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

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identical to those of the rotor sample chamber. All measurements
were recorded at room temperature (295 K) in a magnetic field of
1.35 T. With the PSD in quadrature phase at high gain, resonance
frequencies could be determined precisely from the detected signal
by fine adjustment of the dial settings on the frequency synthesiser.
In obtaining each Knight shift value a series of comparisons
of the resonance frequencies of the metal and reference material
were made. After a small correction of up to 10 Hz to account
for a very slight difference in the magnetic fields at the rotor
and the reference sample (see Section 5.5) the Knight shifts were
calculated from the equation (m
- vr)/vr' Several separate deter-
minations were made, changing the samples used and also varying
slightly the rotation rate and the magnetic field. From eight
different series of measurements the mean value of the aluminium
Knight shift was 1640.0 ppm and the extreme values were 1638.9 and
1640.7 ppm. The estimated possible error in the measured resonance
frequency of each sample was ±5 Hz. Similar errors in determin-
ing the magnitude of the applied correction lead to an overall esti-
mate of the total possible error in each determination of (v - Vr)
of ±20 Hz. Hence with yr equal to 15 MHz the maximum possible
error in any single Knight shift estimate was approximately ±1.3 ppm.
Three specimens of aluminium powder were used in the measure-
ments: (a), (b) and (d). Powders prepared from the two grades of
aluminium wire ((a) and (b)) were sieved so that the maximum part-
icle size was limited to 44 pm. Originally the powder specimens
were encapsulated in epoxy resin in the manner described in Section
5.9. This was later found to be unnecessary so the powders were
packed directly into the rotor sample chambers which were then sealed