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"The Toroid Cavity NMR Detector"
K. Woelk, J.
W. Rathke, and R. J. Klingler, J. Magn. Reson. A 109,
137-146 (1994).
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A cylindrical toroid cavity has been developed as an NMR detector for investigations
at high temperature and high pressure in metal vessel probes. With toroid
cavity detectors, resonance frequencies up to 400 MHz can easily be attained,
which makes them particularly useful for high- field 1H
and 19F spectroscopy. Typically, static
half-height linewidths of 1.5 Hz are achieved, as measured on 1H
with standard solutions in cylindrical pressure vessels.
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Based on the radial dependency of the B1
field inside a toroid detector, a mathematical equation was derived that
precisely predicts the signal intensity as a function of the pulse width.
Inversion-recovery measurements of the T1
relaxation time of compressed gases (methane and hydrogen) were conducted
by using composite inversion pulses. The results demonstrated the utility
of toroid cavities for quantitative measurements in pressure probes. Pressures
up to 300 bar have been used successfully.
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Because of the strength and regularity of the B1
gradient, the toroid cavity detector is also suitable for one-dimensional
rotating frame NMR microscopy. A spatial resolution down to a few micrometers
can be achieved. The spin concentration and spatial distribution of a chloroform
solution were accurately reconstructed from two-dimensional 1H
NMR data.
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Another, similarly accurate but even stronger B1
gradient evolves as a result of the skin effect during high- frequency
current transmission inside the central conductor. This gradient makes
it possible to perform rotating frame microscopy inside the central conductor,
as demonstrated with 63Cu-NMR spectroscopy.
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K. Woelk, October
19, 1998