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

8.4 CADMIUM
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8.4.1 INTRODUCTION TO THE MEASUREMENTS
Cadmium has two magnetic isotopes
111 Cd and 113 Cd which
have natural abundances of 12.86% and 12.34% respectively. Both
isotopes have spin I so, although cadmium metal has a hexagonal
ctystal structure, the nuclear resonance spectra are unaffected by
electric field gradients caused by the deviation from cubic symmetry.
At high magnetic fields the main source of broadening in powdered
specimens arises from the anisotropic Knight shift interaction due
to the non-s-character of the conduction electrons. Because of
the asymmetry of the nuclear lineshapes, there exists some uncert-
ainty in defining the exact centre frequency and hence the isotropic
Knight shift. Rapid sample rotation about the magic axis removes
this anisotropy and narrows the lineshape, thus enabling the centre
frequency to be determined accurately. Under the condition of a
sufficiently rapid rotation rate the residual central spectrum is
determined by the isotropic part of the indirect exchange interact-
ion between nuclear spins and by lifetime broadening. Typical
spectra of static and rotated cadmium powder are shown in Figure 8.2.
The NMR sensitivity of both the 111Cd and 113Cd resonances in
cadmium metal is less than a thousandth of that of aluminium. Although
the anisotropic broadening of the nuclear lineshapes is greater at
higher magnetic fields, the signal to noise ratio of the detected
signals is significantly improved by the use of higher rf frequencies.
Consequently measurements were undertaken employing as high an applied
magnetic field as could reasonably be achieved with the M12 electro-
magnet. The powdered metal samples used in this work were obtained