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

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circuit was triggered by the leading edge of the preceding pulse.
The trailing edge of the resulting pulse then triggered the delay
on the next pulse unit, so the total delay time was the sum of
the two settings. In this way pulse sequences requiring three
pulses could be performed even when the delay time between the
first and second pulses was shorter than that between the second
and third. Such is the case for the Jeener and Brockaert pulse
sequences as described in Sections 5.7 and 5.8.
5.3 VARIABLE TEMPERATURE MEASUREMENTS
For measurements other than at room temperature the temper-
ature*of the sample could be controlled using a Bruker B-ST 100/
700 temperature control system. In this system nitrogen gas is
passed through a heat exchanger immersed in liquid nitrogen and
then to a glass dewar probe insert via a heater. The temperature
of the gas immediately underneath the sample tube in the insert
is monitored by a copper-constantan junction connected to the
control unit. This automatically adjusts the current td the heat-
ing coil so that the temperature of the junction corresponds with
that selected.
The low temperature measurements on the cuprous halides were
performed using a small gas flow cryostat. The assembly shown in
Figure 5.2 was supported in a glass dewar in the magnet gap.
Although the Bruker control unit was used to adjust the flow of
cooling gas the temperature immediately above and below the sample
was measured independently via two calibrated copper-constantan
thermocouples and a DVM. The lowest temperature was attained by