The MEX2 beamline is a bend magnet source X-ray absorption spectroscopy beamline covering the energy range of 1.7 to 3.3 keV, specifically targeting the elements phosphorous, sulfur, chlorine, and silicon. A design goal of MEX2 is to provide a large (3 mm V x 5 mm H), collimated (i.e. not focussed) homogeneous, low-flux density beam suitable for analysis of homogeneous, beam-sensitive samples. 

MEX2 offers simultaneous drain current and fluorescence-mode analysis under high vacuum (1E-5) for solid samples or fluorescence-mode for liquid samples under a helium atmosphere. For solids, powder samples are thinly spread on carbon tape already adhered to the sample holder, allowing up to 30 samples to be mounted onto one holder. This loading technique is especially efficient for environmental scientists measuring soils.

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MEX2 Overview

Note: In response to requests for a motorised sample rotation stage integrated into the beamline control system, which would double the capacity for automated operation, a motorised stage has been ordered and is scheduled for integration in early 2025.

An example of work conducted at the MEX2 beamline by CSIRO scientists: This study demonstrated that Fe(II)-catalyzed recrystallization significantly improves phosphorus (P) extractability by redistributing it within the mineral structure and onto the mineral surface or into the aqueous solution. The findings have important implications for developing sustainable methods for phosphorus extraction in mining and for managing phosphorus behavior in natural environments.

MEX2 can make measurements in:

• Drain current (electron yield) mode

• Fluorescence mode

It is best suited to probing edge features (that are mostly rich in this energy range) to study the speciation via XANES. EXAFS in this energy range is currently very difficult.

MEX2 has adopted the magnetic mounting ruler type sample holder (“sticks”) used on the Soft X-ray fast NEXAFS chamber. See Sample Holder section for more information.

Drain Current (electron yield)

This is essentially the easiest measurement to make, providing your sample is relatively electrically conductive and concentrated. The data will have good signal to noise, but depending on the concentration of the element of interest the signal to background might suffer. Unlike fluorescence and transmission, highly concentrated samples work well in drain current.

As in a soft X-ray experiment, drain current measurements must be made under vacuum, so the sample must not outgas. Drain current measurements are also highly surface sensitive (~ several nanometres). A sparing smear of power is best, as the powder film needs to be as thin as possible and forced into the Carbon tape to aid electrical conductivity.

Drain current is a great alternative to transmission for higher concentration samples, keeping in mind that it is very surface sensitive if that’s important for your materials.

Fluorescence

This is analogous to a hard X-ray XAFS measurements, and works on most types of samples. We have a 4 element Silicon drift detector, and measurements can be made either in vacuum (will get slightly better data) or Helium ambient. It is best not to cover the surface of the sample with any material (eg avoid Kapton tape encapsulation unless absolutely necessary).

As for drain current, a thin smearing on Carbon tape works well for powders. In the MEX2 energy range, the measurements are sensitive to the ~ few microns from the surface due to the X-ray attenuation. Ideally, the sample is thin (less than ~ micron) and dilute of the element of interest (~1000-2000 ppm), because self absorption (the absorption of the fluorescent X-rays by the sample itself) becomes a major experimental issue that will suppress the intensity of peaks in the XAS spectrum.

Drain current and fluorescence spectra can be collected at the same time.