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Proton magnetic moment flip angle 54.7 in an external magnetic field

  1. Nov 27, 2013 #1

    mkj

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    i am reading about MRI. i have seen several in several textbooks that the angle between the direction of the proton magnetic moment μ and the direction of the external magnetic field is 54.7
    how did they find that theoretical or experimental
    if theoretical how can we calculate that
     
  2. jcsd
  3. Nov 27, 2013 #2

    tiny-tim

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    hi mkj! :smile:

    loosely speaking, the angular momentum P is fixed

    if there's no magnetic field, there's no problem … the "spin axis" can be in any direction

    if there is a magnetic field in the z direction, the component Pz is quantised, and (for a spin-half particle) must be P/√3

    so the angle between P and z must be cos-11/√3, = 54.74°

    (see eg http://www.chem.wisc.edu/areas/reich/nmr/05-hmr-00-nmr.htm)
     
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