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 2θ axis.
This condition is typically verifyed by comparing X-ray burns
made on X-ray sensitive paper with
2θ near the undeflected beam direction and with 2θ offset by 180°.
- Set 2θ so that
the undeflected X-ray beam direction corresponds to the zero of 2θ.
- Align the χ rotation axis with the laboratory
y
axis to set
the zero of θ.
- Align the φ rotation axis with the θ rotation axis to set the zero of χ.
One way to do (4) and (5) is as follows:
- Mount a Si(111) wafer so that the (111) direction is (approximately)
along the φ axis.
- Find the (111) Bragg reflection.
Note the values of θ and χ.
Call them θ[1] and χ[1].
- Rotate φ by 180°.
- Find the Bragg reflection again.
Note thes values of θ and χ.
Call them θ[2] and χ[2].
- ½(χ[1]+χ[2]) corresponds to χ=90° in a correctly aligned spectrometer; ½(θ[1]+θ[2]) corresponds to θ=½2θ.
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 χ circle should have dial readings and user readings that are in opposite senses. If the θ 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 θ and 2θ circles. This configuration is compatible with the four-circle code if the tilt stage immediately adjacent to the θ circle is χ and the other is φ. When the first tilt stage is zero, χ is at 90°.