During the course of the ASA-DH project (see chapter 1) the need
to stabilize the crystals in the X-ray beam became essential to obtain
data of good quality. A low temperature device, or cryostat, was
constructed from an old MSC/AFC diffractometer cryostat. The principle
of the device is to cool nitrogen gas to cryo-temperature by passing it
through a copper coil immersed in liquid nitrogen, the heat exchange
unit, and blow it at a steady rate onto the crystal in the X-ray
camera. The following is a description of this machine which is now in
use for all the projects requiring cryo-temperature data collection at
Brandeis University.
The supply of gaseous nitrogen is a set of two high pressure,
240 liter liquid nitrogen containers (New England Air and Gas, NILG
240). One is used to draw gas while the other is a backup reserve. A
pressure-sensitive automated device (see appendix C, C.1, for the
automatic switch logic-circuit) switches the withdrawal from one tank to
the other when the pressure in the first tank drops, an indication that
the liquid nitrogen in it is below working pressure(fig 1.9).
The gas coming from the automatic switching device enters a box
that splits it to three exits:
- The cold flow: leads to the heat exchange unit and from there
to the transfer-tube through the nozzle and onto the crystal.
- The warm flow: leads to the nozzle and feeds the
engulfing stream or dry shroud (see nozzle fig. 1.11).
- The auxiliary
flow: leads to the transfer-tube T-section and feeds the flow to the
crystal if higher temperatures are desired.
The nozzle is design to prevent moisture from condensation on and around the
crystal. The cryo-stream flows inside a room-temperature, dry nitrogen sleeve
engulfing it all around. The two co-axial flows have an effective distance of
3cm at optimal flow.
The cryo-stream is most efficient when pointing down. It is important to
prevent the goniometer head from freezing, an event that might lead to x-y
translation-slides and spindle arrest. This is afforded by inserting a thin
flexible Kapton heater (Omegalux(TM) model KHLV 0502/10 by Omega) under the top
translation-slide in the basic Charles Supper goniometer head (fig
1.12). Figure C.6 in appendix C shows the heated goniometer head in action
during IPM-DH data collection.
Figure 1.13: The final arrangement of the cryostat nozzle
in use over the
heated goniometer head.
To minimize the need for human intervention to once in every two
days, the system is fitted with a liquid nitrogen level controller
(Gordinier Electronics and Cryogenics, model LL459). This device will
maintain the level of liquid nitrogen in the cryostat container within a
set range and will sound an alarm if the level is over or below high and
low limits respectively (fig. 1.10).
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