NNR IN RAPIDLY ROTATED METALS By - Nottingham eTheses ...

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(c) Bulk Susceptibility Effects.
. The magnetic field experienced
at any nuclear site is modified slightly from the applied external
field by the bulk susceptibility of the sample material. The
effective magnetic field H0 is given by (97)
Ho [l + (g - a) Xv1
where XV is the magnetic susceptibility per unit volume and a is
a numerical factor depending upon the shape of the sample. For
a spherical sample a= 41T/3 so Hä = Ho whereas for an infinitely
long rod a= 211. In the case considered here the sample is a
cylinder inclined at an angle of 540 44' to the applied magnetic
field and with a length slightly greater than the diameter. An
exact calculation of the effect of bulk susceptibility for the
aluminium powder samples is precluded by ignorance of the approp-
riate value of a for this configuration. However, an estimate
of the induced magnetic field can be made by approximating the
sample to a prolate ellipsoid aligned perpendicularly to the magnetic
field. If the ratio of the polar to equatorial axes is set as 3: 2,
a is then equal to Or x 0.38
(98)
.
The quantity of metal in the centrifuged resin slugs was found
to be equivalent to that of the closely packed powders in the rotor
sample chambers. The amount of metal by volume was about 30% in
both cases. Taking the susceptibility per unit mass for aluminium
as 0.65 x 10-6 cgs units (but ignoring a small diamagnetic contrib-
ution of the resin matrix) the volume susceptibility of the metal
samples is given by 0.6 x lÖ-6 cgs units. When the above values
for a and Xv are substituted in equation (7.1) the resulting contrib-
ution to the magnetic field, and hence the resonance frequency, is
(7.1)