2015 BFY II Abstract Detail Page

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Abstract Title: Exploring the Rotating Reference Frame in NMR - An Easy Extension of an Experiment You May Already Have
Abstract: Pulsed NMR, with its pi-pulses, spin echos, and T1 and T2 relaxation times presents many students with a challenging set of concepts to digest.  Central to understanding the physics of this important collection of phenomena is the geometry of the rotating reference frame -- the frame that rides with the protons as they precess about the large static B-field.  Students typically learn that to get the largest free-induction-decay (FID) signal, they must tune the RF field to match the resonance frequency, and then set the pulse width to the optimum pi/2 value.  But what happens if the tuning is off?  This poster explores this question.  I show that when the RF field is detuned from resonance, the pulse width needed to maximize the FID signal increases, up to a maximum value of a bit less than twice the width of the on-resonance pulse.  This maximum occurs when the amount of detuning is equal to the Larmor frequency of the RF field's amplitude.  In addition the magnitude of the FID signal remains unchanged up until this limit.  With further detuning, the optimum pulse width decreases and the magnitude of the FID signal also drops.  I will present a simple geometrical analysis that predicts these features, and which shows how to use these easily obtained data to make an independent measurement of the RF ("B1") field amplitude.  The experiment can be performed on the popular TeachSpin apparatus with no extra equipment.
Abstract Type: Poster

Author/Organizer Information

Primary Contact: David B Pengra
University of Washington
Physics/Astronomy Building, Box 351560
Department of Physics
Seattle, WA 98195-1560
Phone: 206-543-4783

Presentation Documents

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