Generation and Application of Ultra-High-Intensity Magnetic Field Gradient Pulses for NMR Spectroscopy

Two different concepts of gradient current power supplies are introduced, which are suitable for the generation of ultra-high intensity pulsed magnetic field gradients of alternating polarity. The first system consists of a directly binary coded current source (DBCCS). It yields current pulses of up to ±120 A and a maximum voltage across the gradient coil of ±400 V. The second system consists of two TECHRON 8606 power supplies in push–pull configuration (PSPPC). It yields current pulses of up to ±100 A and a maximum voltage across the gradient coil of ±300 V. In combination with actively shielded anti-Helmholtz gradient coils, both systems are used routinely in NMR diffusion studies with unipolar pulsed field gradients of up to 35 T/m. Until now, alternating pulsed field gradient experiments were successfully performed with gradient intensities of up to ±25 T/m (DBCCS) and ±35 T/m (PSPPC), respectively. Based on the observation of the NMR spin echo in the presence of a small read gradient, procedures to test the stability and the matching of such ultra-high pulsed field gradient intensities as well as an automated routine for the compensation of possible mismatches are introduced. The results of these procedures are reported for the PSPPC system.     

Single-Sequence Multi-Slice NMR in Strong Gradient Magnetic Fields

We describe and apply a scheme to obtain nuclear magnetic resonance (NMR) signals from multiple regions in space with a single pulse sequence in systems with strong, usually unavoidable, gradient magnetic fields. This is accomplished with multiple frequency irradiation and reception. Applications described include dual-slice NMR of a fluid to enhance S/N, T ₂ measurements of two different samples, and efficient T ₁ measurement sequence by interleaving shorter delays within a longer delay for different slices.     

Demonstrating the Spatial Resolution of Field Gradient NMR

In recent years, field gradient NMR has become a method of increasing importance in measuring very small dislocations of molecules. Rough estimations indicate that, by utilizing large field gradients, one should be able to detect motions down to the 10-nm scale. However, this limit has not yet been experimentally detected. In this paper, we present a method that allows the direct measurement of the spatial resolution of field gradient NMR. For our experimental setup, utilizing an extremely large static field gradient of about 180 T m⁻¹, we find a lower limit of 7 nm, thus for the first time confirming the expected resolution. Furthermore, we discuss the effect of unwanted vibrations as a limiting factor of the method.     

胆石病人胆汁的光谱及稳定性分析

采用红外光谱及二阶导数谱、紫外光谱、动态激光光散射粒度分析、Zeta电位等测试与分析方法,对胆石病人的原胆汁及经离心处理后的胆汁进行了分析。结果表明,胆石病人的胆汁是一个非均匀体系, 处于相对不稳定状态;离心后的胆汁聚沉物主要含有胆固醇、胆红素及其钙盐、蛋白、磷脂等物质,与该病人胆石核心的主要成分非常相似。胆汁中磷脂类囊泡的破坏以及一些难溶性钙盐的生成是影响胆汁稳定性及胆石形成的主要因素。磷脂和蛋白等有机基质在该病人结石的成核过程中可能起诱导和调控作用。     

NMR in Magnetic Field of the Earth: Pre-Polarization of Nuclei with Alternating Magnetic Field

The polarization of nuclei in the low static magnetic field \(B_0\) with an alternating magnetic field \(B^{*} (B^{*} \gg B_0)\) at a very low frequency \(f_m\) (but \(f_m\gg 1\) / \({T_1}\) , where \(T_1\) is the spin-lattice relaxation time) has been investigated. The question of the optimization of the energy consumption during the pre-polarization is also considered. The possibilities of the method are illustrated by the observation of nuclear magnetic resonance signals from a few liquids.     

氧气在梯度磁场中流动及扩散行为研究

氧气是顺磁性气体,氮气是抗磁性气体,其在梯度磁场中具有不同的磁特性。氮气与氧气在受到梯度磁场影响时,其流动及扩散情况会呈现不同的行为特性。本文建立多通道层叠磁体阵列结构试验系统,并根据高纯氮气在多通道层叠磁体阵列结构气流通道中流动时气体中氧含量的变化,进行空气中氧气以及高纯氮气在具有梯度磁场作用时的流动及扩散情况的研究...     

Compensation of Magnetic Field Instabilities in Field Cycling NMR by Reference Deconvolution

Performing a nuclear magnetic resonance (NMR) experiment in an unstable magnetic field causes fluctuations in the NMR frequency, leading to a loss of reproducibility and an effective shortening of the free induction decay after data averaging. Reference deconvolution allows the compensation of field fluctuations via simultaneous measurement of an internal or external reference signal. The technique was applied to compensate the effect of field fluctuations in a resistive electromagnet used for fast field cycling NMR. An external sample was chosen as the reference.     

High Magnetic Field Consequences on the NMR Hyperfine Shifts in Solution

Pseudocontact shifts arise from the isotropic reorientational average of the dipolar coupling between unpaired electron and nuclei, in the presence of magnetic susceptibility anisotropy. The effect of residual orientation due to high magnetic fields on pseudocontact shifts is evaluated here. The effect is found to be smaller and of opposite sign with respect to another novel effect of high magnetic fields on hyperfine shifts due to saturation of the electron spin magnetic moment as described by the Brillouin equation.     

磁镊系统搭建及其磁场分析

微操作生物或物理样品实验中使用单面磁镊来吸引样品,而实际应用中往往需要对样品进行双头牵引.对传统的磁镊平台进行改进,搭建了创新的双面磁镊系统,易于有相关科研需求的学生进行实验操作并根据不同实验需求改进系统.并采用ANSYS对磁场进行有限元模拟,使学生更直观地理解磁镊的工作原理.     

Magnetometry Diagnostics of Defects in High-Temperature Superconducting Tapes in a Gradient Magnetic Field

Technical Physics - A new technique for contactless magnetometric determination of the local critical current in high-temperature superconducting tapes is proposed. In contrast to conventional...     

磁致伸缩材料弱磁场响应特性的实验研究

针对磁致伸缩材料在弱磁场传感器领域的应用需要,采用迈克耳逊干涉原理实验测量了零应力条件下Tb-Dy-Fe材料和Fe-Ga合金的磁场响应灵敏度,以及不同应力下Fe-Ga合金的磁场响应特性和温度响应特性.实验结果表明：在零应力,外加磁场16 mT条件下,Fe-Ga合金的磁场响应灵敏度远高于Tb-Dy-Fe材料,更合适作为弱磁场传感器敏感材料;同时,在1.2 MPa预应力和26 mT偏置磁场下,Fe-Ga合金材料具有较好的磁场响应灵敏度和较大的饱和磁致伸缩系数,因而处在最佳工作状态.所得到的材料的磁场和温度响应曲线可作为弱磁场传感器参量设计的参考依据.     

Detection of Magnetic Field Gradient and Single Spin Using Optically Levitated Nano-Particle in Vacuum

Optically levitated nano-particle with spins is a promising system for high-precision measurement and quantum information processing. We theoretically analyze the ratio between the fluctuation of particle's displacement caused by spins in magnetic field and caused by molecular collisions of the residual air. When the ratio is larger than unity, the displacement fluctuation of spins flipping can be remarkably detected. By theoretical analysis and numerical simulation, we propose and validate a scheme for the detection of gradient of the magnetic field by levitating ferromagnetic nano-particle, and also put forward a realizable detection scheme of the single spin by levitating nano-diamond particle with single nitrogen-vacancy(NV) centers.     

Detection of Magnetic Field Gradient and Single Spin Using Optically Levitated Nano-Particle in Vacuum

Optically levitated nano-particle with spins is a promising system for high-precision measurement and quantum information processing. We theoretically analyze the ratio between the fluctuation of particle's displacement caused by spins in magnetic field and caused by molecular collisions of the residual air. When the ratio is larger than unity, the displacement fluctuation of spins flipping can be remarkably detected. By theoretical analysis and numerical simulation, we propose and validate a scheme for the detection of gradient of the magnetic field by levitating ferromagnetic nano-particle, and also put forward a realizable detection scheme of the single spin by levitating nano-diamond particle with single nitrogen-vacancy(NV) centers.     

Magnetic Field Effects on Quantum-Dot Spin Valves

We study the magnetic field effects on the spin-polarized transport of the quantum dot (QD) spin valve in the sequential tunneling regime. A set of generalized master equation is derived. Based on that, we discuss the collinear and noncollinear magnetic field effects, respectively. In the collinear magnetic field case,we find that the Zeeman splitting can induce a negative differential conductance (NDC), which is quite different from the one found in previous studies. It has a critical polarization in the parallel arrangement and will disappear in the antiparallelconfiguration. In the noncollinear magnetic field case, the current shows two plateaus and their angular dependence is analyzed. Although sometimes the two current plateaus have similar angular dependence, their mechanisms are different. Our formalism is also suitable for calculating the transport in magnetic molecules, in which the spin splitting is induced not by a magnetic field but by the intrinsic magnetization.     

Reduction of Effect of Concomitant Gradients in Low Magnetic Field MRI via Optimization of Gradient Magnetic System

The effect of concomitant magnetic fields emerging in conjunction with encoding gradients, which is important in the process of the magnetic resonance imaging in low fields, has been considered. The manifestations of concomitant magnetic fields in a concrete gradient system, namely in the system of two coaxial gradient coils, have been thoroughly analyzed. It has been suggested to improve the gradient system via optimization of the interspace between coils on the basis of the standard criterion of the minimum of root-mean-square deviation of the encoding field dependence from a linear one. It has been shown that the optimal interspace is not the Maxwell condition.     

Pulsed Field Gradient Selection in Two-Dimensional Magic Angle Spinning NMR Spectroscopy of Dipolar Solids

The utility of gradient selection in MAS spectroscopy of dipolar solids is explored in two examples. In the first, rotor-synchronized gradients of appropriate strength and duration are applied to select¹H double-quantum coherences. The resulting DQ MAS spectrum of adamantane is compared with that acquired by the corresponding phase-cycling technique. As a second example, a¹H 2D exchange MAS experiment is performed on an elastomer sample. In this experiment, a gradient is applied to remove undesired coherences that would otherwise distort the spectrum for short mixing times. The diagonal-peak intensities in the resulting spectrum show a linear decrease with increasing mixing time indicating cross-relaxation by slow chain motions as the relevant process. Both types of experiments demonstrate the potential of gradient-selection techniques for MAS spectroscopy of dipolar solids.     

Reservoir Pore Structure Classification Technology of Carbonate Rock Based on NMR T 2 Spectrum Decomposition

The carbonate reservoir has a number of properties such as multi-type pore space, strong heterogeneity, and complex pore structure, which make the classification of reservoir pore structure extremely difficult. According to nuclear magnetic resonance (NMR) T ₂ spectrum characteristics of carbonate rock, an automatic pore structure classification and discrimination method based on the T ₂ spectrum decomposition is proposed. The objective function is constructed based on the multi-variate Gaussian distribution properties of the NMR T ₂ spectrum. The particle swarm optimization algorithm was used to solve the objective function and get the initial values and then the generalized reduced gradient algorithm was proposed for solving the objective function, which ensured the stability and convergence of the solution. Based on the featured parameters of the Gaussian function such as normalized weights, spectrum peaks and standard deviations, the combinatory spectrum parameters (by multiplying peak value and normalized weight for every peak) are constructed. According to the principle of fuzzy clustering, the carbonate rock pore structure is classified automatically and the discrimination function of each pore structure type is obtained using Fisher discrimination analysis. The classification results were analyzed with the corresponding casting thin section and scanning electron microscopy. The study shows that the type of the pore structure based on the NMR T ₂ spectrum decomposition is strongly consistent with other methods, which provides a good basis for the quantitative characterization of the carbonate rock reservoir pore space and lays a foundation of the carbonate rock reservoir classification based on NMR logging.     

Monitoring Lithology Variations in Drilled Rock Formations Using NMR Apparent Magnetic Susceptibility Contrast

Applied Magnetic Resonance - Nuclear magnetic resonance (NMR) is a powerful technique for determining the petrophysical properties (porosity, permeability, and fluid mobility) of subsurface...     

Internal Magnetic Field Gradients in Heterogeneous Porous Systems: Comparison Between Spin-Echo and Diffusion Decay Internal Field (DDIF) Method

Two techniques used for evaluating internal magnetic field gradient (G _i), spin-echo (SE) and diffusion decay internal field (DDIF), were investigated at 9.4 T and compared in porous systems characterized by different pores size ranging from 4 to 96 μm with magnetic susceptibility difference between solid and liquid phase, \(\Delta \chi\)  ≈ 1.6 ppm. Since diffusion of a fluid in a solid porous matrix plays a role in both SE and DDIF methods, we investigated these two different methods by highlighting their dependence on characteristic parameters and length scales used to describe diffusion behavior of fluids in porous systems. Therefore, G _i behavior as a function of the dephasing length (l _g), diffusion length (l _d) and pores size (l _s) was obtained. Moreover G _i was evaluated by using both free diffusion and measured apparent diffusion coefficient of water, to quantify diffusion effect in different porous samples. This study gives more insight into the physical dynamics process to explain contrast mechanisms recently exploited by DDIF and SE applications for cancellous bone quality measurements.