The experimental set-up
Contact: richard.bean@xfel.eu
The SPB/SFX instrument is primarily concerned with 3D structure determination of both crystalline and non-crystalline micrometre-scale and smaller objects. A particular emphasis is placed on biological objects, including viruses, biomolecules and protein crystals. Nevertheless, the instrument is also capable of investigating non-biological samples using similar techniques. The instrument exploits the unique high repetition rate of the facility and brings benefit to these structural biology applications [1].
The SPB/SFX instrument is designed to operate at photon energies from 3 to 16 keV, with peak performance expected between about 6 and 15 keV. The SPB/SFX instrument is located behind the SASE1 undulator, with X-ray beam delivered via a dedicated branch in the XTD9 photon tunnel. The SPB/SFX instrument has two interaction regions where sample interactions with X-rays and laser sources are measured, one termed Interaction Region Upstream (IRU) and the other termed Interaction Region Downstream (IRD) [1]. The general layout of the SPB/SFX instrument is shown in Fig. 1. The X-rays propagate from the source to their focal point at IRU entirely through a vacuum, arriving with a full width at half-maximum (fwhm) focused diameter of a few micrometres or a few hundred nanometres using either of two Kirkpatrick–Baez mirror systems [2]. A set of compound refractive lens refocusing optics (providing a micrometer-sized focus) are installed after IRU to enable measurements at IRD in an air/helium or vacuum environment [2].
Schematic of SPB/SFX instrument layout.
The SPB/SFX instrument deploys three main classes of sample delivery: liquid jets for delivering (primarily) small crystals to the X-ray FEL beam for serial crystallography, focused aerosol beams for delivering (primarily) non-crystalline particles to the X-ray FEL beam for single particle imaging, and samples arranged on fixed targets, which may be crystalline or non-crystalline. The IRU at SPB/ SFX uses the Adaptive Gain Integrating Pixel Detector (AGIPD) [4], a hybrid pixel array, silicon sensor-based detector for collecting the forward diffraction and scattering data.
Illustration of X-ray path and diffracted X-ray beam from a water jet onto the AGIPD detector.
The experiment to study crystallization of water was performed at IRU of the SPB/SFX instrument. The sample for this experiment, water, was delivered to the interaction region using a liquid jet delivery system [5] with some slight modifications. The X-rays were focused to about 100’s of nm at IRU using the nano KB focusing system to achieve the very high fluence required for resolving the structural information. By controlling the distance of the nozzle for the liquid jet to the interaction point, the water to ice transition region was probed using X-ray diffraction. The diffraction data was collected using the AGIPD detector with up to 1.1 MHz repetition rate. The interaction of water/ice with X-rays was also imaged using the pump-probe laser [6].
[1] Wiedorn, M.O., et al. Megahertz serial crystallography. Nat Commun 9, 4025 (2018)
[2] Mancuso, et al. J. Synchrotron Rad. 26, 660-676 (2019)
[3] Bean, et al. J. Opt. 18, 074011 (2016).
[4] Allahgholi, et al. J. Synchrotron Rad. 26, 74-82 (2019)
[5] Schulz, et al. J. Synchrotron Rad. 26, 339-345 (2019)
[6] Koliyadu, et al. J. Synchrotron Rad. 29, 1273-1283 (2022)