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

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point on the FID following a third 'read' pulse is then directly
proportional
to MM(t).
The delay T was set using the delay unit described previously
and the signal intensity after the third pulse was monitored with
the PGI. The form of the FID was found by stepping out the time
t between the first and second pulses. However, the effectiveness
of this pulse sequence was limited to times (t) greater than the
width of the 90 ° pulses.
5.8 MEASUREMENT OF RELAXATION TIMES
5.8.1 SPIN-LATTICE RELAXATION TIME (T1)
Both 180-T-90 and 90-T-90 pulse sequences were used to
measure spin-lattice relaxation times. In these sequences the
first pulse removes the magnetization from the z direction and the
amplitude of the FID following the second pulse determines that
fraction that has returned after a time T. Assuming a single
relaxation time the exponential recovery envelope plotted out in
this way is of the form
vio -VT=
exp(-T/T1)
where V[0] is the signal magnitude following a single 900 pulse;
K takes the values V[0] and 2V [0] for the 90-T-90 sequence and
180-T-90 sequence respectively. T1 may then be found from the
linear graphical plot: ln[V[01 - V[T]] versus -T/T1.
" From equation (5.1) it follows that after a time = 5T1, the
magnetization has returned to equilibrium along the z axis and the
I
(5.1)