Beamline optics and Experimental station

Even after the EBS upgrade, BM23 has retained its original, very stable optical scheme composed of a cryo-cooled double-crystal fixed exit monochromator, followed by a double mirror for higher harmonics rejection. A new 2-pole wiggler source was installed in replacement of the 0.85 Tesla dipole to match the multi-bend achromat lattice of the EBS. This led to a substantial increase of the photon flux by a factor 2 to 3 depending on the energy range. In January 2024, the old Kohzu monochromator, which has been in operation for almost 30 years, was replaced by a high-performance double-crystal monochromator (DCM) designed at the ESRF. This new DCM drastically increases the performance of BM23 in terms of energy and position stability and reproducibility for EXAFS data acquisitions (i.e., over an E scan of 30° corresponding to a ΔE of 45 keV the angular stability during the scan is 0.05 µrad, the Bragg angular position repeatability is 0.1 µrad and with a micron spot position stability at sample). The principal characteristics of the BM23 beamline are listed in the table below and more details will be presented elsewhere (Mathon et al. in prep).

The experimental hutch located at ~40 meters from the X-ray source hosts two high precision goniometers: 1) the µXAS station for focused operation and 2) the general duty stage for unfocused operation. Both goniometers can be combined with a large variety of state-of-the-art detectors (ionization chambers, fluorescence detectors and a 2D diffraction detector) and can accommodate diverse sample environments (DACs, infrared system, cryostats, ovens, capillary reactors) (see Figure below). The µXAS station is very well suited for studies at extreme P/T conditions studies.

The experimental station of BM23. Left panel: Drawing of the experimental station of BM23 and its granite block. The µXAS station is placed close to the incoming beam comprising the KB-system, the independent goniometer and sample stage as well as the mobile detector block that can hold the ionization chambers, PRL and the Pilatus 1M XRD detector. The µXAS station can be completely moved out of the incoming beam using motorized horizontal translations. The detector block is mobile and can be entirely removed and placed at several position on the experimental table. Right panel: Photograph of the BM23 µXAS station. The vacuum box of the restively heated DAC is installed on the high precision goniometer. It is combined with an XRF detector in quasi-backscattering geometry coupled to a polycapillary and the detector block downstream in transmission geometry holding the Pilatus XRD detector, the ionization chambers and the PRL (figure modified from Rosa et al., 2024)

As shown in the Figure above, the µXAS station is based on a set of Pt-coated mirrors in KB geometry, a high-precision goniometer, and XAS, XRF (Vortex coupled to a  polycapillary) and XRD (Pilatus 1M) detectors. The system further includes a compact spectrometer for pressure measurements by ruby fluorescence (PRL) that also serves as visible-light microscope. The incidence angle of the two KB mirrors can be varied between 2 to 8 mrad allowing XAS operation in focused mode from 5 to 45 keV. The KB mirrors produce a stable focal spot of less than 3 x 3 µm² (FWHM in horizontal and vertical direction) at a distance of 0.5 m from the centre of the second horizontal focusing mirror. The µXAS station can easily accommodate bulky sample environments, such as helium-flow high-pressure cryostats, the Paris-Edinburg press, vacuum chambers for resistively heated DACs. The motor stage comprises independent x, y, z motors below and above a vertical rotation axis with a positioning resolution of 1 µm for motors below the rotation axis and 0.2 µm for motors above the rotation axis, respectively. This motor stage allows repositioning of the micrometric samples in the DAC on the focal plane and on the rotation axis. This configuration permits further the acquisition of X-ray diffraction data during sample rotation either in continuous mode or step-scan mode, which increases the probed reciprocal space and thus statistics for powder or single-crystal XRD data.

General publication:

O. Mathon et al., A. Beteva, J. Borrel, D. Bugnazet, S. Gatla, R. Hino, I. Kantor, T. Mairs, M. Munoz, S. Pasternak, F. Perrin, S. Pascarelli, J. Synchrot. Radiat. 2015, 22, 1548.

For extreme condition setups:

A.D. Rosa, G. Garbarino, J.E. Rodrigues, E. Mijit, ..., K.A. Lomachenko, O Mathon (2024) New opportunities for high pressure X-ray absorption spectroscopy at ID24-DCM and BM23 with the Extremely Brilliant Source of the ESRF High Pressure Research 44 (3), 248-276 https://doi.org/10.1080/08957959.2024.2364281