Revontuli's configurations
This section describes the different configurations available with Revontuli. The proper configuration depends on the experiment to be performed.
Within each subsection (aka configuration) must be found:
- the description of the configuration with schematic (from geogebra)
- discussion of the symmetric and asymmetric cases if relevant
- recall of the main equations governing source, focal points (meridional, sagittal and confusion circle) and analyzer positions in the user referential (that must be properly defined too)
- one can also add specific description of each component of the spectrometer (motors and its controler, etc) with pictures and schematics.
- description of the corresponding config file, with all parts explained
- Examples of simple but working configuration files must be added for each configuration in the repository to allow an easy starting for newcomers.
Configuration for transmission XAS
With a fixed source
With a fixed detector
The geometry and equations of the configuration can be inspected in a Geogebra document (see screenshot below). The theta angle and asymmetric alpha angle can be varied with the sliders.
The detector is positioned at the origin F=(0,0), and stays fixed. The source (S) can be moved in the x and y directions, as well as rotated to ensure its position remains on the Rowland circle. The monochromator crystal (C) can be moved in the x direction, and rotated. It is important to note that in this configuration, the Rowland circle itself moves when scanning through the Bragg angles, which can be seen in the Geogebra document.
Configuration for fluorescence XAS
Using a SDD detector
Using a mini-spectrometer
Please refer to the RIXS corresponding subsection
Configuration for XES
In transmission like setting
In reflectivity like setting
Configuration for RIXS
Symmetric configuration
Revontuli can operate in a "symmetric" configuration, in which the crystal surface is aligned with the diffraction planes of the selected lattice. This geometry is centered: the source-crystal and crystal-detector distances are equal, and the Bragg angle is measured relative to the crystal surface.
It is the standard operating mode in XAS and XES measurements.
Asymmetric configuration
The spherically bent crystals have a multitude of diffraction planes at various angles due to their curvature. These crystals are cut such that the main lattice plane is centered. However, it is also possible to use others reflections than the main one by introducing a small angle, α, between the diffraction plane and the crystal surface. This way of operating is called "asymmetric". It allows access to different lattice planes with the same crystal, thereby extending the accessible energy range.
The main effect of asymmetry is reducing the signal intensity, due to the decreased effective crystal thickness interacting with the beam and increased absorption along the longer path through the crystal lattice. The intensity loss is compensated by the improved angular resolution achievable. This results in a sharper diffraction peak and enhanced energy resolution, crucial for resolving closely spaced spectral lines. Also, asymmetric configurations lead to a beam expansion or compression which can improve illumination of the detector and thus increase the number of photons collected.
Add explanation on phi and α movements + Laue picture of a crystal.
Add a bibliography section of articles summarizing XAs, XES, symmetric and asymmetric configurations. Just a few references.
Old documentation to update
Component names
The spectrometer is composed of 3 main components:
- X-rays source: either an X-ray tube (for XAS measurements) or the sample itself (for XES measurements).
- Crystal used as wavelength dispersive, according to Bragg's law.
- Detector to collect X photons.
Figure of components in Rowland circle without motor names + picture of each on Revontuli or schem designed on OnShape
Some auxiliary components can be added to the spectrometer to enhance the measurement:
- collimators: narrow the beam to reduce divergence and improve angular resolution.
- slits: define the beam size to increase the energy resolution.
- aluminum filters: attenuate low-energy X-rays.
💬 The role of each auxiliary components and where to place will be explained in 3.1 Auxiliary components.
Software glossary %label needed to make a cross-reference
The motors are controlled via borealis environment in an Anaconda terminal. Motor positions can be displayed either via commandq in borealis, directly on the motor controller or using NAME? software.
To display X-rays and scan informations (X-rays beam, scans, energies, MCA, Bragg angle):
Pixet is a software by Advacam. It allows to visualize the X-rays beam by connecting with the MiniPix.
Prospect is a software provide by Ketek, which is the detector. It displays the X-rays signal.
PyMca is an open-source software suite designed for the analysis of X-ray emission and X-ray absorption data. For example, it can be used as:
- Database of emission and absorption lines
- Real time display of the Multi-Channel Analyzer (MCA) spectra
- XAS support: pre-processing of XAS data (spectrum addition, normalization)
💬 Do not use Pixet while running a scan: the detector cannot count in multiple softwares simultaneously, and this may cause errors. PyMca can be used to display scans while running.
Experiment workflow
The workflow for XAS and XES measurements is as follows:
- Energy range selection: determine the energy range of interest based on target element's absorption (XAS) or emission (XES) edges.
- Crystal selection and mounting: choose the appropriate crystal by applying Bragg's law. Mount the crystal onto the device.
- Alignment of the beam: align the beam to optimize resolution and intensity.
- Measurement: acquire spectra. Measure the samples, I_0 and a metallic foil to calibrate in energy.
- Data treatment: export data and treat it with codes, PyMca or Athena.
Each step is detailed in the following sections.
💬 Both alignment and measurement steps are performed in borealis.
Quick explanation: Difference XAS and XES on spectrometer mounting + pictures of tube for each mounting will be in the proper section.
X-ray settings
To produce X-rays, an X-ray tube is used. This X-ray tube emits a continuous Bremsstrahlung spectrum whose maximum energy corresponds to the applied voltage. From the user’s point of view, the main adjustable settings are voltage (keV) and electric current (mA). Both influence the X-rays flux. To preserve the lifetime of the X-ray tube, the following operating limits are recommended:
- Maximum voltage: 22~kV
- Maximum current: 1~mA
💬 Good note is to measure with approximately twice the energy of the binding edge when possible. Note that characteristic emission lines of the anode material (W) may be present depending on the energy.
Add how to connect the tube to the computer.
Spectrometer configuration
Symmetric configuration
Revontuli can operate in a ‘symmetric’ configuration, in which the crystal surface is aligned with the diffraction planes of the selected lattice. This geometry is centered: the source-crystal and crystal-detector distances are equal, and the Bragg angle is measured relative to the crystal surface.
It is the standard operating mode in XAS and XES measurements.
Asymmetric configuration
The spherically bent crystals used have a multitude of diffraction plane at various angle due to their curvature. These crystals are cut such as the main lattice plane is centered. However, it is also possible to use others reflections than the main one by introducing a small angle, \alpha, between the diffraction plane and the crystal surface. This way of operating is called ‘asymmetric’. It allows access to different lattice planes with the same crystal, thereby extending the accessible energy range.
The main effect of asymmetry is reducing the signal intensity, due to the decreased effective crystal thickness interacting with the beam and increased absorption along the longer path through the crystal lattice. The intensity loss is compensated by the improved angular resolution achievable. This results in a sharper diffraction peak and enhanced energy resolution, crucial for resolving closely spaced spectral lines. Also, asymmetric configurations lead to a beam expansion or compression which can improve illumination of the detector and thus increase the number of photons collected.
Simplify the figure and make it coherent with symmetric configuration. + explanation on phi and alfa movements. + Laue picture of a crystal.
Add a bibliography section of articles summarizing XAs, XES, symmetric and asymmetric configurations. Just few references.