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

- 108 -
process for vanadium the measurements were all undertaken on un-
annealed specimens. The loose powder samples used in some measure-
ments were suspended in a light oil. This served to damp out
mechanical vibrations induced in the individual crystallites by
the rf pulse which otherwise distorted the detected decay signals.
Other measurements were performed on solid slugs consisting of the
metal powder suspended in a resin matrix, the manufacture of which
was described in Section 5.9.
The spin-lattice relaxation time of 51V was measured in these
samples. The value of T1 obtained at a room temperature of 295 K
and a resonance frequency of 15 MHz was 2.7 ± 0.1 ms. This value
is in good agreement with that computed from the expression TT1 a 0.788
as determined by Butterworth
(117).
A typical FID recorded with the signal averager from a powder
sample is shown in Figure 8.1, together with the corresponding
computed spectrum in the frequency domain. (The time resolution
of the measured decay envelope was set by the maximum sweep speed
of the signal averager. ) The two lineshapes drawn relate to two
different assumptions about the form of the FID during the 12 ps
dead time. An exponential fit to the first few detected data points
is shown by the full line and the dashed line represents a Gaussian
approximation. It is suspected that the exponential fit is closer
to the true situation. Quadrupole echoes (118,119)
were observed
following pairs of closely spaced pulses, indicating that the absorpt-
ion lineshapes were broadened by first order quadrupole interactions.
This finding is consistent with the Lorentzian type wings of the
frequency spectrum defined by the solid line in Figure 8.1.
The large linewidth of the resonance spectrum prevented the
macroscopic rotation technique used for aluminium from being applied