Energy Selection for Mail In
- Diffraction Cluster (Unlicensed)
- AS Diffraction Wiki
When applying for PD Mail in experiments, Users are asked to select an energy at which the measurements will be collected. Available energies are 16 and 21 KeV. These correspond to wavelengths of ~ 0.775 and 0.59Å respectively.
If you still have questions after reading this, please contact the beamline.
Why collect at a particular energy?
There are several reasons to choose a particular energy for measurements and choosing an energy can be a balanced choice between the following factors.
Sample Absorption
Samples with high Z elements will absorb X-rays and not allow transmission of diffracted X-rays. You can calculate absorption for capillary based samples here:
https://11bm.xray.aps.anl.gov/absorb/absorb.php
Patterns with high absorption have a rising background at low and high 2theta values, giving the pattern a characteristic “smile”, relative intensities will be affected and in extreme cases, there may be shifts in 2theta. You can dilute your samples with either a known phase or an amorphous substance to help address this. Ideally, to ensure that absorption corrections can be applied to relative intensities, the absorption coefficient muR, should be <1.
As a general rule, higher energy X-rays are more penetrating so a higher energy can help absorption effects.
Example of a pattern with absorption effects. |
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Avoiding Fluorescence/ Absorption edges
The Mythen detector used at PD is very sensitive to fluorescence and so data must be collected far enough away from an edge to avoid the detector being swamped by fluorescence.
The data should be collected either below an edge, or at least 6keV above an edge.
Examples:
For samples containing Fe, where K-alpha is ~7.1keV, samples must be collected at energies >13keV
For samples containing Zn, where K-alpha is at ~9.7keV, samples must be collected at energies>16keV
For samples containing Zr, where k-alpha is at ~17.99keV, samples should be collected at <18keV
The position of edges can be calculated using this tool:
https://physics.nist.gov/PhysRefData/FFast/html/form.html
by plotting the Mass Absorption Coefficient between 10 and 22 keV to cover the beamline energy range.
An example of a compound containing Y (K-edge of 17 keV) collected at 18 keV |
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Compared to the same samples at 15 keV: |
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Beamline flux
The flux of the beamline is maximised between 12 - 16 keV, so you might need to count longer at higher energies as can be seen from this figure.
Large unit cells with low angle peaks
In this case, you may want to balance your energy choice to ensure that any expected low angle peaks are at a reasonable 2theta value >~3 or 4° to ensure they are not hidden by the beamstop.