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Optimizing Sudden Passage in the Earth's-Field NMR Technique

✍ Scribed by B.F. Melton; V.L. Pollak

Publisher: Elsevier Science
Year: 1996
Tongue: English
Weight: 209 KB
Volume: 122
Category: Article
ISSN: 1064-1858
DOI: 10.1006/jmra.1996.0172

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✦ Synopsis

The equation of motion dM/dt Å gM 1 B(t) is solved numeriing which limiting case applies, and we analyze the motion cally for the case B(t) Å jB p (t) / kB e . The field B e is a small of M under the second assumption. We show that by introstatic field, typically the earth's field. The field B p (t) is a damped ducing suitable damping it is possible to design the system oscillation having frequency greater than, or on the order of, the so that all important criteria are met: initial cutoff rate not precession frequency in field B e . Such oscillation inevitably occurs critical, circuit heavily damped so that ringing dies out rapat the end of the rapid cutoff of the coil current used to polarize idly, coil impedance high enough to achieve low noise figure, the sample. It is assumed that B p (t) is initially large compared to and free precession signal is maximized. This last condition B e , and that magnetization M is initially along the resultant field is achieved by utilizing the damping transient to tip the B. This is the usual situation in the earth's-field NMR technique magnetization back into the x-y plane. when the polarizing field is produced by a coil of moderate to high impedance. It is shown that, when properly damped, the transient can be used to restore the magnetization to the x-y plane, thereby THE RINGING TRANSIENT maximizing the amplitude of the subsequent free precession signal. The damping required is close to critical damping, so that the Figure 1 is a schematic diagram of a typical circuit used problem of circuit ringing when the coil is switched to receiver for switching the polarizing current on and off in an earth'smode is also eliminated. ᭧ 1996 Academic Press, Inc.

field NMR experiment (3). Capacitor C represents the stray capacitance of the coil and of the cable used to connect the coil to the remainder of the apparatus. In some systems

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