Project leader: René Bès   Personnel

X-ray absorption spectroscopy (XAS) is a non-destructive method allowing the direct characteriza-tion of the electronic structure (degrees of oxidation) and the local environment (coordination ge-ometry) of a given element in any kind of samples (solid, liquid, gas). During its century long histo-ry, XAS has demonstrated itself as an essential approach for the elemental analysis of complex ma-terials, and was therefore successfully used in many fields of science, ranging from physics and chemistry to materials science or medicine and biology. However, on the contrary to other struc-ture elucidation methods such as X-ray Diffraction (XRD) or X-ray Photoelectron Spectroscopy (XPS) for example, the XAS measurements are performed almost exclusively at synchrotron radia-tion sources.

Thanks to the recent developments in optics, X-ray sources and detectors, the renewal of labora-tory XAS instruments with performance complementing the synchrotrons is now a reality. These new instruments overcome the most serious disadvantage of synchrotron radiation research: its very limited access, which is making often difficult to measure samples relevant to current re-search within acceptable times and exclude de facto routine experiments. Despite their quickly demonstrated performance and versatility, still two main issues restrain the laboratory-based spectrometer: the non-simultaneous data collection strategy and the limited energy resolution of solid states detectors.

The IDEAL project aims to investigate proper answers to those two limitations, focusing essentially on the development of new design for the detection system. The research plan is as follows:

  1. At a synchrotron, both incoming and sample transmitted photon intensities are meas-ure simultaneously with, e.g., ionization chambers placed before and after the sample. In laboratory-based instruments, they are usually collected separately by performing the experiment sequentially: with and without a sample. This strategy assumes inher-ently that the stability of the incident flux. This is generally justified because the radia-tion spectrum and flux of modern X-ray tubes are very stable. However, such an ap-proach, nevertheless, suffers from the fact that it requires two separate measurements instead of one, which increases the overall time required for the data collection as well as the risk that any instability remains unnoted or wrongly attributed to the sample. In addition, if the sample is enclosed in a complex environment such as an operando elec-trochemical or catalytic reactor or otherwise has to be enclosed in a nontrivial cell (such as for radioactive samples), removal of the sample and its environment may have unpredictable effects on the spectrum. Then, it may be difficult if not impossible to ob-tain reliable data with the two-phase measurement strategy. The main reason behind this two-step approach is the limited amount of available photons produced by X-ray tubes, with intensities several order of magnitude below the ones observed at synchro-tron. Indeed, all photons spared for the actual measurements are important to keep the data collection time reasonable. More efficient detectors are thus required, but due to the presence of harmonics because of the use of single Bragg’s optic, detectors having an energy resolution worse than 400 eV is excluded. The IDEAL project will de-velop and evaluate alternative detection solutions to provide the simultaneous meas-urements. Thanks to the expertise of the HIP detector laboratory, we will seek for solu-tions having an energy resolution good enough to ensure the separation of harmonics (i.e. < 400 eV), good efficiency and transmission properties (< 10% of photon loss), and a large area (about 3-4 cm2). For now, none exists in the detector market.
  2. XAS measurements in transmission mode requires sample thickness to be reduced enough to allow X-ray to be transmitted through the sample. This is not always possible and therefore alternative measurement strategies such as fluorescence mode, are widely performed instead. Similarly to what happen at synchrotron in fluorescence mode, interferences between the emission lines of the different elements present in the samples can limit the sensitivity of quantitative measurements, especially when studying low concentrated elements in chemically complex materials such as environ-mental samples, high entropy alloys or nuclear fuels. The recent instrumental devel-opments carried out on synchrotron beamlines have demonstrated that the implemen-tation of an X-ray emission spectrometer instead of the usual detectors can circumvent this issue. Indeed, emission spectrometers achieve usually energy resolutions of less than 1-2 eV against approx. 150 eV for conventional fluorescence detectors. An alterna-tive and arising technology, the transient edge sensor (TES), is even better in terms of energy resolution and detection efficiency, but is still by far more expensive and com-plicated to handle due to the required cryogenic temperature. These are strong limiting factors for their quick adoption at laboratory. Considering the success of energy disper-sive detectors such as SDD, compact emission spectrometer are therefore the perfect answer to fill the gap between SDD and TES, with many potential applications from PIXE to XRF and XES. The IDEAL project aims to provide a compact enough solution to compete with the state of the art solid-state detectors, while keeping the energy reso-lution below 15 eV and the detection efficiency in acceptable range for use at laborato-ry. To achieve this goal, small bending radius optics are keys because they increase the solid angle while improving compactness. Prototypes of such optics were developed at ESRF especially for the XTREME project. But, the detector component is still missing be-cause of the ideal energy resolution (< 500 eV) and active area (3-4 cm2) required. We are thus proposing to develop new type of SDD with large active area in strong collabo-ration with the detector laboratory of HIP, especially adapted for the creation of the proposed compact spectrometer and its potential commercialization.