4.2. - Diffractometer Alignment
This section presents a guide on how to set up a four-circle spectrometer.
This summary applies whether the scattering
plane is horizontal or vertical.
The first step, which should need only be done the first time
fourc
is used with the diffractometer,
is to
ensure each diffractometer motor is set up with
the correct name, mnemonic, rotation sense, steps-per-degree, etc.
The
config macro is generally used for this purpose.
For stepping motors,
the rotation sense of an axis
depends on the details of the motor controller and cable connections.
If the rotation sense isn't as described in the
Introduction,
change the
sign_of_user_×_dial
parameter in the motor configuration file.
For each motor,
fourc
keeps track of a both a dial and a user position.
The dial position is meant to agree with the readout of the
physical dial on the spectrometer.
The value and the sign of the
steps_per_unit
parameter should be
chosen so that the dial position and its direction in the computer agree
with the
physical dial reading.
Use the
set_dial macro to set the dial positions.
The user positions should correspond to
the underlying
"true"
orientation angles of the spectrometer that
satisfy the constraints given above.
Use the
set macro to set the user positions.
Once properly configured, diffractometer alignment proceeds as follows.
- Arrange for
the X-ray beam to go through the center of rotation.
Generally, the center of rotation is found with a pin and a telescope.
- Arrange for the X-ray beam to be perpendicular to the 2
 axis.
This condition is typically verifyed by comparing X-ray burns
made on X-ray sensitive paper with
2 near the undeflected beam direction and with 2 offset by 180°.
- Set 2 so that
the undeflected X-ray beam direction corresponds to the zero of 2.
- Align the
 rotation axis with the laboratory
y
axis to set
the zero of .
- Align the
 rotation axis with the rotation axis to set the
zero of .
One way to do (4) and (5) is as follows:
- Mount a Si(111) wafer so that the (111) direction is (approximately)
along the axis.
- Find the (111) Bragg reflection.
Note the values of and .
Call them [1] and [1].
- Rotate by 180°.
- Find the Bragg reflection again.
Note thes values of and .
Call them [2] and [2].
- ½([1]+[2]) corresponds to
=90° in a correctly aligned spectrometer;
½([1]+[2]) corresponds to
=½2.
The Huber four-circle diffractometer is an example of an instrument
that works with
fourc.
(
The four-circle Huber has dial readings with all
right-handed rotations, so the circle should have dial readings and
user readings that are in opposite senses.
If the circle is
offset by 180° then the dial readings and the user readings
of all angles can have the same sense.
)
Another common spectrometer configuration
has two crossed ± 20° tilt stages on top of full and
2 circles.
This configuration is compatible with the four-circle code if
the tilt stage immediately adjacent to the circle is
and
the other is .
When the first tilt stage is zero, is at 90°.
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