rd  - 1962 - ENC Conference

large, the echo signal amplitude falls off much more rapidly than
would be the case if the decay was purely determined by T 2. The
equation derived by Carr and Purcell for the echo signal decay is
given in the figure. Carr and Purcell 9 were able to show that by
applying a large number of 180 ° pulses following the 90 ° pulse, the
echo signal amplitude could be made to follow the real T 2 decay curve.
A Carr-Purcell sequence is given in Fig. 5b.
Spin-echo apparatus is particularly useful in providing an easy and
quick method of determining T I. For the T 1 measurement, a 180 ° pulse
is first applied which rotates the moment into the negative Z direc-
tion as shown in Fig. 6. If the apparatus is correctly tuned, no free
precession signal should be observed following this 180 ° pulse. After
a time Z~., a 90 ° pulse is applied and a free precession signal is
observed which in the figure we assume to be detected with a phase-
sensitive detector. As the time ~between the 180 ° and 90 ° pulses is
increased, the amplitude of the free precession signal is observed to
go to zero and then increase in the positive direction. The time qT 0
for which the free precession signal is zero is related to T 1 by the
expression T 1 = i~44 ~0-
In Fig. 7, an apparatus which might be used in the observation of
spin-echo signals is described in block form. The apparatus consists
of a pulse programmer which provides pulses for the 90 ° and 180 ° gates
at the times desired. The 90 ° and 180 ° gates are univibrators or one-
shots which can be varied to produce the correct 90 ° and 180 ° gate
widths. The two gates from the univibrators are combined in an ampli-
fier and are used to control an r.f. gate connecting the r.f. oscil-
lator with the sample coil assembly. The signals following the r.f.
pulses are amplified and observed with a wide-band oscilloscope which
can be triggered at the desired times with pulses from the programmer.
In Fig. 8, a detailed block diagram of a spin-echo pulse programmer
is given which can be assembled from transistorized computer logic.
The timing of all pulses in the unit is controlled by a i00 Kc.
crystal oscillator which drives a 6-place counter. Any time interval
from i0 ~seconds to i0 seconds in units of i0 pseconds may be selected
with a 6-place preset giving rise to signal G" The selected time T
is the basic timing unit of the pulse programmer. Another 6-place
preset (i) is used to select a time interval less than T which deter-
mines the position of the oscilloscope trigger. Signal~drives a
flip-flop, output Qof which is used to drive a latching pulse cir-
cuit which produces the 90 ° pulse. The latching circuit has a
characteristic that it lets the first pulse of Qthrough but will
not let any subsequent pulses through until it is reset with reset
voltage B. Signal Qon the other side of the flip-flop is used to
(* T. J. Calvert and R. S. Codrington: To be published)
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