Fprime: X-ray cross-section estimators

The design of an x-ray powder diffraction experiment should consider the effects of both absorption and resonant scattering. Poorly chosen x-ray wavelengths or incorrect sample dimensions can lead to excessive absorption that can invalidate the experiment by making the data unanalyzable. Carefully chosen x-ray wavelengths that enhance resonant scattering for elements of similar atomic number can yield data that allows discrimination between them that is otherwise not possible with x-ray diffraction. To help address these issues we have developed two Python GUI programs to compute approximate resonant x-ray scattering and absorption cross sections (f, f', f" and µ) for individual elements using the Cromer & Liberman algorithm (Acta Cryst. 1981 v.A37, p.267). FPRIME computes and plots elemental resonant scattering factors. ABSORB computes scattering and absorption for a given composition and makes an attempt to estimate density as well.

Most commonly the functionality in ABSORB is accessed via a web utility. The web utility was designed to screen potential 11-BM samples for experimental feasibility, but is now widely used by synchrotron beamlines around the world.

Limitations: these computations are performed for elements between Li - Cf and in the X-ray wavelength range 0.05 - 3.0Å (4.13-248 keV) using the Cromer & Liberman algorithm and orbital cross-section tables. Note that the Cromer - Liberman algorithm fails in computing f' for wavelengths < 0.16 Å (> 77.48 keV) for the heaviest elements (Au-Cf) and fails to correctly compute f', f" and mu for wavelengths > 2.67 Å (< 4.64 keV) for very heavy elements (Am-Cf).

 

Distribution & Impact

No statistics are maintained for downloading or installation of the Python utility programs. Most usage is believed to occur via the web application, which received ~500 visits/month when last tracked, with relatively comparable levels of use from the US, UK, France and Germany.

 

 

Funding Source

This project has been produced using operational funding from the APS, contract DE-AC02-06CH11357.

 

 
Future Work
  • This project is considered mature and no future development is planned.