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

- 90 -
at the top with epoxy glue. There was no measurable difference
in the Knight shift values obtained from any of the three powder
samples whether they were encapsulated in the resin or not.
Various aqueous solutions of A1C13 were also compared, but
the value of the reference frequency v) was found to be unaffected
by concentration or by use of 99% reagent grade A1C13 instead of
the pure salt. In fact no change in resonant frequency was observed
even when aqueous solutions of reagent grade AlBr3 were substituted.
The value of the aluminium Knight shift quoted above is therefore
largely insensitive to the concentration of impurities in the metal
sample or the choice of reference solution.
We now consider other factors which might affect the Knight
shift value obtained.
7.2.3 POSSIBLE FACTORS INFLUENCING THE EXPERIMENTAL KNIGHT
SHIFT VALUE
(a) Temperature. Although at first sight all measurements were
undertaken at laboratory temperature, the temperature of the rotors
could not be monitored precisely. Due to cooling upon expansion
the temperature of the driving gas immediately after passing through
the jets in the stator was several degrees below room temperature.
However, as would be expected, any variation from room temperature
of the temperature of the samples was found to be small - less than
3 K. The temperature dependence of the aluminium Knight shift is
relatively small. The most recent reported measurements by Kushida
and Murphy (96) and El-Hanany and Zamir(95) indicate that an error
of about 15 K would be required to produce an uncertainty of 1 ppm.