Xenon Partial Pressure Research Articles

Application of a pulsed OPO seeded by a CW OPO to investigation of linewidth effects on TALIF excitation efficiency

In this article we present a widely tunable laser architecture for high-resolution nonlinear spectroscopy which incorporates a continuous wave optical parametric oscillator (OPO) for injection-seeding of a singly resonant actively-locked β-Ba(BO2)2 nanosecond OPO. Such an instrumental configuration is designed to provide near-transform-limited pulses over 450–650 nm signal, and 225–325 nm after signal doubling. Novel features that enable frequency agility and ramp-lock injection-seeding over a wide frequency range are described. System performance was demonstrated through investigations of two-photon laser induced fluorescence (TALIF) and the influence of excitation linewidth on the signal strength of the 5p6S0 - 6p[3/2]2 transition of atomic xenon at 252.49 nm. Measurements conducted at xenon partial pressures of 1×10-1-4×10-4 Torr, with and without active cavity locking and injection seeding, resulted in a 17× increased TALIF fluorescence yield during injection-seeded operation. The results are found to be consistent with modeling, taking into account the effects of lineshape, energy, and beam spatial cross-section. Modeling was informed by direct single-shot measurements of the signal beam lineshape in seeded and free-running operation using a virtually imaged phased array (VIPA) spectrometer.

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.

Scintillation and ionization yields of helium–xenon gas mixture for application in neutron detectors

The scintillation and ionization yields of helium–xenon gas mixture were measured using a gridded ionization chamber with 241Am alpha particles to study the potential of the mixture to be applied in neutron detectors. The ionization yields for various ratios of the mixture gases were obtained with gas pressures of 1.0, 0.657, and 0.100 MPa. The yield data were explained well using the ratio of the number of excited atoms to that of ionized atoms in helium, i.e. The scintillation yield increased with xenon partial pressure and saturated at 10% of the total pressure, while the ionization yield increased up to 1.59 times of the initial value for pure helium at 1% of the total pressure then gradually increased to 1.8 times. The helium gas with added xenon at 10% of its pressure was suggested as a medium of a neutron detector with signal readouts from both scintillation and ionization, since it increases the scintillation yield to more than 1.1 times of pure xenon and increases the ionization yield to more than increases 1.8 times of pure helium.

Multidimensional mapping of spin-exchange optical pumping in clinical-scale batch-mode 129Xe hyperpolarizers.

We present a systematic, multiparameter study of Rb/129Xe spin-exchange optical pumping (SEOP) in the regimes of high xenon pressure and photon flux using a 3D-printed, clinical-scale stopped-flow hyperpolarizer. In situ NMR detection was used to study the dynamics of 129Xe polarization as a function of SEOP-cell operating temperature, photon flux, and xenon partial pressure to maximize 129Xe polarization (PXe). PXe values of 95 ± 9%, 73 ± 4%, 60 ± 2%, 41 ± 1%, and 31 ± 1% at 275, 515, 1000, 1500, and 2000 Torr Xe partial pressure were achieved. These PXe polarization values were separately validated by ejecting the hyperpolarized 129Xe gas and performing low-field MRI at 47.5 mT. It is shown that PXe in this high-pressure regime can be increased beyond already record levels with higher photon flux and better SEOP thermal management, as well as optimization of the polarization dynamics, pointing the way to further improvements in hyperpolarized 129Xe production efficiency.

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.

Configuration and Performance of a Mobile 129Xe Polarizer

A stand-alone, self-contained and transportable system for the polarization of 129Xe by spin exchange optical pumping with Rb is described. This mobile polarizer may be operated in batch or continuous flow modes with medium amounts of hyperpolarized 129Xe for spectroscopic or small animal applications. A key element is an online nuclear magnetic resonance module which facilitates continuous monitoring of polarization generation in the pumping cell as well as the calculation of the absolute 129Xe polarization. The performance of the polarizer with respect to the crucial parameters temperature, xenon and nitrogen partial pressures, and the total gas flow is discussed. In batch mode the highest 129Xe polarization of PXe = 40 % was achieved using 0.1 mbar xenon partial pressure. For a xenon flow of 6.5 and 26 mln/min, P Xe = 25 % and P Xe = 13 % were reached, respectively. The mobile polarizer may be a practical and efficient means to make the applicability of hyperpolarized 129Xe more widespread.

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.

Interdependence of in-cell xenon density and temperature during Rb/129Xe spin-exchange optical pumping using VHG-narrowed laser diode arrays

The 129Xe nuclear spin polarization (PXe) that can be achieved via spin-exchange optical pumping (SEOP) is typically limited at high in-cell xenon densities ([Xe]cell), due primarily to corresponding reductions in the alkali metal electron spin polarization (e.g. PRb) caused by increased non-spin-conserving Rb–Xe collisions. While demonstrating the utility of volume holographic grating (VHG)-narrowed lasers for Rb/129Xe SEOP, we recently reported [P. Nikolaou et al., JMR 197 (2009) 249] an anomalous dependence of the observed PXe on the in-cell xenon partial pressure (pXe), wherein PXe values were abnormally low at decreased pXe, peaked at moderate pXe (∼300torr), and remained surprisingly elevated at relatively high pXe values (>1000torr). Using in situ low-field 129Xe NMR, it is shown that the above effects result from an unexpected, inverse relationship between the xenon partial pressure and the optimal cell temperature (TOPT) for Rb/129Xe SEOP. This interdependence appears to result directly from changes in the efficiency of one or more components of the Rb/129Xe SEOP process, and can be exploited to achieve improved PXe with relatively high xenon densities measured at high field (including averaged PXe values of ∼52%, ∼31%, ∼22%, and ∼11% at 50, 300, 500, and 2000torr, respectively).

Hyperpolarized 131Xe NMR spectroscopy

Hyperpolarized (hp) 131Xe with up to 2.2% spin polarization (i.e., 5000-fold signal enhancement at 9.4T) 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 T1 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 131Xe 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 129Xe SEOP. 131Xe 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 131Xe 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 131Xe spectrum shows differential line broadening, suggesting the presence of strong adsorption sites. Beyond hp 131Xe NMR spectroscopy studies, a general equation for the high temperature, thermal spin polarization, P, for spin I⩾1/2 nuclei is presented.

P‐94: Discharge Efficiency Study in SMPDP

AbstractThe relationship between IR and VUV emission was investigated in an opposed PDP discharge at high Xe content and high voltage levels. The ratio of VIS/IR and the ratio of Efficacy/VIS/IR were determined to analyze the behavior of the xenon excitation efficiency and the electron excitation efficiency as a function of driving voltage and xenon content. We see that the xenon excitation efficiency is almost independent from the driving voltage, and is increasing approximately linearly with the xenon partial pressure.

Generation of laser-polarized xenon using fiber-coupled laser-diode arrays narrowed with integrated volume holographic gratings

Volume holographic gratings (VHGs) can be exploited to narrow the spectral output of high-power laser-diode arrays (LDAs) by nearly an order of magnitude, permitting more efficient generation of laser-polarized noble gases for various applications. A ∼3-fold improvement in 129Xe nuclear spin polarization, P Xe, (compared to a conventional LDA) was achieved with the VHG-LDA’s center wavelength tuned to a wing of the Rb D 1 line. Additionally, an anomalous dependence of P Xe on the xenon density within the OP cell is reported—including high P Xe values (>10%) at high xenon partial pressures (∼1000 torr).

Xenon Adsorption on Modified ETS-10

The interaction of xenon with silver ETS-10 is found to be unusually strong. Xenon adsorption was studied on Na-ETS-10 and its silver exchanged counterpart, Ag-ETS-10, by gas chromatography and gravimetric adsorption. High adsorption capacities were observed even at low pressure (6 wt % Xe at 0.5 Torr and 25 °C). High isosteric heats of adsorption for xenon on Ag-ETS-10 were observed, higher than on any other adsorbent reported to date. High selectivity of xenon over nitrogen and oxygen is also observed, especially at low xenon partial pressures. The great affinity of this adsorbent for xenon is attributed to the presence of silver nanoparticles, which grow on the surface of the molecular sieve after heat treatment of Ag exchanged material.

Feasibility and Safety of Delivering Xenon to Patients Undergoing Coronary Artery Bypass Graft Surgery While on Cardiopulmonary Bypass

Postoperative neurocognitive deficit is prevalent after cardiac surgery. Xenon may prevent or ameliorate acute neuronal injury, but it also may aggravate injury during cardiac surgery by increasing bubble embolism. Before embarking on a randomized clinical trial to test the safety and efficacy of xenon for postoperative neurocognitive deficit, we undertook a phase I study to investigate the safety of administering xenon to patients undergoing coronary artery bypass grafting while on cardiopulmonary bypass and to assess the practicability of our xenon delivery system. Sixteen patients scheduled for coronary artery bypass grafting surgery with hypothermic cardiopulmonary bypass gave their informed consent to participate in an open-label dose-escalation study (0, 20, 35, 50% xenon in oxygen and air). Xenon was delivered throughout surgery using both a standard anesthetic breathing circuit and the oxygenator. Gaseous and blood xenon partial pressures were measured five times before, during, and after cardiopulmonary bypass. Middle cerebral artery Doppler was used to assess embolic load, and major organ system function was assessed before and after surgery. Middle cerebral artery Doppler showed no evidence of increased emboli with xenon. Patients receiving xenon had no major organ dysfunction: Troponin I and S100beta levels tended to be lower in patients receiving xenon. Up to 25 l xenon was used per patient. Xenon partial pressure in the blood tracked the delivered concentration throughout. Xenon was safely and efficiently delivered to coronary artery bypass grafting patients while on cardiopulmonary bypass. Prevention of nervous system injury by xenon should be tested in a large placebo-controlled, randomized clinical trial.

Production of hyperpolarized xenon in a static pump cell: Numerical simulations and experiments

Hyperpolarized $^{129}\mathrm{Xe}$ finds numerous applications in NMR spectroscopy and magnetic resonance imaging. The production of hyperpolarized $^{129}\mathrm{Xe}$ by spin-exchange optical pumping is therefore an important experimental issue. We model the three-dimensional transport processes within a so-called batch mode pump cell via numerical finite element method simulations and compare the results with experimental data. In particular, the influence of different experimental parameters, such as temperature, xenon and nitrogen partial pressure, laser power, and radius-to-length ratio of a cylindrical pump cell, is evaluated. The developed numerical method is capable of describing the spin-exchange optical pumping process in a realistic manner.

Production of XeI(B) and I2(B) molecules in the plasma of a steady electric discharge in Xe/I2 mixtures

The production of excited xenon iodides and iodine dimers in the plasma of a longitudinal dc glow discharge is investigated. The discharge was ignited in iodine vapor and Xe/I2 mixtures at xenon pressures of P(Xe)=0.1–1.5 kPa and deposited powers of 10–100 W. The current-voltage characteristics of a glow discharge, the plasma emission spectra in the spectral range of 200–650 nm, and the intensities of spectral lines and molecular bands are studied as functions of the deposited power and the xenon partial pressure in a Xe/I2 mixture. It is found that the discharge plasma emits within the spectral range of 206–343 nm, which includes the 206-nm resonant line of atomic iodine and the XeI(B-X) 253-nm and I2(B-X) 343-nm molecular bands. The power deposited in the plasma and the xenon pressure P(Xe) are optimized to achieve the maximum UV emission intensity. The 7-W total UV power emitted from the entire surface of the cylindrical discharge tube is achieved with an efficiency of ≤5%.

Competition with Xenon Elicits Ligand Migration and Escape Pathways in Myoglobin

Evidence for ligand migration toward the xenon-binding cavities in myoglobin comes from a number of laser photolysis studies of MbO 2 including mutants and from cryo- and time-resolved crystallography of MbCO. To explore ligand migration in greater detail, we investigated the rebinding kinetics of both MbO 2 and MbCO under a xenon partial pressure ranging from 1 to 16 atm over the temperature range (293–77 K). Below 180 K xenon affects to a significant, but minor, extent the thermodynamic parameters for rebinding from the primary docking site in each Mb taxonomic substate. Above 200 K the ligand migrates to the proximal Xe1 site but when the latter is occupied by xenon a new kinetic process appears. It is attributed to rebinding from transient docking sites located on the path between the primary and the secondary docking site of both ligands. Ligand escape exhibits a more complicated pattern than expected. At room temperature O 2 and CO escape appears to take place exclusively from the primary site. In contrast, at T ≈ 250 K, roughly 50% of the CO molecules that have escaped from the protein originate from the Xe1 secondary site.

Spin-exchange optical pumping of high-density xenon-129

Gas mixtures with high xenon densities are explored for continuous flow spin-exchange optical pumping. It is shown that the Xe-NMR129 signal increases significantly with increasing xenon partial pressures up to about 200 kPa, despite a decreasing spin-polarization. Comparison of the rubidium infrared D2 emission with the xenon polarization demonstrates that radiation quenching by molecular nitrogen is of no substantial benefit for the pumping process at xenon pressures above 100 kPa. This reflects a diminished importance of spin-depolarization by radiation trapping due to the increased significance of spin-relaxation by rubidium-xenon collisions at high xenon densities. A quantitative expression for this effect is provided.

Subnormal glow discharge in a Xe/Cl2 mixture in a narrow discharge tube

Electrical and optical characteristics of a subnormal glow discharge in a short (L=10 cm) discharge tube with an inner diameter of 5 mm are investigated. The dependences of the discharge current-voltage characteristic, the energy deposition in the discharge, the plasma spectral characteristics in the 130-to 350-nm wavelength range, the emission intensities of the XeCl(D-X) 236-nm and XeCl(B-X) 308-nm bands, and the total emission intensity in the range 180–340 nm on the pressure and composition of the Xe/Cl2 mixture are studied. Two modes of glow discharge are shown to exist: the low-current mode at a discharge current of I ch ≤2 mA and the high-current mode at I ch >2 mA. The transition from one mode to another occurs in a stepwise manner. The increase in the chlorine content causes the discharge voltage and the energy deposition in the plasma to increase. At low pressures of the Xe/Cl2 mixture (P≤0.7 kPa), stationary strata form in the cathode region. The lower the discharge current, the greater the volume occupied by the strata. This longitudinal discharge acts as a powerful source of continuous broadband emission in the range 180–340 nm, which forms due to overlapping the XeCl(D, B-X) and Cl 2 * bands with edges at λ=236, 308, and 258 nm. The intensity of the 236-nm band is at most 20% of the total intensity of UV radiation. The maximum power of UV radiation (3 W at an efficiency of 8%) is attained at a xenon partial pressure of 250–320 Pa and a total pressure of the mixture of 2 kPa.

Parametric studies of x-ray preionized discharge XeCl laser at single shot and at high pulse rate frequency (1 kHz)

The design and performance of a high repetition rate (1 kHz) and high average power (200 W) XeCl discharge pumped laser (λ=308 nm) using a cold cathode x-ray gun or a wire ion plasma gun for preionization are presented. The dependence of the output energy and of the average output power at low and high pulsed repetition frequency (PRF) on xenon partial pressure is studied. The discharge stability at high PRF is better with lower xenon partial pressure. The influence of preionization level and temporal delay between x-ray pulse and laser discharge is discussed, as well as the required preionization level at higher PRF.

Excitation transfer, lifetimes, and satellite bands in Cs(7p)–Xe collisions

Observation of the time-dependence of excitation transfer between Cs 7p 2P1/2 and 2P3/2 levels caused by collisions with xenon are reported. The 2P levels were selectively excited with an N2 laser-pumped dye laser, and the spectrum of the fluorescence radiation was observed with 5 nsec time resolution. The lifetimes of the levels were found to be (2P1/2) 154±2 nsec and (2P3/2) 135±2 nsec, and the 2P1/2 to 2P3/2 transfer cross section at 325±10 °K is 0.150±0.002 Å2. An analysis of the time-dependent decay versus xenon partial pressure shows that the cross section for quenching or collision induced radiation is less than 4×10−3 Å2. This demonstrates that collision induced transitions cannot be important in the formation of near red satellites on the Cs 6s–7p lines perturbed by Xe. In addition, the radiative decay constant for the satellite is identical to that for the line within experimental uncertainty. The correspondence of satellite and line decay curves provides a method of identifying satellites with their parent line.