Spin-exchange Optical Pumping Cells Research Articles

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.

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.

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.

XeUS: A second-generation automated open-source batch-mode clinical-scale hyperpolarizer

We present a second-generation open-source automated batch-mode 129Xe hyperpolarizer (XeUS GEN-2), designed for clinical-scale hyperpolarized (HP) 129Xe production via spin-exchange optical pumping (SEOP) in the regimes of high Xe density (0.66–2.5 atm partial pressure) and resonant photon flux (~170 W, Δλ = 0.154 nm FWHM), without the need for cryo-collection typically employed by continuous-flow hyperpolarizers. An Arduino micro-controller was used for hyperpolarizer operation. Processing open-source software was employed to program a custom graphical user interface (GUI), capable of remote automation. The Arduino Integrated Development Environment (IDE) was used to design a variety of customized automation sequences such as temperature ramping, NMR signal acquisition, and SEOP cell refilling for increased reliability. A polycarbonate 3D-printed oven equipped with a thermo-electric cooler/heater provides thermal stability for SEOP for both binary (Xe/N2) and ternary (4He-containing) SEOP cell gas mixtures. Quantitative studies of the 129Xe hyperpolarization process demonstrate that near-unity polarization can be achieved in a 0.5 L SEOP cell. For example, %PXe of 93.2 ± 2.9% is achieved at 0.66 atm Xe pressure with polarization build-up rate constant γSEOP = 0.040 ± 0.005 min−1, giving a max dose equivalent ≈ 0.11 L/h 100% hyperpolarized, 100% enriched 129Xe; %PXe of 72.6 ± 1.4% is achieved at 1.75 atm Xe pressure with γSEOP of 0.041 ± 0.001 min−1, yielding a corresponding max dose equivalent of 0.27 L/h. Quality assurance studies on this device have demonstrated the potential to refill SEOP cells hundreds of times without significant losses in performance, with average %PXe = 71.7%, (standard deviation σP = 1.52%) and mean polarization lifetime T1 = 90.5 min, (standard deviation σT = 10.3 min) over the first ~200 gas mixture refills, with sufficient performance maintained across a further ~700 refills. These findings highlight numerous technological developments and have significant translational relevance for efficient production of gaseous HP 129Xe contrast agents for use in clinical imaging and bio-sensing techniques.

Pilot multi-site quality assurance study of batch-mode clinical-scale automated xenon-129 hyperpolarizers

We present a pilot quality assurance (QA) study of spin-exchange optical pumping (SEOP) performed on two nearly identical second-generation (GEN-2) automated batch-mode clinical-scale 129Xe hyperpolarizers, each utilizing a convective forced air oven, high-power (~170 W) continuous pump laser irradiation, and xenon-rich gas mixtures (~1.30 atm partial pressure). In one study, the repeatability of SEOP in a 1000 Torr Xe/900 Torr N2/100 Torr 4He (2000 Torr total pressure) gas mixture is evaluated over the course of ~700 gas loading cycles, with negligible decrease in performance during the first ~200 cycles, and with high 129Xe polarization levels (avg. %PXe = 71.7% with standard deviation σPXe = 1.5%), build-up rates (avg. γSEOP = 0.019 min−1 with standard deviation σγ = 0.003 min−1) and polarization lifetimes (avg. T1 = 90.5 min with standard deviation σT = 10.3 min) reported at moderate oven temperature of ~70 °C. Although the SEOP cell in this study exhibited a detectable performance decrease after 400 cycles, the cell continued to produce potentially useable HP 129Xe with %PXe = 42.3 ± 0.6% even after nearly 700 refill cycles. The possibility of “regenerating” “dormant” (i.e., not used for an extended period of time) SEOP cells using repeated temperature cycling methods to recover %PXe is also demonstrated. The quality and consistency of results show significant promise for translation to clinical-scale production of hyperpolarized 129Xe contrast agents for imaging and bio-sensing applications.

Helium-rich mixtures for improved batch-mode clinical-scale spin-exchange optical pumping of Xenon-129

We present studies of spin-exchange optical pumping (SEOP) using ternary xenon-nitrogen-helium gas mixtures at high xenon partial pressures (up to 1330 Torr partial pressure at loading, out of 2660 Torr total pressure) in a 500-mL volume SEOP cell, using two automated batch-mode clinical-scale 129Xe hyperpolarizers operating under continuous high-power (~170 W) pump laser irradiation. In this pilot study, we explore SEOP in gas mixtures with up to 45% 4He content under a wide range of experimental conditions. When an aluminum jacket cooling/heating design was employed (GEN-3 hyperpolarizer), 129Xe polarization (%PXe) of 55.9 ± 0.9% was observed with mono-exponential build-up rate γSEOP of 0.049 ± 0.001 min−1 for the 4He-rich mixture (1000 Torr Xe/900 Torr He, 100 Torr N2), compared to %PXe of 49.3 ± 3.3% at γSEOP of 0.035 ± 0.004 min−1 for the N2-rich gas mixture (1000 Torr Xe/100 Torr He, 900 Torr N2). When forced-air cooling/heating was used (GEN-2 hyperpolarizer), %PXe of 83.9 ± 2.7% was observed at γSEOP of 0.045 ± 0.005 min−1 for the 4He-rich mixture (1000 Torr Xe/900 Torr He, 100 Torr N2), compared to %PXe of 73.5 ± 1.3% at γSEOP of 0.028 ± 0.001 min−1 for the N2-rich gas mixture (1000 Torr Xe and 1000 Torr N2). Additionally, %PXe of 72.6 ± 1.4% was observed at a build-up rate γSEOP of 0.041 ± 0.003 min−1 for a super-high-density 4He-rich mixture (1330 Torr Xe/1200 Torr 4He/130 Torr N2), compared to %PXe = 56.6 ± 1.3% at a build-up rate of γSEOP of 0.034 ± 0.002 min−1 for an N2-rich mixture (1330 Torr Xe/1330 Torr N2) using forced air cooling/heating. The observed SEOP hyperpolarization performance under these conditions corresponds to %PXe improvement by a factor of 1.14 ± 0.04 at 1000 Torr Xe density and by up to a factor of 1.28 ± 0.04 at 1330 Torr Xe density at improved SEOP build-up rates by factors of 1.61 ± 0.18 and 1.21 ± 0.11 respectively. Record %PXe levels have been obtained here: 83.9 ± 2.7% at 1000 Torr Xe partial pressure and 72.6 ± 1.4% at 1330 Torr Xe partial pressure. In addition to improved thermal stability for SEOP, the use of 4He-rich gas mixtures also reduces the overall density of produced inhalable HP contrast agents; this property may be desirable for HP 129Xe inhalation by human subjects in clinical settings—especially in populations with heavily impaired lung function. The described approach should enjoy ready application in the production of inhalable 129Xe contrast agent with near-unity 129Xe nuclear spin polarization.

Batch-Mode Clinical-Scale Optical Hyperpolarization of Xenon-129 Using an Aluminum Jacket with Rapid Temperature Ramping

We present spin-exchange optical pumping (SEOP) using a third-generation (GEN-3) automated batch-mode clinical-scale 129Xe hyperpolarizer utilizing continuous high-power (~170 W) pump laser irradiation and a novel aluminum jacket design for rapid temperature ramping of xenon-rich gas mixtures (up to 2 atm partial pressure). The aluminum jacket design is capable of heating SEOP cells from ambient temperature (typically 25 °C) to 70 °C (temperature of the SEOP process) in 4 minutes, and perform cooling of the SEOP gas mixture to the temperature at which the hyperpolarized gas mixture can be released from the hyperpolarizer (with negligible quantity of Rb metal leaving the cell) in approximately 4 minutes-substantially faster (by a factor of six) than previous hyperpolarizer designs relying on air heat exchange versus aluminum as employed here. These decreases in temperature cycling time will likely be highly beneficial for the overall increase of production rates of batch-mode (i.e., stopped-flow) 129Xe hyperpolarizers, which is particularly beneficial for clinical applications. The additional advantage of the presented design is significantly improved thermal management of the SEOP cell. Accompanying the heating jacket design and performance, we also evaluate the repeatability of SEOP experiments conducted using this new architecture, and present typically achievable hyperpolarization levels exceeding 40% at exponential build-up rates on the order of 0.1 min-1.

Characterizing and modeling the efficiency limits in large-scale production of hyperpolarized 129Xe.

The ability to produce liter volumes of highly spin-polarized 129Xe enables a wide range of investigations, most notably in the fields of materials science and biomedical MRI. However, for nearly all polarizers built to date, both peak 129Xe polarization and the rate at which it is produced fall far below those predicted by the standard model of Rb metal vapor, spin-exchange optical pumping (SEOP). In this work, we comprehensively characterized a high-volume, flow-through 129Xe polarizer using three different SEOP cells with internal volumes of 100, 200 and 300 cc and two types of optical sources: a broad-spectrum 111-W laser (FWHM = 1.92 nm) and a line-narrowed 71-W laser (FWHM = 0.39 nm). By measuring 129Xe polarization as a function of gas flow rate, we extracted peak polarization and polarization production rate across a wide range of laser absorption levels. Peak polarization for all cells consistently remained a factor of 2-3 times lower than predicted at all absorption levels. Moreover, although production rates increased with laser absorption, they did so much more slowly than predicted by the standard theoretical model and basic spin exchange efficiency arguments. Underperformance was most notable in the smallest optical cells. We propose that all these systematic deviations from theory can be explained by invoking the presence of paramagnetic Rb clusters within the vapor. Cluster formation within saturated alkali vapors is well established and their interaction with resonant laser light was recently shown to create plasma-like conditions. Such cluster systems cause both Rb and 129Xe depolarization, as well as excess photon scattering. These effects were incorporated into the SEOP model by assuming that clusters are activated in proportion to excited-state Rb number density and by further estimating physically reasonable values for the nanocluster-induced, velocity-averaged spin-destruction cross-section for Rb (<σcluster-Rbv> ≈4×10-7 cm3s-1), 129Xe relaxation cross-section (<σcluster-Xev> ≈ 4×10-13 cm3s-1), and a non-wavelength-specific, photon-scattering cross-section (σcluster ≈ 1×10-12 cm2). The resulting modified SEOP model now closely matches experimental observations.

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.

Comment on “Enhanced polarization and mechanisms in optically pumped hyperpolarized3Hein the presence of4He”

Chen et al. [Phys. Rev. A 81, 033422 (2010)] claim that adding ${}^{4}\mathrm{He}$ to spin-exchange optical pumping (SEOP) cells confines ${}^{3}\mathrm{He}$ atoms to a diffusion-limited region, which effectively reduces the wall relaxation factor $X$. Here $X$ is a phenomenological parameter used to characterize typical SEOP cells; it quantifies the observation that the measured linear slope of the relaxation rate with alkali-metal-atom density exceeds the spin-exchange rate. In addition they claim to demonstrate that magnetic-field gradients cause a diffusive motion of ${}^{3}\mathrm{He}$. We argue that these claims and other central conclusions drawn by Chen et al. [Phys. Rev. A 81, 033422 (2010)] are inconsistent with the physics of diffusion or should not be made from the data as presented. We also draw attention to several additional issues associated with the results, analysis, and presentation of data in Chen et al. [Phys. Rev. A 81, 033422 (2010)] that confound clear interpretation.

Polarized 3He cell development and application at NIST

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.

Low-field orientation dependence of3Herelaxation in spin-exchange cells

We have observed a significant dependence of ${}^{3}\mathrm{He}$ longitudinal relaxation times in glass spin-exchange optical pumping (SEOP) cells due only to the physical orientation of the cell in a 3 mT (30 G) applied magnetic field. The cells had no previous exposure to higher fields or were thoroughly degaussed prior to being measured. The presence of rubidium metal and heating of the cells associated with the SEOP process is necessary to produce this low-field orientation dependence. Our data suggest that the magnetic relaxation sites at the glass wall involved here may be the dominant cause of wall relaxation in SEOP cells at any field.