Optical Pumping Cell Research Articles

Resolving the discrepancy between theoretical and experimental polarization of hyperpolarized 129Xe using numerical simulations and in situ optical spectroscopy

For emerging biomedical applications of hyperpolarized xenon, the ability to obtain reliably high nuclear spin polarization levels is paramount. Yet, experimental nuclear spin polarization levels of xenon are highly variable and, more than often than not, well below what theory predicts. Despite rigorous and well-studied theoretical models for hyperpolarization and continuous-flow spin-exchange optical pumping (SEOP), there remains a substantial discrepancy between the theoretical and experimental polarization of 129Xe; inexplicably, seemingly similar experimental parameters can yield very different polarization values. In this paper, the validity of the assumptions typically made about the thermodynamic state of the Rb vapor inside the optical pumping cell and the gas dynamics are investigated through finite element analysis simulations of realistic optical pumping cell models, while in situ optical and nuclear magnetic resonance spectroscopy measurements are used to validate the results of the simulations. Our results show that shorter xenon gas residence times and lower Rb vapor densities than those predicted by empirical saturated vapor pressure curves, along with incorrect SEOP parameters, are the primary cause of the discrepancy between theoretical and experimental polarization values reported in the literature.

On the theory of a pulse laser with microwave pumping

The construction of a laser with a microwave pumping based on the light-pumping cell that uses radiation of sulfur vapor exposed to electromagnetic waves of the microwave band is discussed. The advantages of this device are high efficiency of the optical pumping cell, simple structure, the possibility of cooling the pumping element, and easiness of modifying the emission spectrum of the optical pumping by changing the admixtures in the light-emitting cell. A mathematical tool for the simulation of the short-pulse operation of this laser is proposed basing on the solving a generalized wave equation. The microwave waveguide with the sulfur vapors is interpreted as a regular transmission line with essentially nonlinear dispersion characteristic and substantial dissipation. Possible techniques for the numerical solving the generalized wave equation are described. The continuous approximation of the regular dispersive line is used. In the Fourier approach, the electric field in the system is calculated as a series in the longitudinal wavenumber. As an alternative, the D’Alembert approach may be used, with evaluating the electric field as a series in the frequency. Three-layer explicit and implicit second-order approximation schemes for the solving generalized wave equation in the Fourier approach are constructed. The modeling of radio pulse propagation in the “cold” regular electrodynamic system was performed. Stability of the algorithm and qualitative agreement between the numerical and analytic results confirm the correctness of the base equations as well as their solutions based at the finite-difference schemes.

Electron microscopic observations of Rb particles and pitting in 129Xe spin-exchange optical pumping cells.

High-volume production of hyperpolarized 129Xe by spin-exchange optical pumping (SEOP) has historically fallen short of theoretical predictions. Recently, this shortfall was proposed to be caused by the formation of alkali metal clusters during optical pumping. However, this hypothesis has yet to be verified experimentally. Here, we seek to detect the presence of alkali particles using a combination of both transmission (TEM) and scanning (SEM) electron microscopy. From TEM studies, we observe the presence of particles exhibiting sizes ranging from approximately 0.2 to 1 μm and present at densities of order 10 s of particles per 100 square microns. Particle formation was more closely associated with extensive cell usage history than short-term ([Formula: see text]1 h) SEOP exposure. From the SEM studies, we observe pits on the cell surface. These pits are remarkably smooth, were frequently found adjacent to Rb particles, and located predominantly on the front face of the cells; they range in size from 1 to 5 μm. Together, these findings suggest that Rb particles do form during the SEOP process and at times can impart sufficient energy to locally alter the Pyrex surface.

A 3D-Printed High Power Nuclear Spin Polarizer

Three-dimensional printing with high-temperature plastic is used to enable spin exchange optical pumping (SEOP) and hyperpolarization of xenon-129 gas. The use of 3D printed structures increases the simplicity of integration of the following key components with a variable temperature SEOP probe: (i) in situ NMR circuit operating at 84 kHz (Larmor frequencies of (129)Xe and (1)H nuclear spins), (ii) <0.3 nm narrowed 200 W laser source, (iii) in situ high-resolution near-IR spectroscopy, (iv) thermoelectric temperature control, (v) retroreflection optics, and (vi) optomechanical alignment system. The rapid prototyping endowed by 3D printing dramatically reduces production time and expenses while allowing reproducibility and integration of "off-the-shelf" components and enables the concept of printing on demand. The utility of this SEOP setup is demonstrated here to obtain near-unity (129)Xe polarization values in a 0.5 L optical pumping cell, including ∼74 ± 7% at 1000 Torr xenon partial pressure, a record value at such high Xe density. Values for the (129)Xe polarization exponential build-up rate [(3.63 ± 0.15) × 10(-2) min(-1)] and in-cell (129)Xe spin-lattice relaxation time (T1 = 2.19 ± 0.06 h) for 1000 Torr Xe were in excellent agreement with the ratio of the gas-phase polarizations for (129)Xe and Rb (PRb ∼ 96%). Hyperpolarization-enhanced (129)Xe gas imaging was demonstrated with a spherical phantom following automated gas transfer from the polarizer. Taken together, these results support the development of a wide range of chemical, biochemical, material science, and biomedical applications.

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.

High-volume 100 Liter-per-day SEOP Polarization of 3He

Abstract We describe a novel apparatus for large-scale production of polarized 3He using Spin-Exchange Optical Pumping (SEOP). The large optical pumping cell is enclosed inside a pressure containment vessel to equalize the differential pressure across the glass cell walls. Numerical simulations of laser absorption and spin-transfer guided our choice of pressure, temperature, and laser power. Computational fluid dynamics simulations of the two-zone thermal bath environment revealed buoyancy-induced flow which favored operation inclined at an angle. We prepared and tested three separate 8.5 liter cells at internal pressures up to six amagat (50 bar-liters) with hybrid alkali ratios of K:Rb of 10:1 by mass (4.4:1 by number ratio) and at temperatures up to 250°C. An early prototype used a 1.4 kW broadband laser, while our most recent prototype incorporated a new 2.2kW spectrally narrowed external cavity laser. Using small surface coil NMR to measure polarization, we obtained spin-up rates greater than 20% per hour consistent with our numerical simulations. Unfortunately, each cell installation to date has been compromised, limiting T1 relaxation times to 12 hours and asymptotic polarizations to values below 50%. If a cell could be installed with 50 hour lifetime and X-factor of 0.2, this polarizer will deliver 50 liters of over 70% polarized 3He twice each day.

An accurate optical technique for measuring the nuclear polarisation of 3He gas

In the metastability exchange optical pumping cells of our on-site production unit and of our other experimental set-ups, we use a light absorption technique to measure the 3He nuclear polarisation. It involves weak probe beams at 1083 nm, that are either perpendicular or parallel to the magnetic field and cell axis, with suitable light polarisations. When metastability exchange collisions control the populations of the sublevels in the 23S state, absolute values of the 3He ground state nuclear polarisation are directly inferred from the ratio of the absorption rates measured for these probe beams. Our report focuses on the transverse detection scheme for which this ratio, measured at low magnetic field for σ and π light polarisations, hardly depends on gas pressure or the presence of an intense pump beam. This technique has been systematically tested both in pure 3He and isotopic mixtures and it is routinely used for accurate control of the optical pumping efficiency as well as for calibration of the NMR system.

Optimization of the pumping laser beam spatial profile in the MEOP experiment performed at elevated3He pressures

The efficiency of the metastability exchange optical pumping (MEOP) performed at elevated 3 He gas pressures in high magnetic field was improved by modifying the spatial profile of the pumping laser beam in such a way that it matched the nonuniform distribution of plasma in the optical pumping cell. Comparing to a conventional Gaussian profile, at the 3 He gas pressure equal to 267 mbar, both the achieved nuclear polarization (equal to 26%) and the total magnetization in the cell (equal to 1.37 sccfp) are 60% higher when the annular laser beam profile is applied.

Characterization of a low-pressure high-capacityX129eflow-through polarizer

We describe a low-pressure flow-through apparatus for generating hyperpolarized $^{129}\text{X}\text{e}$ and report its performance by examining both the output $^{129}\text{X}\text{e}$ polarization ${P}_{\text{Xe}}$ by NMR and the in situ Rb polarization profile by optically detected electron paramagnetic resonance. The polarizer is based on a previously presented design employing a long optical pumping cell, lean Xe mixture at low pressure, Rb presaturation, and counterflow of gas with respect to the direction of light propagation. The numerical model to which we compare the polarizer's performance includes the temperature dependence of the $\text{Rb-}^{129}\text{X}\text{e}$ spin-exchange rate, which has not previously been treated. The qualitative trends in the data mostly follow those in the model, although the model predicts ${P}_{\text{Xe}}$ to be up to a factor of two higher than observed. This discrepancy cannot be attributed to low Rb polarization: the model and the optically detected electron paramagnetic resonance data (acquired at six points along the length of the heated portion of the optical pumping cell) are in reasonable agreement and show typical values of 85%--95%, although measurements also reveal an anomalous region of depressed Rb polarization near the middle of the cell. The highest output $^{129}\text{X}\text{e}$ polarization ${P}_{\text{Xe}}=84\ifmmode\pm\else\textpm\fi{}16%$, was recorded using $\ensuremath{\approx}60\text{ }\text{W}$ of frequency-narrowed laser light at a Xe partial pressure (referenced to $20\text{ }\ifmmode^\circ\else\textdegree\fi{}\text{C}$) of $1.1\ifmmode\pm\else\textpm\fi{}0.2\text{ }\text{mbar}$, flowing at 1 sccm of Xe; typical values were ${P}_{\text{Xe}}\ensuremath{\approx}20%$ flowing at 10 sccm of Xe with $\ensuremath{\approx}30\text{ }\text{W}$ of laser light.

Polarization and relaxation of 209Rn

The study of the nuclear polarization of radon is motivated by the expected large enhancement of sensitivity to a CP-violating electric dipole moment (EDM) in isotopes with octupole deformation or vibrational strength. In preparation for EDM measurements, the polarization of radon by spin exchange with laser-polarized alkali metals is studied. The measurement of the alignment of 209Rn using HPGe detectors to observe the resulting anisotropy in the 337 and 745 keV gamma rays emitted following electron-capture decay of 209Rn to 209At is demonstrated. Radon is polarized via spin-exchange collisions with rubidium atoms in an uncoated Pyrex optical pumping cell. Anisotropy measurements at several temperatures are used to study polarization and relaxation .

Measurements of the 3He spin relaxation below room temperature

The 3He spin relaxation was measured below room temperature for spin-exchange optical pumping (SEOP) cells made of quartz glass. Although a large variation was observed among cells, the spin relaxation rates were found to have a similar temperature dependence. Below room temperature, the relaxation rate decreases slowly with temperature while it increases rather rapidly above room temperature. The phenomena below room temperature may be interpreted by adsorption of 3He to the cell wall.

Characterization of violet emission from Rb optical pumping cells used in laser-polarized xenon NMR experiments

Visible emission from Rb optical pumping cells was characterized under a range of conditions relevant to the production of laser-polarized xenon (including temperature, partial pressures, and D 1-resonant 795 nm laser power). Bright 421 nm (6P → 5S) emission was consistent with energy-pooling processes of the type: Rb ∗(5P 1/2) + Rb ∗(5P 1/2,3/2) → Rb ∗(6P 1/2,3/2) + Rb(5S 1/2), with processes transiting through 5D states likely contributing at higher temperatures/lower N 2 partial pressures. Under such conditions a number of Rb lines may be observed, indicating population of Rb states to ⩾9D (∼31 822 cm −1). Such energies exceed those required for efficient production of laser-induced plasma.

Limits to the Polarization for Spin-Exchange Optical Pumping ofHe3

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.

Regionally specific hyperfine polarization of Rb atoms in the vicinity(∼10−5cm)of surfaces

We made regionally specific measurement of the hyperfine polarization $⟨\mathbit{S}∙\mathbit{I}⟩$ of Rb atoms in the vicinity $(\ensuremath{\sim}{10}^{\ensuremath{-}5}\phantom{\rule{0.3em}{0ex}}\mathrm{cm})$ of coated and uncoated Pyrex glass surfaces in optical pumping cells. This is in contrast to the previous hyperfine polarization studies, where the quantity measured is the bulk hyperfine polarization, which depends on surface interactions averaged over the entire cell surfaces. We probe the hyperfine polarization of the Rb atoms in the vicinity of cell surfaces using the evanescent wave of a weak laser beam. We find that the polarization in the vicinity of uncoated surfaces is significantly lower than that in the bulk. The polarization decreases rapidly with decreasing distance from the surface. By contrast, the polarization in the vicinity of a silicone-coated Pyrex glass surface is independent of the distance from the cell surface and is equal to the bulk polarization. Regionally specific measurement of the hyperfine polarization as a function of the penetration depth of the evanescent wave allows us to deduce the hyperfine polarization, its normal gradient, as well as the normal gradient coefficient ${\ensuremath{\mu}}_{\mathbit{S}∙\mathbit{I}}$ at the cell surface. We present the values of these quantities for three representative uncoated cells.

Developments of apparatus for hyperpolarization of Xe-129: optimized structure of cell for efficient spin exchange at a high concentration of optically pumped rubidium

A flow-through-type apparatus for nuclear spin polarization of Xe-129, which is installed immediately before the nuclear magnetic resonance detector and is free from He buffer gas and cold-trap storage, is presented. When the amount of light absorption can be increased, the proportion of ineffective gas which flows in a region with low light intensity can be decreased by decreasing the optical-path length of the optical pumping cell. In other words, the generation efficiency of hyperpolarized noble gas is increased by increasing the partial pressure of rubidium vapor that absorbs the light in the flow apparatus. This design enables the device to continuously generate hyperpolarized Xe gas. The flow-through-type apparatus for the polarization of noble gas nuclear spins using high-power diode laser arrays was constructed. The performance for the polarization of natural abundant Xe gas was evaluated. The values of polarization rate obtained were not so large but the efficiency of the polarization was better than that of the batch-type production. The flow-through-type production methods overcame the limitation of the batch-type production, that is, the polarized Xe-129 depolarizes when it collides with the cell wall.

Mapping surfaces using regionally specific hyperfine polarization

We show that the hyperfine polarization in the vicinity $(\ensuremath{\sim}{10}^{\ensuremath{-}5}\phantom{\rule{0.3em}{0ex}}\mathrm{cm})$ of cell surfaces can be used as a way to quantify and map the regional surface property inside optical pumping cells. In silicone-coated cells our method allows us to image the regional quality of the coatings, revealing the existence of areas that have strikingly different qualities.

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.

Development of a Hyperpolarized 129Xe System on 3T for the Rat Lungs

MRI (magnetic resonance imaging) with 129Xe has gained much attention as a diagnostic methodology because of its affinity for lipids and possible polarization. The quantitative estimation of net detectability and stability of hyperpolarized 129Xe in the dissolved phase in vivo is valuable to the development of clinical applications. The goal of this study was to develop a stable hyperpolarized 129Xe experimental 3T system to statistically analyze the dissolved-phase 129Xe signal in the rat lungs. The polarization of 129Xe with buffer gases at the optical pumping cell was measured under adiabatic fast passage against the temperature of an oven and laser absorption at the cell. The gases were insufflated into the lungs of Sprague-Dawley rats (n = 15, 400-550 g) through an endotracheal tube under spontaneous respiration. Frequency-selective spectroscopy was performed for the gas phase and dissolved phase. We analyzed the 129Xe signal in the dissolved phase to measure the chemical shift, T2*, delay and its ratio in a rat lungs on 3T. The polarizer was able to produce polarized gas (1.1+/-0.47%, 120 cm3) hundreds of times with the laser absorption ratio (25%) kept constant at the cell. The optimal buffer gas ratio of 25-50% rendered the maximum signal in the dissolved phase. Two dominant peaks of 211.8+/-0.9 and 201.1+/-0.6 ppm were observed with a delay of 0.4+/-0.9 and 0.9+/-1.0 s from the gas phase spectra. The ratios of their average signal to that of the gas phase were 5.6+/-5.2% and 4.4+/-4.7%, respectively. The T2* of the air space in the lungs was 2.5+/-0.5 ms, which was 3.8 times shorter than that in a syringe. We developed a hyperpolarized 129Xe experimental system using a 3T MRI scanner that yields sufficient volume and polarization and quantitatively analyzed the dissolved-phase 129Xe signal in the rat lungs.

Progress of the 3He neutron spin filter project at HMI

Recent progress in the various fields related to the project will be described. On the one hand, the development of suitable and reliable filter cells is a crucial task. Especially further decrease of the wall relaxation rate continues to be important. Therefore, we embarked on coatings made out of Cs suboxides, in particular, Cs 7O, and compared such cells in an elementary neutron experiment with those simply coated with Cs. To enable wide-angle applications of the 3He neutron spin filter technique, like reflectometry or small angle scattering, the cell design has been modified in order to get access to scattering angles up to 18°. A new type of transportation unit, well adapted to the requirements of those cells, has been constructed and tested. On the other hand, the construction of a filling station to supply filter cells is indispensable. Although a 3He polarization degree of 80% during optical pumping had been achieved reliably, further improvements for a novel optical pumping cell has been evaluated prepared and tested. Moreover, progress in the compressor completion is discussed. To integrate it, the rebuilding of the filling station for a flow-through type of operation has been evaluated and afterwards the construction of certain parts upgraded, allowing determination of the polarization in each of the four optical pumping cells which will be part of the filling station in the future.

Skew light propagation in optically thick optical pumping cells

Skew light propagation in optically thick optical pumping cells