frequency shifts (well known at low temperatures) in Na-Rb-3He and K-Rb-3He spin-exchange cells. The lowered Rb vapor pressure of these “hybrid” spin-exchange cells also allowed us to extend the measured temperature dependence of the Rb EPR frequency shifts up to 350 °C. This work presents measurements of the EPR frequency shift for Na in 3He and significantly extends the temperature range of previous Rb and K EPR frequency shift measurements. These results are critical to the performance of accurate 3He EPR polarimetry in spin-exchange cells.
We describe the development of a polarized neutron device that combines a 3He neutron spin filter and a neutron spin flipper using adiabatic fast passage (AFP), to adiabatically reverse the 3He polarization and thus the neutron polarization with near perfect symmetry. A typical AFP sequence takes place in 2.5–7.5 ms, with the time for the 3He transition from P to -P much less, thus the neutron polarization is nearly perfectly reversed very quickly with only a 2×10-5 loss in 3He polarization per flip. We believe this device, the 3He “flipperizer” can become a standard option wherever a 3He spin filter is already in use. Our first on beam test was performed on MIRA at the new FRM-2 reactor in Garching using polarized 3He from HELIOS. We also briefly describe tests of a new neutron flipper based on AFP. This broad band neutron RF flipper was shown to create neutron flipping efficiencies of >99% at a neutron wavelength of 0.4 Å. Neutron tests were performed on D3 (ILL) and on ROTAX (ISIS).
We have designed and demonstrated a compact solenoid 3He neutron spin filter with a fast spin flip capability. The 3He spin is flipped by adiabatic fast passage (AFP) in a short time and with very little polarization loss, which is essential for polarized neuron scattering measurements in cancellation of systematic errors. A solenoid coil and compensation coils provide a uniform magnetic field for the polarized 3He to minimize the longitudinal spin relaxation. This neutron spin filter can be installed rather easily in neutron scattering spectrometers as a neutron polarizer as well as a neutron spin analyzer. We describe the design and the performance of the device tested by AFP-NMR and with a pulsed neutron beam at KEK.
We have tested two 3He neutron spin filters (NSF), one for the polarizer and one for the analyzer, in conjunction with a doubly focusing pyrolytic graphite (PG) monochromator on the state-of-the-art BT-7 thermal triple axis spectrometer (TAS) at the National Institute of Standards and Technology (NIST) Center for Neutron Research (NCNR). This system will provide significantly better neutronic performance for polarization analysis over a conventional TAS with Heusler crystals. We discuss the scheme for employing NSFs on the TAS instrument, including the 3He cell design, spin-exchange optical pumping (SEOP) of these large 3He cells, and the holding fields on the spectrometer. Using Rb/K hybrid SEOP, we have produced 75% 3He polarization for the 11 cm diameter cells for TAS in less than two days.
A new procedure for calibrating the efficiencies of the polarisation sensitive components in slow neutron polarised beam instruments is described. The measurement of one “flipping ratio” and two “shim ratios” allows the effective polarisation, the spin flipping efficiency, and a depolarising shim efficiency to be determined unambiguously. Results of its application to the calibration of a neutron time-of-flight polarised beam reflectometer are presented.
We present a concept for a compact magnetostatic cavity called a “Magic box” that provides a very homogeneous magnetic environment for a polarised 3He neutron spin-filter cell in the presence of moderate magnetic stray fields. Based on this concept different systems have been built and tested. The concept is ideal for the transport or hosting of a spin-filter on a neutron beam-line.
A large area polarized 3He neutron spin filter has been developed and tested in long-term operation. The polarizer cells have area 50 cm2 and larger allowing access to a large area neutron beam. Polarization of 3He up to 57±1% was measured by monitoring the neutron transmission through the 3He. The polarization was maintained over the course of several weeks, but the maximum attainable polarization declined over the course of a year-long run as the cell degraded, possibly due to long-term exposure to the neutron beam at high temperatures. The resulting neutron polarization and transmission are velocity dependent and are optimized for 5 Å. Cell construction, polarizer design, performance, and polarimetry are described in this paper.
The D17 neutron reflectometer at the Institut Laue-Langevin, Grenoble, can now carry out neutron polarization analysis experiments with a polarized 3He gas filter. The cell containing the gas is housed in the evacuated detector tank. The gas polarization decays with time and must be replenished periodically. A novel method to accomplish this in situ, known as local filling, has been developed to minimize down-time on the instrument. The instrument is now optimized for the rapid collection of off-specular scattering with polarization analysis, complementary to a polarizing supermirror analyser which is used for measurements of specular reflectivity. Cell performance, test and experimental data are presented, along with methods for analysing the data collected using the filter.
The capability of performing accurate measurements of neutron beam polarization opens a number of exciting opportunities in fundamental neutron physics and in neutron scattering. At the LANSCE pulsed neutron source we have measured the neutron beam polarization with absolute accuracy of 0.3% in the neutron energy range from 40 meV to 10 eV using an optically-pumped polarized 3He spin filter and a relative transmission measurement technique. 3He was polarized using the Rb spin-exchange method. We describe the measurement technique, present our results, and discuss some of the systematic effects associated with the method.
Four channel off-specular reflectivity data from the Fe– 58 Fe superlattice grating, using the SEOP in situ spin filter as an analyser. The data have been corrected for polarisation efficiency and 3 He transmission changes with wavelength
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Fig. 4. Four channel off-specular reflectivity data from the Fe– 58 Fe superlattice grating, using the SEOP in situ spin filter as an analyser. The data have been corrected for
polarisation efficiency and 3 He transmission changes with wavelength.
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