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The closed-cycle He gas refrigerators used at the NCNR are not properly
Displex devices, since that term is a trade name. I mention the term
``Displex'' though because the
closed-cycle He gas refrigerators at the NCNR are commonly and incorrectly
referred to using that name.
Most units can be used to control sample temperatures over the range 10-300 K,
but a few units have been modified to allow operation to 450 K or higher.
Do not use the standard units above 310 K as this can destroy the cooling head.
While it is possible to manually set the temperature controller
attached to the refrigerators, the
ICP program can control the temperature of the refrigerator,
so a series of measurements
can be made at different temperatures automatically. The steps to be followed
for automatic usage are outlined below and then will be discussed subsequently
in more detail.
Note that the function
key references (e.g. F17) refer to the labels on the
console computer, not to the the letters on the keys.
- Load a sample, pump down and cool.
- Connect the temperature controller to the VAX.
- Set the temperature controller type in ICP (TDEV).
- Confirm communication (PT).
- Define parameters for run in ``Prepare mode.''
- Set the data collection time using Automon.
- Copy and edit the parameters for additional runs.
- Set up a run sequence (RS=...).
- Switch to control mode (F17) and check the run sequence
- Check and log the neutron monitor (MRAT) as well as
document the sample on: a sample tag, the white board and in the log book.
- Start the run sequence (RS).
- Remove the refrigerator from the instrument.
- You should ask for help loading a sample, if you are not comfortable
with the task. It is better demonstrated than described.
Samples are usually sealed with at least a few percent
He gas to promote thermal equilibration and then sealed with a indium gasket,
but note that indium melts at temperatures in the operating range of the
Attach the sample can to the copper block with
screws that are the correct length as screws that
are too long can damage the heating element or temperature sensor.
Also, make sure these screws are tight, so that the screws do not loosen
due to vibration.
The inner heat shield cans should not be screwed on tightly,
as they may be impossible
to remove after temperature cycling. Back them off by a quarter turn.
Attach the outer vacuum can and pump down with a turbopump. As soon as the
pressure is in the range of torr, the cooling can be started. It
will typically take about 3 hours to bring the sample to
the minimum temperature.
I do not recommend attempting to collect room temperature
data with the set point at 295 K, while the displex is initially cooled as
temperature control may not be good.
When I collect data over a range of temperatures, I
tend to start at the lowest temperature first, but I cannot defend this choice
to be the best.
For runs of a day or so, one can usually operate
the refrigerator without continual pumping, but cooling can fail if the
pressure rises, so it is usually worth the effort to leave a pump attached
while data are collected. Always use a pump with the high-temperature
refrigerator, when operating above room temperature in case of outgassing.
- Physically connect the BT-1 temperature controller
RS-232 connector to the refrigerator. The cable from the VAX is marked
``BT1 Temperature Control'' and is connected either to the rear
or the top of the refrigerator temperature controller. The cable can
usually be found in the vicinity of the shield wall with
the ``white board'' attached. Take care not to use the nearly identical
``BT1 it Magnet Control'' cable.
- Instruct ICP (in Control Mode) which temperature controller you
will be using with a tdev command (see Figures
5 and 6). Note that the tdev command
must be reentered each time ICP is restarted and must be reentered if you
change the controller model when a new refrigerator is mounted.
Menu of temperature controllers from the tdev command.
Selecting a temperature controller with the tdev command.
The tdev command sets the T+ flag in ICP, so that the temperature is
recorded at each data point.
- Check that the temperature controller is reading correctly using the pt command (optional, see Figure 6).
- In ``Prepare Mode,'' define a run to be measured
in what is called an ``increment buffer'' (see Figure 7).
Each ``buffer'' line defines the parameters for a single diffractometer scan.
Defining a single run in ``Prepare Mode.''
For use with a temperature controller,
you will typically need to set the following fields in the buffer:
Comment, T0, Wait, Err, Hld, Monit, Prefac. and M-typ,
which are used as follows:
It is very unlikely that you will want change the default values
for some fields:
A3-beg, A3-end, Inc-3, A4-beg, A4-end, Inc-4 and #pts. The Col field informs
ICP of the in-pile collimation (15' or 7'). The default, 15' is usually
correct. Note that the A4-*, #pts and Col values are reset every time
a buffer is edited. There is one exception to this. If you are setting up
runs while the instrument is collecting data and
plan to use a
different monochromator than the one that is currently in use, you may need to
change the A4-beg and A4-end values to match the monochromator you plan to use.
Use 3-13 degrees for Cu311 and Si531 and 1.3-11.3 degrees
for Ge311. Note that the
A- command in control mode turns off the automatic resetting of A4-beg
- This sets a 1-line file header and the name of the data collection file. Be
sure to use letters and numbers (A-Z and 0-9) and no other characters
for the first five letters of the Comment as this is used for the file name.
If an invalid name is used the file will be named DEFLTxxx.BT1, where xxx is
a number in the range 001 to 999.
- This specifies the nominal temperature for data collection. This value is sent
as the set point to the temperature controller. For the controllers attached
to the He refrigerators, this is a temperature in K. If T0 is set to 0, and
the T+ flag is set, the temperature will be recorded, but will not be changed
and the Wait and Hld0 terms (below) are ignored.
- This specifies a maximum amount of time in minutes that ICP will wait for the
sample temperature to be in range (see ERR, below), before starting data
collection time. If the desired sample temperature is reached in less
time, the remaining time wait is not used. Typical practice is to use a
wait that is much longer than the
expected time needed to reach the desired temperature, for example 120 to 180
minutes. If you do not want data collection to wait for the temperature to
be reached, Wait can be set to 0.
- The temperature is considered ``in range'' if the temperature is between T0+ERR
Note that the value for ERR does not affect
the actual stability of the temperature (which is determined by the PID
parameters set in the temperature controller) so setting ERR to a small number,
can cause data collection to be suspended for long periods when temperature
control is flaky. Typical values for ERR are 2 to 5 K for low temperature
measurements, but may be 5 to 10 K near room temperature or above.
- This specifies an amount of time in minutes to wait for temperature
to equilibrate after the temperature is reached (or Wait expires) before
data collection is started. The desired value for this
parameter will depend on the experiment to be performed.
A value of 20 minutes is common, but so is 0 as well as longer times.
- Is either ``NEUT'' or ``TIME''. NEUT is used for most data collection, where
the data collection time is adjusted to match the neutron flux on the sample.
- Each data point is measured ``Prefac'' times and if Prefac is 4 or greater,
the measurements are checked for statistical agreement, so that significant
noise spikes can be discarded. A rule of thumb is that Prefac should be 4 for
runs of 6 hours or less. It may be desirable to increase Prefac by 1 for
each additional 6 hours of length, but 4 is a good default value regardless
of the data collection time.
- This value, along with Prefac, determines the length of the data
collection period. If M-typ=TIME, this specifies a count time in seconds. Most
commonly, M-typ=NEUT and Monit is set using the AUTOMON (AMON) feature.
Note that two fields, Hld and Inc-T, should always be 0. Inc-T causes the
temperature to be changed for each data point and Hld creates a delay that
is executed at each data point. These processes are almost never
of use at BT-1.
- Determine the data collection time using the Automon feature. The
appropriate monitor value is computed so that the current run
will finish at a specified time.
Automon is initiated by moving the cursor to the AMON field and pressing
Enter. The screen shown in Figure 8 then appears.
Using Automon to compute a run length.
computation can either use or ignore the Wait and Hld0 values. If you
answer Y for ``Use Holds,'' the time needed for the Hld0 (and Hld) hold
is included in the run length computation. If you
answer Y for ``Use TempWait,'' the entire Wait period is included in the
run length computation. Since the entire Wait period is usually not used,
it is best to say N for ``Use TempWait'' but the answer for ``Use Holds''
is a matter of personal convenience. The number of days and the end time for
the run are then entered. Use 1 for delta-days if the run will go
past midnight even if the run length is only a few hours. A run starting
at 21:00 (9 pm) and ending at 9:00 (9 am) the next morning, would be entered as
delta-days=1 and Time=9:00. The computed Monit value is set when Automon
- Duplicate the run information for other temperatures that you will wish to
run. This is done either by highlighting the buffer to be copied and
then pressing the F14 key (actually the F6 key), which will copy the
information to all
other buffers, or (preferably) the buffer can be copied selectively by
pressing the F18 key (actually F10) to enter the ``Buffer Ops'' mode where
a buffer can be copied by entering a command such as COPY 2,3
(see Figure 9).
Copying a buffer in ``Buffer Ops'' mode.
copies the parameters in buffer #2 into buffer #3. Exit ``Buffer Ops'' mode
by pressing the F20 key (actually F12).
Each buffer can then be quickly modified to
change the temperature (T0) and possibly the Comment, Hld0, Err and Wait
- Once a series of runs has been defined, a ``run sequence'' can be defined
by pressing the F19 key (actually the F11 key). This brings up the menu
shown in Figure 10. If a previous sequence is present, it can be
cleared by typing DEL and return.
Beginning a ``Run Sequence.''
Commands are added to the run sequence by typing RI#, where
is the buffer number of the run in the list. Commands may be entered one a time
or several commands may be entered at once, separated by semicolons (;).
The run sequence in Figure 11 will cause buffer #1 to be collected
three times and then buffer #2 to be collected twice. Note that the files
will all be named NALICxxx.BT1, so if no other files exist, the data files
will be named NALIC001.BT1, NALIC002.BT1 and NALIC003.BT1 for 15 K runs and
NALIC004.BT1 and NALIC005.BT1 for the 295 K runs. Exit the RS menu with the
F20 key (actually F12).
Entering a ``run sequence.''
- Switch to control mode by pressing the F17 (actually F9) key.
The length of a run sequence can be estimated using the HOWLONG/RS
command (see Figure 12).
Using the HOWLONG/RS command to determine the expected run time for a ``run sequence.''
Note that the estimated lengths will be estimated assuming the maximum delay
allowed by Wait and the minimum assumes that all Wait times are negligible.
In the example shown in Figure 12, there are no actual
temperature changes, except between the third run and the fourth, and perhaps
before the first run.
Assuming that the sample
is already at the appropriate temperature and the refrigerator will need
approximately one hour to heat from 15 K to 295 K, a good estimate is that
the runs will require 32.2 hours. Your mileage may vary.
- Before starting the run be sure to:
- be sure the shutter is open
- measure the monitor using the MRAT command
- enter the sample composition and contact info on the white board
- put the sample tag in the holder on the white board
- enter the sample information in the log book
(see Figure 12).
- The run sequence is started with a RS
command, as shown in Figure 13.
Starting a ``run sequence'' with the RS command.
Note that it is possible to modify the run sequence or change
the measurement parameters for the runs that have not been started in another
(ExtraICP) window while the data collection is in progress.
- When removing the refrigerator, close the vacuum valve
before turning off the vacuum pump. Let the pump vent completely
before removing the vacuum hose - this takes 5 to 10 minutes.
The compressor may be turned off at any time. If you plan to unload your
sample as soon as possible, you may wish to set the temperature set-point
to 295 K, though the cold-head will remain very cold for
many hours even when the sample has reached room temperature.
When possible, let the refrigerator warm up slowly by letting it sit
for a day or so before releasing the vacuum and removing the sample. Remember
to leave the sample tag on the refrigerator, so that the identity of the sample
If samples will be changed quickly, it may be necessary to use a heat gun to
drive off condensation, but be careful not to heat the cold-head or sample
stage to much more than room temperature. The refrigerator can
be severely damaged by heating it above about 50 C.
Next: Using the six-position sample
Up: A User's Guide to
Previous: Running a single sample