Hybrid optical pumping of optically dense alkali-metal vapor without quenching gas

Optical pumping of an optically thick atomic vapor typically requires a quenching buffer gas, such as N2, to prevent radiation trapping of unpolarized photons which would depolarize the atoms. We show that optical pumping of a trace contamination of Rb present in K metal results in a 4.5 times higher polarization of K than direct optical pumping of K in the absence of N2. Such spin-exchange polarization transfer from optically thin species is useful in a variety of areas, including spin-polarized nuclear scattering targets and electron beams, quantum-nondemolition spin measurements, and ultrasensitive magnetometry.     

Optical pumping system design for large production of hyperpolarized

We present a design for a spin-exchange optical pumping system to produce large quantities of highly polarized 129Xe. Low xenon concentrations in the flowing gas mixture allow the laser to maintain high Rb polarization. The large spin-exchange rate between Rb and 129Xe through the long-lived van der Waals molecules at low pressure, combined with a high flow rate, results in large production rates of hyperpolarized xenon. We report a maximum polarization of 64% achieved for a 0.3 l/h Xe flow rate, and maximum magnetization output of 6 l/h at 22% polarization. Our findings regarding the polarization dependence on temperature, nitrogen partial pressure, and gas mixture flow velocity are also reported.     

Limits to the polarization for spin-exchange optical pumping of 3He

Based on measurements of the temperature dependence of 3He relaxation in a wide range of spin-exchange optical pumping cells, we report evidence for a previously unrecognized surface relaxation process. The relaxation rate was found to be linearly proportional to the alkali-metal density with a slope that exceeds the spin-exchange rate, which limits the polarization for current applications, including neutron spin filters, polarized targets, and polarized gas magnetic resonance imaging. We find that the magnitude of this excess relaxation can vary widely between cells, and that the variation is larger for cells of higher surface to volume ratio. We have observed 3He polarization as high as 81%, but further improvements require understanding the origin of this relaxation.     

Hybrid spin-exchange optical pumping of 3He

We demonstrate spin-exchange optical pumping of 3He using a "hybrid" K-Rb vapor mixture. The Rb atoms absorb light from a standard laser at 795 nm, then collisionally polarize the potassium atoms. Spin-exchange collisions of K and 3He atoms then transfer the angular momentum to the 3He with much greater efficiency than Rb-3He. For a K-rich vapor, the efficiency of the hybrid spin-exchange collisions approaches 1/4, an order of magnitude greater than achieved by pure Rb pumping. We present the first measurements of actual photon efficiencies (polarized nuclei produced per absorbed photon), and show that a new parasitic absorption process limits the total efficiencies for both hybrid and pure Rb pumping.     

Performance assessment of a new laser system for efficient spin exchange optical pumping in a spin maser measurement of <Superscript>129</Superscript>Xe EDM

We demonstrate spin-exchange optical pumping of ¹²⁹Xe atoms with our newly made laser system. The new laser system was prepared to provide higher laser power required for the stable operation of spin maser oscillations in the ¹²⁹Xe EDM experiment. We studied the optimum cell temperature and pumping laser power to improve the degree of ¹²⁹Xe spin polarization. The best performance was achieved at the cell temperature of 100 ^°C with the presently available laser power of 1 W. The results show that a more intense laser is required for further improvement of the spin polarization at higher cell temperatures in our experiment.     

Wall relaxation of 3He in spin-exchange cells

The 3He longitudinal spin-relaxation rate T1-1 is crucial for production of highly polarized 3He by spin-exchange optical pumping. We show that T1-1 is increased by a factor of 2-20 solely by exposure of spin-exchange cells to a few-kG magnetic field. The original T1-1 can be restored by degaussing the cell. The effect is attributed to magnetic surface sites and has been observed in both Pyrex and aluminosilicate-glass cells. Our results both advance the understanding of wall relaxation and demonstrate the use of 3He as an extremely sensitive probe of surface magnetism.     

Increasing the pump-up rate to polarize He gas using spin-exchange optical pumping method

In recent years, polarized ³He gas has increasingly been used as neutron polarizers and polarization analyzers. Two of the leading methods to polarize the ³He gas are the spin-exchange optical pumping (SEOP) method and the meta-stable exchange optical pumping (MEOP) method. At present, the SEOP setup is comparatively compact due to the fact that it does not require the sophisticated compressor system used in the MEOP method. The temperature and the laser power available determine the speed, at which the SEOP method polarizes the ³He gas. For the quantity of gas typically used in neutron scattering work, this speed is independent of the quantity of the gas required, whereas the polarizing time using the MEOP method is proportional to the quantity of gas required. Currently, using the SEOP method to polarize several bar-liters of ³He to 70% polarization would require 20−40 h. This is an order of magnitude longer than the MEOP method for the same quantity of gas and polarization. It would therefore be advantageous to speed up the SEOP process. In this article, we analyze the requirements for temperature, laser power, and the type of alkali used in order to shorten the time required to polarize ³He gas using the SEOP method.     

Development and test of SEOP neutron spin filter in Japan

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.     

External cavity design of high-power diode laser for polarized helium-3 neutron spin filters based on spin-exchange optical pumping

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.     

A wide angle neutron spin filter system using polarized He

Polarized ³He neutron spin filters can operate over a wide neutron energy range and provide a large angular acceptance. A compact ³He neutron spin filter system has been developed for the Multi-Axis Crystal Spectrometer at the National Institute of Standards and Technology (NIST) Center for Neutron Research. Sealed ³He cells, polarized by spin-exchange optical pumping, are used as polarizer and analyzer. The polarization of the neutrons incident on the sample is inverted by flipping the polarization of the ³He gas in the polarizer, with only a small effect on the analyzer cells. The cell fabrication process, ³He spin flipper, and the holding magnetic field are discussed and we present the results of a first on-linetest.     

Applications of He neutron spin filters at the NCNR

At the NIST Center for Neutron Research (NCNR), we have applied ³He neutron spin filters (NSFs) to the instruments where ³He NSFs are advantageous, such as thermal triple-axis spectrometry, small-angle neutron scattering, and diffuse reflectometry. We present the status of our development and application of this method, including polarized gas production by spin-exchange optical pumping, magnetostatic cavities for storage of the polarized gas on the beam line, and nuclear magnetic resonance (NMR)-based, on-line monitoring and reversal of the ³He polarization. We present the status of developing user-friendly interfaces incorporated into the instrument software to handle these ³He neutron spin filters while taking data and performing data analysis. Finally we discuss the status of development of a polarization capability on the multi-axis crystal spectrometer, which requires polarization analysis over a 220° angular range.     

Comparative study of in situ N2 rotational Raman spectroscopy methods for probing energy thermalisation processes during spin-exchange optical pumping

Spin-exchange optical pumping (SEOP) has been widely used to produce enhancements in nuclear spin polarisation for hyperpolarised noble gases. However, some key fundamental physical processes underlying SEOP remain poorly understood, particularly in regards to how pump laser energy absorbed during SEOP is thermalised, distributed and dissipated. This study uses in situ ultra-low frequency Raman spectroscopy to probe rotational temperatures of nitrogen buffer gas during optical pumping under conditions of high resonant laser flux and binary Xe/N₂ gas mixtures. We compare two methods of collecting the Raman scattering signal from the SEOP cell: a conventional orthogonal arrangement combining intrinsic spatial filtering with the utilisation of the internal baffles of the Raman spectrometer, eliminating probe laser light and Rayleigh scattering, versus a new in-line modular design that uses ultra-narrowband notch filters to remove such unwanted contributions. We report a ~23-fold improvement in detection sensitivity using the in-line module, which leads to faster data acquisition and more accurate real-time monitoring of energy transport processes during optical pumping. The utility of this approach is demonstrated via measurements of the local internal gas temperature (which can greatly exceed the externally measured temperature) as a function of incident laser power and position within the cell.     

Feedback control for manipulating magnetization in spin-exchange optical pumping system

Control of magnetization plays an important role in the scientific and technological field of manipulating spin systems. In this work, we study the problem of manipulating nuclear magnetization in the spin-exchange optical pumping system, including accelerating the recovery of nuclear polarization and fixing it on a specific desired state. A real-time feedback control strategy is exploited here. We have also done some numerical simulations, with the results clearly demonstrating the effectiveness of our method, that the nuclear magnetization is able to be driven towards the equilibrium state at a much faster speed and also can be stabilized to a target state. We expect that our feedback control method can find applications in gyro experiments.     

Hyperpolarized (131)Xe NMR spectroscopy

Hyperpolarized (hp) (131)Xe with up to 2.2% spin polarization (i.e., 5000-fold signal enhancement at 9.4 T) was obtained after separation from the rubidium vapor of the spin-exchange optical pumping (SEOP) process. The SEOP was applied for several minutes in a stopped-flow mode, and the fast, quadrupolar-driven T(1) relaxation of this spin I = 3/2 noble gas isotope required a rapid subsequent rubidium removal and swift transfer into the high magnetic field region for NMR detection. Because of the xenon density dependent (131)Xe quadrupolar relaxation in the gas phase, the SEOP polarization build-up exhibits an even more pronounced dependence on xenon partial pressure than that observed in (129)Xe SEOP. (131)Xe is the only stable noble gas isotope with a positive gyromagnetic ratio and shows therefore a different relative phase between hp signal and thermal signal compared to all other noble gases. The gas phase (131)Xe NMR spectrum displays a surface and magnetic field dependent quadrupolar splitting that was found to have additional gas pressure and gas composition dependence. The splitting was reduced by the presence of water vapor that presumably influences xenon-surface interactions. The hp (131)Xe spectrum shows differential line broadening, suggesting the presence of strong adsorption sites. Beyond hp (131)Xe NMR spectroscopy studies, a general equation for the high temperature, thermal spin polarization, P, for spin I ≥ 1/2 nuclei is presented.     

Polarized He neutron spin filter program at the JCNS

Polarized neutron instruments will occupy about 80% of the Jülich Centre for Neutron Science (JCNS) instrument park. A successful polarized ³He program will be integral to many of these instruments. We have been focusing the developments on spin-exchange optical pumping (SEOP) to polarize the ³He gas in situ. Where possible, in situ polarization using the SEOP method will provide higher time averaged performance of the instruments. Further this allows a custom-built and independent source of polarized ³He to be developed optimized for each instruments demands. In this paper we will: present an argument for the advantages of in situ polarization; describe an in situ polarizer we have constructed, and initial tests of its performance; describe testing of polarization analysis for small angle neutron scattering on biological samples, and our plans for an in situ polarizer for this application.     

Using frequency-narrowed,tunable laser diode arrays with integrated volume holographic gratings for spin-exchange optical pumping at high resonant fluxes and xenon densities

Next-generation laser diode arrays with integrated ‘on-chip’ volume holographic gratings can provide high power with spectrally narrowed output that can be tuned about the rubidium D₁ line—without causing significant changes to the laser’s flux or spectral profile. These properties were exploited to independently evaluate the effects of varying the laser centroid wavelength and power on batch-mode Rb/¹²⁹Xe spin-exchange optical pumping (SEOP) as functions of xenon partial pressure and cell temperature. Locally optimized SEOP was often achieved with the laser tuned to either the red or blue side of the Rb D₁ line; global optimization of SEOP was observed at lower cell temperatures and followed the D₁ absorption profile, which was asymmetrically broadened and red-shifted from the nominal wavelength. The complex dependence of the optimal wavelength for laser excitation on the cell temperature and Xe density appears to result from an interplay between cell illumination and the Rb/¹²⁹Xe spin-exchange rate, as well as [Xe]_cell-dependent changes to the Rb absorption lineshape that are in qualitative agreement with expectations based on previous work [Romalis et al., Phys. Rev. A, 56:4569–4578, (1997)], but significantly greater in magnitude. These next-generation lasers provide a ∼2–3-fold improvement in ¹²⁹Xe polarization compared to conventional broadband lasers.     

超极化129Xe磁共振分子影像学

磁共振分子影像学发展的主要瓶颈之一在于灵敏度的限制,基于激光光泵和自旋交换技术能获得增强4~5个量级的超极化¹²⁹Xe磁共振信号,因此超极化¹²⁹Xe磁共振分子影像学相对于传统MRI在灵敏度上表现出巨大的优势. 围绕提高灵敏度这一核心MRI问题及其在科学研究中的应用,该文介绍了目前基于超极化¹²⁹Xe的生物分子探针的基本结构和原理,阐述了与之相关的分子影像学方法和技术,同时评述了当前的最新研究进展和发展方向.     

Enhanced NMR signal from the laser-polarized129Xe produced by spin exchange at a high magnetic field

Nuclear-magnetic-resonance (NMR) measurement of laser-polarized gaseous¹²⁹Xe produced by spin-exchange optical pumping with a narrow-linewidth laser at a high magnetic field of 4.7 T is reported. The samples are contained in the glass tubes. The nuclear spin polarization of the laserpolarized¹²⁹Xe is 3.9%, and this corresponds to an enhancement of 9· 10³ compared to the equilibrium value at 311 K and at the same magnetic field. The laser-enhanced¹²⁹Xe NMR signals can be used in MR imaging.     

基于振动弛豫理论提高光抽运太赫兹激光器输出功率的研究

以半经典密度矩阵理论和分子振动弛豫理论为基础,研究添加适当比例缓冲气体与适当减小波导芯径对光抽运太赫兹激光器输出光强的影响.计算结果表明,加入适当比例缓冲气体或适当减小波导的芯径均能提高太赫兹激光的输出光强;同时优化两个参数能进一步提高抽运激光能量转化为太赫兹激光能量的效率,延长工作腔中的有效激活区,延缓抽运饱和效应的出现,提高太赫兹激光输出光强.该研究对提高光抽运太赫兹激光器的能量转化效率、提高光抽运太赫兹激光器的输出功率及实现光抽运太赫兹激光器的小型化有重要的指导意义.     

Development of a Spin-Exchange Optical Pumping-Based Polarized ~3He System at the China Spallation Neutron Source(CSNS)

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 ~3 He 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.