Sample and detector geometry¶
The experiment chamber is spherical. The X-ray beam enters from upstream, is defined by slits, and illuminates a sample plate. A goniometer arm carries both an Axis Photonique CMOS area detector and a GaAs photodiode for complementary measurements.
Sample coordinate system¶
The sample is positioned with four primary motors:
| Axis | Role |
|---|---|
| Sample X | Translation along the goniometer axis (colinear with Sample Theta). Positive typically moves the plate toward the motor side; negative toward the chamber door. |
| Sample Y | Translation perpendicular to the beam when Sample Theta is at 0°. Positive moves the plate away from the beam; negative toward the beam. |
| Sample Z | Translation normal to the sample plate. At Sample Theta = 0°, positive is up in the lab frame and negative is down. Changing Z also changes sample height relative to the beam and, except at Sample Theta = 0°, couples to sample–detector distance. |
| Sample Theta | Rotation of the plate about the axis colinear with Sample X. 90° places the surface normal to the incident beam (grazing-incidence experiments use other angles). 0° and 180° correspond to plate facing up vs down in the lab frame. |
Together, Sample X, Y, and Z form a right-handed system that rotates with Sample Theta for Y and Z, while X stays on the rotation axis.
Additional motors (for example Sample Azimuthal Rotation, piezo fine stages, or numbered SampleRot axes) provide extra degrees of freedom for specialized holders.
Detector arm (CCD prefix)¶
The same goniometer arm carries:
- An Axis Photonique CMOS area detector for scattering patterns and imaging (this station no longer uses the previous CCD camera; frame size and pixel pitch follow the active CMOS mode and ROI).
- A 5 mm × 5 mm GaAs photodiode for direct-beam or high-flux monitoring when the detector is moved out of the beam.
Arm motors use the CCD prefix:
| Axis | Role |
|---|---|
| CCD Theta | Rotation of the arm in the scattering plane. At 0° the arm can be aligned with the direct beam from the chamber-door perspective; positive rotation follows the same handedness as Sample Theta over roughly 120° of travel. |
| CCD Y | Sample–detector distance. Positive moves the detector farther from the sample; negative closer. This is the primary lever for Q or camera length in scattering. |
| CCD X | Lateral shift of the detector, colinear with the same direction as Sample X. |
Pollux CCD X/Y name alternative encoder or staging positions when a Pollux-style detector path is configured.
Photodiode vs beamstop photodiode¶
- Main photodiode (GaAs): placed in the direct beam when the arm is at the photodiode preset (typically large CCD Y and small CCD X with CCD Theta near 0°). Used for alignment, normalization, and NEXAFS-style measurements with the CMOS detector out of the way.
- Beamstop photodiode (small Si diode on the beamstop): used when the CMOS detector is in scattering geometry to monitor transmitted or scattered flux without saturating the main diode.
T-2T and reflectivity¶
T-2T couples Sample Theta and CCD Theta so the detector tracks at twice the sample angle for specular reflectivity (θ–2θ geometry).
Beam stop¶
The Beam Stop motor positions a stop that blocks the direct beam from hitting the CMOS sensor during scattering, protecting the detector and defining shadowing for certain geometries.
Alignment order (experimental practice)¶
- Instrument alignment: put the photodiode in the direct beam using the CCD arm; optimize CCD Theta (and related positions) so the diode reads the beam cleanly.
- Sample alignment: iterate Sample Z (often half-maximum of absorption or scattering) and Sample Theta (centroid or symmetry) so the sample intersects the beam as intended.
This repository’s higher-level scan helpers assume sensible shutter and exposure defaults; see Digital I/O and Analog inputs for what each signal represents.