B.H. Suits

Three-frequency nuclear quadrupole resonance of spin-1 nuclei

Abstract We introduce a new nuclear quadrupole resonance (NQR) method for the detection of spin-1 nuclei, where the transition excited and directly detected is not irradiated at all. It is demonstrated, theoretically and experimentally, that the irradiation of a powder sample containing spin-1 nuclei by two of the three characteristic NQR frequencies can result in free induction decay (FID) and echo signals at the third NQR frequency. We present the optimal conditions for such three-frequency NQR experiments and compare theory with experiment using 14 N ( I =1) in a powder sample of sodium nitrite.

Nuclear magnetic resonance imaging of temperature profiles

Nuclear magnetic resonance imaging using 81Br is used to obtain measurements of an inhomogeneous temperature distribution in KBr. Possible applications of the technique are presented.

14N magnetic resonance for materials detection in the field.

Nitrogen is prevalent in many materials, both naturally occurring and man-made. In particular, it is found in many explosives and other contraband materials. One technique for the detection of such materials in the field is the use of the magnetic resonance signal from the nearly 100% abundant, spin-1, 14N nuclei. Some of the difficulties with such measurements in the field include spurious signals from acoustic resonances, radio-frequency interference, and generally low signal-to-noise ratios. A summary of recent work by the authors to help mitigate these difficulties is presented.

Optimizing surface coils and the self-shielded gradiometer

A computational method for optimizing both single- and two-layer surface coils is presented that is relatively easy to implement in practice using either a variational approach or through standard numerical matrix diagonalization techniques. The technique is applied, in particular, to develop a self-shielded gradiometer that is relatively immune to radiofrequency (rf) interference from distant sources with a minimal compromise to its ability to sense rf from nearby sources, properties necessary for nuclear quadrupole resonance detection of buried land mines. Results of the optimization procedure are evaluated using finite-element numerical techniques and measurements on prototype coils for a number of configurations. These results show that practical coils can be constructed that approximate the theoretically optimized coils quite well. In addition, the trade off between the surface coil sensitivity and noise immunity is presented for the self-shielded gradiometer configuration.

Hardness measurements of nanophase Al/Al-oxide consolidated composites

Abstract Materials with grain sizes less than 100 nm, called nanophase materials, typically exhibit large changes in their physical properties. This work reports hardness measurements for composites made by consolidation of nanoscale aluminum particles which have been exposed to air. The results presented here point to a low density high strength material with significant porosity. A general trend of increasing hardness with decreasing grain size is also indicated. Some of the smaller grained samples show an increase in hardness with an accompanying decrease in mass density with increasing consolidation pressure.

Secondary echoes in three-frequency nuclear quadrupole resonance of spin-1 nuclei

We demonstrate, theoretically and experimentally, that the irradiation of a powder sample containing spin-1 nuclei by two of the three characteristic nuclear quadrupole resonance (NQR) frequencies can result in several echo signals at the third NQR frequency. One of these echoes, the principal echo, has the same shape and time of occurrence as an echo produced after a pair of single-frequency excitations. The other echoes are not equivalent to any single-frequency echo. These secondary echoes are the focus of this paper. The time of occurrence and shape of the secondary echoes are determined by the correlation of the distribution in one transition frequency to the distribution in a second transition frequency. This correlation is in turn determined by the correlation between the distributions of the electric field gradient components, which is itself determined by the types and concentrations of crystalline defects present. We present the optimal conditions to observe such secondary echoes and compare the...

NMR chemical shifts and the electronic structure of lead in lead halides

Abstract The experimentally observed trends of the isotropic chemical shifts in NMR spectra for powders of the lead halides (PbX 2 ; X = F, Cl, Br, I) and a mixed hallide, PbFCl are analyzed using electronic structure calculations based on the pseudopotential approximation for both isolated molecules and molecules forming a cluster reflecting the site symmetry in the solid. In this approximation, the lead 6 p admixture with both the lead 6 s and the halide p valence levels determines the magnitude of the isotropic chemical shifts, with the trends in the halide series predominantly influenced by the latter. The calculations show that the nature of the environment surrounding a PbX 2 unit has a substantial effect on the magnitude of the chemical shift but not on the trends, which both experimentally and theoretically correlate with anion electronegativity.

NMR Study of nanophase A1/A1-oxide powder and consolidated composites

Abstract 27 Al Nuclear Magnetic Resonance (NMR) measurements from aluminum powders and consolidated nanophase aluminum made from those powders are presented. The signals from the metal and surface oxidation are easily separated and are compared before and after consolidation. The results presented indicate that the oxide coating becomes the interface region within the nanophase composite material and that during consolidation the metal has undergone a deformation equivalent to that seen for bulk material under a compressive strain of between 4% and 8%.

Nuclear magnetic resonance imaging as a tool to characterize defect densities in solids

Nuclear magnetic resonance imaging methods demonstrate the spatial variation of 23Na signal intensity in NaCl which results from an impact‐induced inhomogeneous defect distribution.

NMR measurements of nanophase silver

Abstract The room temperature nuclear magnetic resonance (NMR) spectrum of Ag109 in nanophase silver is presented. In contrast to previous measurements of Cu in nanophase copper, where evidence of a significant distribution of anisotropic Knight shifts is present, the Ag signal can be understood in terms of a distribution of isotropic Knight shift. The width and shape of the response can be interpreted using a simple model for the grain boundary resulting in spatial variations in the electron wavefunction within the material.