Search

After being hyperpolarized using the technique of Metastability Exchange Optical Pumping (MEOP), 3He can be used as a contrast agent for lung magnetic resonance imaging (MRI). MEOP is usually performed at low magnetic field (∼1 mT) and low pressure (∼1 mbar), which results in a low magnetization production rate. Polarization preserving compression with a compression ratio of order 1000 is also required. It was demonstrated in sealed cells that high nuclear polarization values can be obtained at higher pressures with MEOP, if performed at high magnetic field (non-standard conditions). In this work, the feasibility of building a high-field polarizer that operates within a commercial 1.5 T scanner was evaluated. Preliminary measurements of nuclear polarization with sealed cells filled at different 3He gas pressures (1.33 to 267 mbar) were performed. The use of an annular shape for the laser beam increased by 25% the achievable nuclear polarization equilibrium value (Meq) at 32 and 67 mbar as compared to a Gaussian beam shape. Meq values of 66.4% and 31% were obtained at 32 and 267 mbar, respectively, and the magnetization production rate was increased by a factor of 10 compared to the best results obtained under standard conditions. To study the reproducibility of the method in a polarizing system, the same experiments were performed with small cells connected to a gas handling system. Despite careful cleaning procedure, the purity of the 3He gas could not be matched to that of the sealed cells. Consequently, the polarization build-up times were approximately 3 times longer in the 20-30 mbar range of pressure than those obtained for the 32 mbar sealed cell. However, reasonable Meq values of 40%-60% were achieved in a 90 ml open cell. Based on these findings, a novel compact polarizing system was designed and built. Its typical output is a 3He gas flow rate of 15 sccm with a polarization of 33%. In-vivo lung MRI ventilation images (Signal to Noise Ratio (SNR) of approximately 55 for a voxel size of 50 mm × 3 mm × 3 mm) were acquired to demonstrate the polarizer's application. [ABSTRACT FROM AUTHOR]

The transition of the dynamics of ND[sup +, sub 4] ions in ammonium persulphate, dominated at low temperatures by coherent uniaxial rotational tunnelling about one specific N-D bond (the preferred bond) and, at high temperatures, by frequent stochastic jumps about all N-D bonds is elucidated with deuteron spin-lattice relaxation measurements, selective saturation experiments and deuteron NMR line shape analyses. Between 20 and 35 K, the coherent uniaxial tunnelling is superseded by thermally activated stochastic jumps about the same bond with kinetic parameters k[sup 0, sub dyn] = 10[sup (11.5±0.5)] s[sup -1] and E[sup a, sub dyn] (3.6±0.3) kJ/mol. At higher temperatures, thermally activated stochastic jumps about the other N-D bonds set in. Their kinetic parameters are k[sup 0, sub st] 10[sup(12.2±.0·5)]sup -1] and E[sup a, sub st] = = (7.8±0.5) kJ/mol. From the primary and secondary tunnelling observed at low temperatures we infer the heights of the potentials which hinder rotations of the ND[sup +, sub 4] ions about the preferred and any other N-D bond. These heights, minus the rotator's ground state energy, are about 25% larger than, respectively, E[sup a, sub dyn] and E[sup a, sub st]. The kinetic parameters of the two stochastic processes are such that the essentially uniaxial coherent and then incoherent dynamics at low temperatures is superseded at the decomposition temperature of the compound by stochastic reorientational jumps which reflect the basic tetrahedral symmetry of the ammonium ion. [ABSTRACT FROM AUTHOR]

We report on a single-crystal deuteron nuclear magnetic resonance (NMR) spectroscopy study of the low-temperature dynamics of ND[sup +, sub 4] and NH[sub 3]D[sup +] ions in the title compound. The most prominent feature of the dynamics of ND[sup +, sub 4] ions is uniaxial rotational (primary) tunneling of three deuterons about the N-D bond of the fourth. At T< 25 K, the latter deuteron gets localized on the time scale of 10[sup -3]s. We identify the direction of this unique N-D bond. The low-temperature limit of the primary tunneling frequency is 1.6 MHz. In the form of splittings of NMR resonances, our spectra also contain clear evidence of secondary tunneling which, again, is uniaxial. Again we say about which of the other three N-D bonds it takes place. The secondary tunneling frequency is only 4.5 kHz. The deuteron of NH[sub 3]D[sup +] ions gets localized at T<25 K. It can reside in any of the four sites available to the hydrogens of the ion. The dynamics of the three protons depends strongly on which site the deuteron occupies. If it is the site which was identified as unique for ND[sup +][sub 4] ions, the protons reorient stochastically with a rate k > 10[sup 6] S[sup -1] . Very likely, they also undergo tunneling but the stochastic reorientations erase any tunneling features from the spectra. By contrast, if the deuteron occupies any other site, stochastic reorientations of the protons are slow and (proton) tunneling on the scale of 10[sup 5] Hz can be identified. Finally, isotopic ordering is observed. The single deuteron of NH[sub 3]D[sup +] ions goes preferentially into the site identified as unique. Energetically, the preference amounts to 1 meV. [ABSTRACT FROM AUTHOR]

In the view of growing application of hyperpolarized xenon in various fields of science and medicine, the availability of relatively low cost, reliable polarizers is of the utmost importance. The article describes the design and construction of the spin exchange optical pumping (SEOP) polarizer for 129Xe, operating in the stop-flow mode. Using 9 W of laser power at the input of 1 litre optical pumping cell, a single batch of the gas mixture is polarized in about 15 minutes. The xenon nuclear polarization is calibrated against the thermally polarized water and xenon phantoms. It reaches about 40 % in the gas mixture containing 3 % of isotopically enriched xenon. The hyperpolarized xenon is accumulated in the detachable liquid nitrogen dewar containing 0.6 T Halbach magnet with the cold trap inside. All components that are important for the effective operation of the system are described in detail, and possible improvements are pointed out. [ABSTRACT FROM AUTHOR]

We present a general theory of coherent isotropic averaging in nuclear magnetic resonance (NMR). In a zero external field, magnetic-field pulses can selectively average the internal spin Hamiltonians, while preserving the intrinsic invariance of the spectrum with respect to the sample orientation. The theory predicts the limits of the scaling factors for tensor interactions of different ranks. Time reversal is found to be possible for first- and second-rank tensors with scaling factors of -1/3 and -1/4, respectively. Explicit sequences, based on icosahedral symmetry, are given for a number of optimal scaling factors. To illustrate the theory, an experiment is also presented in the special case of rank-selective decoupling. As in high-field NMR, applications can be expected from the introduction of coherent averaging schemes for zero-field techniques: for example, decouplings (by rank or nuclear species), time reversal, and multipolar experiments (zero-field analog of multiple-quantum NMR). © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We present a special case of the theory of coherent isotropic averaging in zero-field NMR, given in part I of this work. In a zero external field, combinations of the magnetic-field pulses restricted to π/2 rotations along the three coordinate axes can selectively average internal spin Hamiltonians while preserving the intrinsic invariance of the spectrum with respect to the sample orientation. Compared with the general case, the limits of the allowed scaling factors of first- and second-rank interactions are slightly reduced. For instance, time reversal is possible for second-rank tensors with a -1/5 scaling factor, instead of -1/4 in general. Finite pulse compensations are analyzed and experimental illustrations are given for two optimum time-reversal sequences. The cubic sequences, though less efficient than the icosahedral sequences, are technically more feasible and may be used in zero-field experiments such as decoupling (by rank or nuclear species), time reversal or multipolar experiments (the zero-field equivalent of multiple-quantum NMR). © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Recent experimental developments of high-resolution NMR in solids (for example, double rotation and dynamic-angle spinning) address the reduction of second-order line broadening effects, particularly in systems involving quadrupolar nuclei such as 23Na, 17O, 27Al, and 14N. However, some aspects of the theoretical description of these systems have not been clearly understood; in particular, the various procedures available to truncate the interactions give incompatible results. We present a general framework, based on static perturbative methods, which provides a natural procedure to derive the correct Hamiltonian for higher-order effects in irreducible tensor form. Applications of this method to coherent averaging techniques (sample motion or radio-frequency irradiation) are described and compared to previous treatments based on average Hamiltonian theory. [ABSTRACT FROM AUTHOR]

Porous clay heterostructures (PCH) have been prepared through the surfactant directed assembly of organosilica in the galleries of synthetic layered clay mineral Laponite. Fe-containing PCH nanostructures with nominal (Si + Mg)/Fe = 10 were prepared by post-synthesis impregnation of porous clay hybrid composites with metal-organic C6H5FeO. After impregnation, the as-synthesized Fe-PCH samples were either directly calcined at 550 °C or, prior to calcination, hydrothermally treated in the high pressure reactor. The impact of the PCH composite doping by the trivalent (Fe3+) cations on the physicochemical properties was investigated with the use of spectroscopic methods. Detailed information on the character and lattice positions occupied by iron, the extent of its incorporation, its local structure and symmetry within samples were provided by comparing the FTIR analysis with the 57Fe Mössbauer spectroscopy and 29Si MAS NMR data. The nature of different Fe species formed after post-synthesis treatments was determined by Mössbauer spectroscopy and HRTEM. High pressure treatment resulted in the alteration of sample morphology and better, more homogenous dispersion of the Fe species in the composite lattice. FTIR indicated slight impairment of the clay structure in the PCH network followed by surface enrichment with Mg species because of Mg cations leaching from the octahedral sheets as confirmed by XPS study. Copyright © 2016 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

The structure of silicate-phosphate glasses from the SiO-PO-KO-CaO-MgO-CuO system acting as slow release fertilizers, containing different amounts of copper ions replacing magnesium or calcium ions, was examined by DSC, XRD, P, and Si MAS-NMR techniques. The chemical activity of the glasses in the 2 mass% citric acid solution was measured by the ICP-AES method. The studies showed that regardless of the kind of replaced components, the Cu ions prefer to associate with the phosphorous Q units, forming the domains composed of orthophosphate with P-O-Cu bonds. The formation of domains with chemically stable P-O-Cu bonds resulted in poorer glass solubility under conditions simulating the natural soil environment. [ABSTRACT FROM AUTHOR]

A theory of deuteron nuclear magnetic resonance (NMR) spectra is outlined for NH3D+ ions. Dipolar interactions among all five spins in the ion contribute to the shape of the deuteron spectra. Dimensions of the ammonium ion can be derived from the structure of the single-crystal spectra. The analysis of the spectra supplies ways to distinguish between cases of rigid, tunnelling or reorienting protons. Single-crystal spectra of partially deuterated ammonium perchlorate measured at 5 K provide examples of the applicability of the theory. Distances between nuclei are derived. Evidence is provided for tunnelling and reorientation of protons in NH3D+ ions. Deuteron spectra indicate that NH3D+ ions exhibit a diverse mobility. There exist ions with immobile deuterons which populate one position in the crystal unit cell (isotopic ordering), while the deuterons in the remaining ions reorient about C3 axes parallel to the N-D bond orientation of the rigid ones. The contribution of the latter increases with increasing temperature. [ABSTRACT FROM AUTHOR]

The theory of deuteron spin-lattice relaxation for free D2 quantum rotors is developed. Relaxation rates are calculated for ortho-D2 and para-D2. The spin-rotational interaction as well as quadrupole and dipole-dipole interactions under interference condition are taken into account. Relaxation rates, to be compared with experiment, are derived as weighted sums of contributions from different rotational states according to their Boltzmann populations. The theory is applied to explain relaxation rates measured in a wide temperature range for D2 molecules in NaY zeolite cages. At high temperatures (above 110 K), a scattering of molecules on the cage walls provides the relaxation mechanism. At low temperatures, molecules stay close to the surface and undergo reorientations in a potential introduced by the adsorption centers. [ABSTRACT FROM AUTHOR]

12-tungstophosphoric acid (PW12) has been supported on dealuminated zeolite Y, containing a secondary pore system with the predominant pore radii of 15 Å. The interaction between the heteropolyacid and the zeolite matrix has been studied using 29Si, 27Al and 31P MAS NMR. Distribution of two types of the Keggin anions, strongly and weakly interacting with the support, has been estimated quantitatively. Increased acidic properties of the PW12/zeolite Y system were demonstrated by a change in the selectivity pattern of m-xylene transformations in the gas phase (significantly increased dealkylation and transalkylation paths at the expense of isomerization). Redox properties were evaluated in oxidation reactions with H2O2 in the liquid phase. It has been shown that the PW12/zeolite Y catalysts can selectively oxidize a number of organic compounds at relatively low temperatures. [ABSTRACT FROM AUTHOR]

The construction and performance of a single stage nuclear demagnetization apparatus utilizing 0.267 mole of praseodymium nickel material is described. An optimization procedure to minimize entropy production during the demagnetization of the hyperfine enhanced material has been developed which enabled the cryostat to cool a liquid-3He sample to a temperature of 350 μK. The effects of measured heat leaks and inferred eddy-current heating are accounted for satisfactorily in a model of this procedure. [ABSTRACT FROM AUTHOR]

The efficiency of the metastability exchange optical pumping (MEOP) performed at elevated 3He 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 3He 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. [ABSTRACT FROM PUBLISHER]