Spin-exchange Optical Pumping Research Articles

129Xe Dynamic Nuclear Polarization Demystified: The Influence of the Glassing Matrix on the Radical Properties.

129Xe dissolution dynamic nuclear polarization (DNP) is a controversial topic. The gold standard technique for hyperpolarized xenon magnetic resonance imaging (MRI) is spin exchange optical pumping, which received FDA approval in 2022. Nevertheless, the versatility of DNP for enhancing the signal of any NMR active nucleus might provide new perspectives for hyperpolarized 129Xe NMR/MRI. Initial publications about 129Xe DNP underlined the increased complexity in the sample preparation and lower polarization levels when compared to more conventional 13C-labeled molecules, at same experimental conditions, despite very close gyromagnetic ratios. Herein, we introduce, using a Custom Fluid Path system, a user-friendly and very robust sample preparation method. Moreover, investigating the radical properties at real DNP conditions by means of LOngitudinal Detected Electron Spin Resonance, we discovered a dramatic shortening of the electron spin longitudinal relaxation time (T1e) of nitroxyl radicals in xenon DNP samples' matrices, with respect to more commonly used water:glycerol ones. Mitigating those challenges through microwave frequency modulation, we achieved over 20% 129Xe polarization without employing any deuterated solvent.

Dramatic improvement in the “Bulk” hyperpolarization of [formula omitted]Xe via spin exchange optical pumping probed using in situ low-field NMR

We report on hyperpolarization of quadrupolar (I=3/2) 131Xe via spin-exchange optical pumping. Observations of the 131Xe polarization dynamics via in situ low-field NMR show that the estimated alkali-metal/131Xe spin-exchange rates can be large enough to compete with 131Xe spin relaxation. 131Xe polarization up to 7.6±1.5% was achieved in ∼8.5×1020 spins—a ∼100-fold improvement in the total spin angular momentum—potentially enabling various applications, including: measurement of spin-dependent neutron-131Xe s-wave scattering; sensitive searches for time-reversal violation in neutron-131Xe interactions beyond the Standard Model; and surface-sensitive pulmonary MRI.

Opportunities and challenges with hyperpolarized bioresponsive probes for functional imaging using magnetic resonance.

The development of hyperpolarized bioresponsive probes for magnetic resonance imaging (MRI) applications is an emerging and rapidly growing topic in chemistry. A wide range of hyperpolarized molecular biosensors for functional MRI have been developed in recent years. These probes comprise many different types of small-molecule reporters that can be hyperpolarized using dissolution dynamic nuclear polarization and parahydrogen-induced polarization or xenon-chelated macromolecular conjugates hyperpolarized using spin-exchange optical pumping. In this Perspective, we discuss how the amplified magnetic resonance signals of these agents are responsive to biologically relevant stimuli such as target proteins, reactive oxygen species, pH or metal ions. We examine how functional MRI using these systems allows a great number of biological processes to be monitored rapidly. Consequently, hyperpolarized bioresponsive probes may play a critical role in functional molecular imaging for observing physiology and pathology in real time.

Analysis of the thermophysical process within the SEOP polarized 3He system

Temperature is a crucial parameter in the spin-exchange optical pumping (SEOP) process of noble gas (3He), but is hard to measure due to its confinement nature. In this paper, we conduct research upon the temperature and gas flow distribution within a sealed SEOP cell through computational fluid dynamics simulation. The simulation result shows that the external heat exchange of the initial heating of the cell becomes a cooling process in the presence of high pumping laser power absorbed by the alkali metal. The heat from the pumping laser would also cause the gas in the cell to reach a much higher temperature than the oven, with the hottest part appearing on the upper side of the cell. These predicted behaviors from the simulation are later confirmed by our experiment measurement, which strongly indicates that a gas flow and heat flow exist within the cell. These results help us to understand the temperature distribution of 3He gas in the cell and provide references for the development and improvement of the future SEOP system.

Optimization of Nuclear Polarization in an Alkali-Noble Gas Comagnetometer

Self-compensated comagnetometers, employing overlapping samples of spin-polarized alkali and noble gases (for example K-${}^{3}\mathrm{He}$) are promising sensors for exotic beyond-the-standard-model fields and high-precision metrology such as rotation sensing. When the comagnetometer operates in the so-called self-compensated regime, the effective field, originating from contact interactions between the alkali valence electrons and the noble-gas nuclei, is compensated with an applied magnetic field. When the comagnetometer begins operation in a given magnetic field, spin-exchange optical pumping establishes an equilibrium between the alkali electron-spin polarization and the nuclear-spin polarization. Subsequently, when the magnetic field is tuned to the compensation point, the spin polarization is brought out of the equilibrium conditions. This causes a practical issue for long measurement times. We report on a method for closed-loop control of the compensation field. This method allows optimization of the operating parameters, especially magnetic field gradients, in spite of the inherently slow (hours to days) dynamics of the system. With the optimization, higher stable nuclear polarization, longer relaxation times, and stronger electron-nuclear coupling are achieved which is useful for nuclear-spin-based quantum memory, spin amplifiers, and gyroscopes. The optimized sensor demonstrates a sensitivity comparable to the best previous comagnetometer but with four times lower noble-gas density. This paves the way for applications in both fundamental and applied science.

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.

Development of polarized neutron techniques at the China Spallation Neutron Source

In this article, the ongoing development progress of the polarized neutron technique developments at the China Spallation Neutron Source (CSNS) is presented. The general goal of establishing polarized neutron capability in parallel to CSNS beamline construction is also explained. The key instrument development projects of developing the Spin-Exchange Optical Pumping (SEOP) 3He systems, polarized neutron devices, and their application on neutron beamlines are demonstrated. Performance parameters of the developed techniques are also discussed.

Status of the new muonic helium atom HFS measurements at J-PARC MUSE

Measurements of the muonic helium atom hyperfine structure (HFS) are a sensitive tool to test the theory of three-body atomic systems and bound-state quantum electrodynamics (QED) and to determine fundamental constants of the negative muon magnetic moment and mass. The world’s most intense pulsed negative muon beam at J-PARC MUSE brings an opportunity to improve previous measurements and test further CPT invariance by comparing the magnetic moments and masses of positive and negative muons. Test measurements at D-line are now in progress utilizing MuSEUM apparatus at zero field. The first results already have better accuracy than previous measurements in the 1980s. Also, the investigation of a new experimental approach to improve HFS measurements by repolarizing muonic helium atoms using a spin-exchange optical pumping (SEOP) technique was started. If successful, this would drastically improve the measurement accuracy.

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.

Balanced Light-Assisted Hybrid Spin Exchange Optical Pumping in SERF Comagnetometers

Balanced Light-Assisted Hybrid Spin Exchange Optical Pumping in SERF Comagnetometers

Investigating Rubidium Density and Temperature Distributions in a High-Throughput 129Xe-Rb Spin-Exchange Optical Pumping Polarizer.

Accurate knowledge of the rubidium (Rb) vapor density, [Rb], is necessary to correctly model the spin dynamics of 129Xe-Rb spin-exchange optical pumping (SEOP). Here we present a systematic evaluation of [Rb] within a high-throughput 129Xe-Rb hyperpolarizer during continuous-flow SEOP. Near-infrared (52S1/2→52P1/2 (D1)/52P3/2 (D2)) and violet (52S1/2→62P1/2/62P3/2) atomic absorption spectroscopy was used to measure [Rb] within 3.5 L cylindrical SEOP cells containing different spatial distributions and amounts of Rb metal. We were able to quantify deviation from the Beer-Lambert law at high optical depth for D2 and 62P3/2 absorption by comparison with measurements of the D1 and 62P1/2 absorption lines, respectively. D2 absorption deviates from the Beer-Lambert law at [Rb]D2>4×1017 m−3 whilst 52S1/2→62P3/2 absorption deviates from the Beer-Lambert law at [Rb]6P3/2>(4.16±0.01)×1019 m−3. The measured [Rb] was used to estimate a 129Xe-Rb spin exchange cross section of γ′=(1.2±0.1)×10−21 m3 s−1, consistent with spin-exchange cross sections from the literature. Significant [Rb] heterogeneity was observed in a SEOP cell containing 1 g of Rb localized at the back of the cell. While [Rb] homogeneity was improved for a greater surface area of the Rb source distribution in the cell, or by using a Rb presaturator, the measured [Rb] was consistently lower than that predicted by saturation Rb vapor density curves. Efforts to optimize [Rb] and thermal management within spin polarizer systems are necessary to maximize potential future enhancements of this technology.

Effect of absorption oscillation of resonant radiation observed in a compact installation for hyper-polarization of 129Xe

This work presents the discovery and study of a new, to the best of our knowledge, effect of oscillation of resonant radiation absorption in optical cells with rubidium vapor in the presence of xenon and nitrogen inside an installation for xenon polarization by spin-exchange optical pumping. This effect was observed at temperatures of 100°C–130°C, partial xenon pressure exceeding 100 kPa, total gas mix pressure of 400 kPa, and power of broad-band ( Δ λ = ∼ 2 n m ) laser radiation over 2 W. The highest registered magnitude of power oscillations of the transmitted radiation exceeded 20%, the oscillation period depending on the temperature and lying in the range of 4.8–7 s. The conducted measurements of the parameters of the transmitted radiation lead to the conclusion that the discovered effect is related to concentration oscillations of the atomic rubidium vapor, which probably arises due to cyclic dimerization of rubidium or an oscillating continuous chemical reaction of rubidium and the optical cell contents.

Collisional electron paramagnetic resonance frequency shifts in Cs-Rb-Xe mixtures

We report a measurement of the ratio of dimensionless enhancement factors ${\ensuremath{\kappa}}_{0}$ for $\mathrm{Cs}\text{\ensuremath{-}}^{129}\mathrm{Xe}$ and $\mathrm{Rb}\text{\ensuremath{-}}^{129}\mathrm{Xe}$ in the temperature range 115--140 ${}^{\ensuremath{\circ}}\mathrm{C}$; both pairs are used in spin-exchange optical pumping (SEOP) to produce hyperpolarized $^{129}\mathrm{Xe}$. ${\ensuremath{\kappa}}_{0}$ characterizes the amplification of the $^{129}\mathrm{Xe}$ magnetization contribution to the alkali-metal electronic effective field, compared to the case of a uniform continuous medium in classical magnetostatics. The measurement was carried out in ``hybrid'' vapor cells containing both Rb and Cs metal in a prescribed ratio, producing approximately the same vapor density for both. Alternating measurements of the optically detected electron paramagnetic resonance (EPR) frequency shifts caused by the SEOP polarization and subsequent sudden destruction of the same quantity of $^{129}\mathrm{Xe}$ magnetization were made for $^{133}\mathrm{Cs}$ and either $^{87}\mathrm{Rb}$ or $^{85}\mathrm{Rb}$. An important source of systematic error caused by power fluctuations in the pump laser that produced variable light shifts in the EPR frequency was characterized and then mitigated by allowing sufficient warm-up time for the pump laser. We measured ${({\ensuremath{\kappa}}_{0})}_{\mathrm{CsXe}}/{({\ensuremath{\kappa}}_{0})}_{\mathrm{RbXe}}=1.215\ifmmode\pm\else\textpm\fi{}0.007$ with no apparent temperature dependence. Based on our previous measurement ${({\ensuremath{\kappa}}_{0})}_{\mathrm{RbXe}}=518\ifmmode\pm\else\textpm\fi{}8$, we determine ${({\ensuremath{\kappa}}_{0})}_{\mathrm{CsXe}}=629\ifmmode\pm\else\textpm\fi{}10$, which is more precise than, but consistent with, a previous measurement made by J. Fang et al. [Chin. Phys. B 23, 063401 (2014)].

Analysis of Complex Mixtures by Chemosensing NMR Using para-Hydrogen-Induced Hyperpolarization.

ConspectusNuclear magnetic resonance (NMR) is a powerful technique for chemical analysis. The use of NMR to investigate dilute analytes in complex systems is, however, hampered by its relatively low sensitivity. An additional obstacle is represented by the NMR signal overlap. Because solutes in a complex mixture are usually not isotopically labeled, NMR studies are often limited to 1H measurements, which, because of the modest dispersion of the 1H resonances (typically ∼10 ppm), can result in challenging signal crowding. The low NMR sensitivity issue can be alleviated by nuclear spin hyperpolarization (i.e., transiently increasing the differences in nuclear spin populations), which determines large NMR signal enhancements. This has been demonstrated for hyperpolarization methods such as dynamic nuclear polarization, spin-exchange optical pumping and para-hydrogen-induced polarization (PHIP). In particular, PHIP has grown into a fast, efficient, and versatile technique since the recent discovery of non-hydrogenative routes to achieve nuclear spin hyperpolarization.For instance, signal amplification by reversible exchange (SABRE) can generate proton as well as heteronuclear spin hyperpolarization in a few seconds in compounds that are able to transiently bind to an iridium catalyst in the presence of para-hydrogen in solution. The hyperpolarization transfer catalyst acts as a chemosensor in the sense that it is selective for analytes that can coordinate to the metal center, such as nitrogen-containing aromatic heterocycles, sulfur heteroaromatic compounds, nitriles, Schiff bases, diaziridines, carboxylic acids, and amines. We have demonstrated that the signal enhancement achieved by SABRE allows rapid NMR detection and quantification of a mixture of substrates down to low-micromolar concentration. Furthermore, in the transient complex, the spin configuration of p-H2 can be easily converted to spin hyperpolarization to produce up to 1000-fold enhanced NMR hydride signals. Because the hydrides’ chemical shifts are highly sensitive to the structure of the analyte associating with the iridium complex, they can be employed as hyperpolarized “probes” to signal the presence of specific compounds in the mixture. This indirect detection of the analytes in solution provides important benefits in the case of complex systems, as hydrides resonate in a region of the 1H spectrum (at ca. −20 ppm) that is generally signal-free. The enhanced sensitivity provided by non-hydrogenative PHIP (nhPHIP), together with the absence of interference from the complex matrix (usually resonating between 0 and 10 ppm), set the detection limit for this NMR chemosensor down to sub-μM concentrations, approximately 3 orders of magnitude lower than for conventional NMR. This nhPHIP approach represents, therefore, a powerful tool for NMR analysis of dilute substrates in complex mixtures as it addresses at once the issues of signal crowding and NMR sensitivity. Importantly, being performed at high field inside the NMR spectrometer, the method allows for rapid acquisition of multiple scans, multidimensional hyperpolarized NMR spectra, in a fashion comparable to that of standard NMR measurements.In this Account, we focus on our chemosensing NMR technology, detailing its principles, advantages, and limitations and presenting a number of applications to real systems such as biofluids, beverages, and natural extracts.

Measurement of rubidium vapor number density based on Faraday modulator

The actual vapor density characterizing the alkali metal spin-exchange rate remains a compelling issue for spin-exchange optical pumping. Based on the deduced relationship between the Faraday rotation angle and the rubidium vapor number density using the electrodynamics theory, we report a measurement of the number density for rubidium vapor sealed inside a cell based on a Faraday modulator. The measurement relies on the optical rotation angle due to rubidium vapor under a bias magnetic field (∼0.08 T) produced by a samarium–cobalt magnet. A Faraday modulator with a lock-in amplifier is used to accurately measure the tiny optical rotation angle in a temperature range of 387–468 K. In addition, a synchronization verification is performed by the photoelastic modulator (PEM). The recurring data showed that the two methods are consistent with each other. Compared with the PEM method, the Faraday modulator detection system does not need to adjust the optical axis difference of 45° in the PEM detection system, thereby reducing the complexity of the experiment and the error caused by the alignment of the optical axis, which showed that the Faraday modulator detection method more advantageous in measuring the alkali metal density.

J-PARCにおける高性能<sup>3</sup>He中性子スピンフィルタの開発とその先導研究

3He spin filter is a useful neutron polarization device as a neutron polarizer and an analyzer in neutron scattering experiments because of its capabilities to polarize neutrons in broad energy range and cover wide solid angle. We are developing 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 J-PARC. A 3He gas-filling station was constructed at J-PARC, and several 3He cells with long spin relaxation times have been fabricated. A compact pumping systems with laser powers of 110 W was developed, and a 3He polarization of 85 % was achieved under real experimental conditions. Several user experiments utilizing the 3He spin filter are now being conducted at several beamlines at MLF. The development and utilization of 3He spin filters at MLF of J-PARC are reported.

Portable Hyperpolarized Xe-129 Apparatus with Long-Time Stable Polarization Mediated by Adaptable Rb Vapor Density.

The extraordinary sensitivity of 129Xe, hyperpolarized by spin-exchange optical pumping, is essential for magnetic resonance imaging and spectroscopy in life and materials sciences. However, fluctuations of the polarization over time still limit the reproducibility and quantification with which the interconnectivity of pore spaces can be analyzed. Here, we present a polarizer that not only produces a continuous stream of hyperpolarized 129Xe but also maintains stable polarization levels on the order of hours, independent of gas flow rates. The polarizer features excellent magnetization production rates of about 70 mL/h and 129Xe polarization values on the order of 40% at moderate system pressures. Key design features include a vertically oriented, large-capacity two-bodied pumping cell and a separate Rb presaturation chamber having its own temperature control, independent of the main pumping cell oven. The separate presaturation chamber allows for precise control of the Rb vapor density by restricting the Rb load and varying the temperature. The polarizer is both compact and transportable─making it easily storable─and adaptable for use in various sample environments. Time-evolved two-dimensional (2D) exchange spectra of 129Xe absorbed in the microporous metal-organic framework CAU-1-AmMe are presented to highlight the quantitative nature of the device.

Pilot Quality-Assurance Study of a Third-Generation Batch-Mode Clinical-Scale Automated Xenon-129 Hyperpolarizer.

We present a pilot quality assurance (QA) study of a clinical-scale, automated, third-generation (GEN-3) 129Xe hyperpolarizer employing batch-mode spin-exchange optical pumping (SEOP) with high-Xe densities (50% natural abundance Xe and 50% N2 in ~2.6 atm total pressure sourced from Nova Gas Technologies) and rapid temperature ramping enabled by an aluminum heating jacket surrounding the 0.5 L SEOP cell. 129Xe hyperpolarization was performed over the course of 700 gas loading cycles of the SEOP cell, simulating long-term hyperpolarized contrast agent production in a clinical lung imaging setting. High levels of 129Xe polarization (avg. %PXe = 51.0% with standard deviation σPXe = 3.0%) were recorded with fast 129Xe polarization build-up time constants (avg. Tb = 25.1 min with standard deviation σTb = 3.1 min) across the first 500 SEOP cell refills, using moderate temperatures of 75 °C. These results demonstrate a more than 2-fold increase in build-up rate relative to previously demonstrated results in a comparable QA study on a second-generation (GEN-2) 129Xe hyperpolarizer device, with only a minor reduction in maximum achievable %PXe and with greater consistency over a larger number of SEOP cell refill processes at a similar polarization lifetime duration (avg. T1 = 82.4 min, standard deviation σT1 = 10.8 min). Additionally, the effects of varying SEOP jacket temperatures, distribution of Rb metal, and preparation and operation of the fluid path are quantified in the context of device installation, performance optimization and maintenance to consistently produce high 129Xe polarization values, build-up rates (Tb as low as 6 min) and lifetimes over the course of a typical high-throughput 129Xe polarization SEOP cell life cycle. The results presented further demonstrate the significant potential for hyperpolarized 129Xe contrast agent in imaging and bio-sensing applications on a clinical scale.

18W ultra-narrow diode laser absolutely locked to the Rb D2 line.

We described a wavelength locked and spectral narrowed high-power diode laser with a Faraday anomalous dispersion optical filter (FADOF). By an external cavity with a 85Rb FADOF, the central wavelength of the diode laser was precisely locked to the Rb resonance D2 line. The bandwidth was narrowed from the free-running 4 nm to 0.002 nm (1.2 GHz, FWHM). At 4.9 A maximal driven current, the laser produced a continuous wave (CW) output of 18 W with an external cavity efficiency of 80%, either the current or the temperature had no impact on the central wavelength of the diode laser. The Rb cell works well without any damage under a long-time running. This ultra-stable and extreme-narrowed high power diode laser would find many applications in alkali lasers pumping, metastable rare gas laser pumping, spin-exchange optical pumping, and quantum optics.

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.