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Nuclear Quadrupole Spin Dynamics: How Weak RF Pulses and Double Resonance Cross-Relaxation Contribute to Explosives Detection.

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dc.contributor.advisor Sauer, Karen L.
dc.contributor.author Prescott, David W.
dc.creator Prescott, David W.
dc.date 2010-04-29
dc.date.accessioned 2010-06-08T14:56:52Z
dc.date.available NO_RESTRICTION en_US
dc.date.available 2010-06-08T14:56:52Z
dc.date.issued 2010-06-08T14:56:52Z
dc.identifier.uri http://hdl.handle.net/1920/5871
dc.description.abstract Nuclear quadrupole resonance (NQR) is a type of radio-frequency (rf) spectroscopy which can detect quadrupolar nuclei (I > 1=2), such as nitrogen, in crystalline solids. NQR spectroscopy is useful for the detection of the many types of explosives containing 14N, however it suffers from a low signal to noise ratio (SNR) particularly in samples with long spin-lattice relaxation times. To improve the SNR the nuclear quadrupole spin dynamics are exploited in two limiting cases: systems with long spin relaxation times and systems where the excitation power is limited. The former is addressed through double resonance effects and the latter through spin echoes created by weak rf pulses. The double resonance effect occurs in samples that also contain a second faster relaxing nuclear species, such as 1H in ammonium nitrate. In this sample an 1H-14N double resonance can be created between the species that improves the SNR. While the focus is on the common case of solids containing both nitrogen and hydrogen, the theory is generally applicable to solids containing spin-1 and spin-1/2 nuclei. A model of this system is developed that treats the motionally averaged secular dipolar Hamiltonian as a perturbation of the combined quadrupole and Zeeman Hamiltonians. This model reveals three types of double resonance conditions, involving static and rf fields, and predicts expressions for the cross-relaxation rate (Wd) between the two species. Using this cross-relaxation rate, in addition to the hydrogen and nitrogen autorelaxation rates, expressions governing the relaxation back to equilibrium in a spin-1/2 and spin-1 system are determined. The three different types of double resonance conditions are created experimentally; one of them for the first time in any system and another for the first time in a solid. Under these double resonance conditions, the increase in Wd and improvements in SNR are explored both theoretically and experimentally using ammonium nitrate. The second effect investigated is the NQR spin echo that forms after excitation of a powder sample by a single weak resonant radio-frequency pulse. This single- pulse echo is identified for the first time, and when applications are limited by a weak rf field, can be used effectively to increase the SNR over conventional detection techniques. en_US
dc.language.iso en_US en_US
dc.subject nuclear quadrupole resonance en_US
dc.subject double resonance en_US
dc.subject spin dynamics en_US
dc.subject cross-relaxation en_US
dc.title Nuclear Quadrupole Spin Dynamics: How Weak RF Pulses and Double Resonance Cross-Relaxation Contribute to Explosives Detection. en_US
dc.type Dissertation en
thesis.degree.name Doctor of Philosophy Physics en_US
thesis.degree.level Doctoral en
thesis.degree.discipline Physics en
thesis.degree.grantor George Mason University en


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