rd  - 1962 - ENC Conference

polyurethane A is obviously narrower than the signal obtained from
polyurethane B. However, it is noted that the signal in the wings of
resonance A is greater than the signal in the wings of B. When the
T 2 decays of the signals are examined, the situation becomes a little
clearer. The straight line decay curve for polymer B indicates that
the line shape of the steady-state signal should be pure Lorentzian.
The faster decay of the signal from polymer A indicates that most of
the protons in the sample are in a broad line phase. The narrow line
observed for polymer A in the derivative steady-state presentation is
actually only derived from about 20~ of the protons in the polymer.
In this paper, a pulse programmer is described which is useful in
carrying out a variety of spin-echo measurements. The unit may be
used with r.f. pulse apparatus discussed in papers by Schwartz ~,
Buchta et al 3, Meiboom and Gill 4, and Blume 5, and has particular
value when used with the integrating circuits described by Holcomb
and Norberg 6, and Blume7 Before discussing the programmer, some of
the terms used in the spin-echo technique will be introduced briefly.
In Fig. 3, a 90 ° pulse is defined. The term 90 ° arises from the fact
that after the moment M 0 has been established in the d.c. magnetic
field direction, an H 1 field is applied for a time just long enough
to rotate the moment through 90 ° , i.e., to a position along the Y
axis. If the HI field is intense enough, the moment will reach the Y
axis unattenuated and in this position will induce a signal in a pick-
up coil which will decay at an exponential rate characterized by T 2.
If T2 is large, however, i.e., if the natural width of the resonance
is less than the homogeneity of the magnet over the sample, the decay
of the signal following a 90 ° pulse is determined by the magnet in-
homogeneity.
In Fig. 4, the echo technique, originally developed by Hahn 8, is des-
cribed which allows the full free precession decay to be monitored in
the presence of an inhomogeneous field. The sequence described by
the four illustrations is as follows: a 90 ° pulse rocks the three
magnetic moments into the direction of the Y axis. After awhile the
moments get out of phase due to the inhomogeneity of the magnetic
field. A 180 ° pulse, twice the length of a 90 ° pulse, is then
applied to the system which swings the dephased moments from the
positive Y to the negative Y direction. Moments which were lagging
in phase are now leading in phase and vice versa. In a time equal to
the time between the 90 ° and 180 ° pulses, the moments will come back
into phase and form an echo signal.
In Fig. 5a, the complete sequence is shown with the peak amplitude
of the echo signal for all positions of the 180 ° pulse indicated by
the dotted line. In liquids, where the diffusion constant is quite
-2-