Recently, a team at the Institute of High Energy Physics has made a breakthrough in fabricating spherically bent crystal analyzers (SBCA), thus paving the way for the construction of the Hard X-ray High Energy Resolution Spectroscopy (“H²O”) beamline of the High Energy Photon Source (HEPS).

To cater to the scientific needs of multiple disciplines such as high pressure science and energy-related sciences, etc. in China, as well as fulfill the eager demands of general users, a world-class X-ray Raman scattering (XRS) spectrometer (named “Qiankun” for the two characters representing heaven and earth in the Bagua Octagon) has been under construction at the H²O beamline at HEPS. 

The spherically bent crystal analyzers (SBCAs), being the crucial optics in the spectrometer, make it possible to focus the X-ray photons with precise energy onto the detectors. The idea of using bent crystals in spectrometers was first proposed by Jesse W. M. DuMond in response to the problem of using crystals as grating in the standard Rowland configuration. DuMond, H. H. Johann and others later constructed curved crystal spectrometers based on this idea. What is most challenging is for the SBCAs to achieve better focusing and higher energy resolution simultaneously. The Qiankun spectrometer hosts six independent modules, consisting of about 90 analyzers with an equal bent radius of 1 m. The energy resolution of SBCAs is required to match the level of core-hole lifetime broadening, or ΔE/E~10^-5. Meanwhile, the spherical mirrors are expected to reach 1:1 focusing. 

A dedicated working group, organized by HEPS management, was formed by synergetic efforts from the H²O beamline together with individuals from the optics design system, fine mechanics system, and beamline control system, etc. at HEPS. Among the group members, Dr. GUO Zhiying, a key member of the H²O beamline, and Dr. DIAO Qianshun, a crystal fabrication specialist at IHEP, made huge efforts to improve crystal fabrication procedures through trial and error over several years. Eventually, the team succeeded in establishing a protocol to fabricate multiple high quality SBCAs with well-balanced energy resolution and focusing properties. 

From Oct. 2–5, 2022, the first batch of 15 SBCAs (Fig.1) fabricated for the Qiankun spectrometer were measured at the 1W2B beamline of the Beijing Synchrotron Radiation Facility (BSRF). The optical layout consisted of a Si(111) double crystal monochromator followed by a Si(220) channel-cut crystal monochromator to sharpen the bandwidth of the incident beam, as well as a poly-capillary lens for micro-focusing. The analyzers were mounted on a prototyped 3-element spectrometer to measure the elastic scattering signals from a plastic scattering sample (Fig. 2). A specially designed EPICS-bluesky control system was employed to smoothly scan the two monochromators together. Using this setup, it was possible to perform high precision and highly efficient characterization of the SBCAs. The bandwidth of the incident beam was optimized to 0.8eV@9.7 keV (FWHM). The net energy resolution of all SBCAs reached about 1eV@9.7 keV (FWHM) (Fig. 3), which is close to the theoretically predicted value. Meanwhile, the focusing properties were also shown to be perfect (Fig. 3 and Fig. 4). All the measured values were within the specifications of the project. 

The H²O beamline is the first dedicated hard X-ray inelastic scattering spectroscopy beamline in China. By exploiting the features of the high energy synchrotron facility HEPS, the beamline offers high energy resolution nuclear resonance scattering (NRS), X-ray Raman scattering (XRS) and resonant inelastic X-ray scattering (RIXS) techniques. These methods focus on detection of nuclear hyperfine interactions, phonon density of states and core level/valence electron excitations. These techniques can be applied to the frontiers of multidisciplinary research fields, e.g., quantum science, energy science, materials science, condensed matter physics, chemistry, biochemistry, geoscience, high pressure science and environmental science, etc. Among them, XRS is an advanced spectroscopic technique based on the inelastic scattering of X-rays. 

Due to the extremely small cross-section of inelastic scattering—4-5 orders of magnitude lower than X-ray absorption—the XRS technique requires a high brilliance light source as well as high detection efficiency using spectrometers with large solid-angle coverage, which is realized by multiple analyzers. Many beamlines are equipped with a large quantity of crystal analyzers: ESRF (72), APS (19), SPring-8 (12), PETRAIII (12), SSRL (40), SOLEIL (40), etc. The Diamond Light Source-II also proposed a flagship beamline equipped with an XRS spectrometer.

Passing of the online test by the first batch of HEPS crystal analyzers represents a milestone for the construction of the HEPS Qiankun spectrometer as well as the H²O beamline, meaning that a large number of SBCAs can now be fabricated to meet the requirements of the project. In addition, the fabrication process can also be used to fabricate crystal analyzers for other types of spectrometers. 

So what is the next step? The team is now not only working on batch production of SBCAs to fill the rest modules of the spectrometer, but also investigating strain-free super high energy resolution crystal analyzers. 

This work is also supported by the Platform of Advanced Photon Source (PAPS) Technology R&D, as well as by beamtime granted by the BSRF-1W2B, 3W1, 4W1A and 4W1B beamlines. 

Fig. 1. HEPS Crystal Analyzers (Image by IHEP)

Fig. 2. Experimental Setup (Image by IHEP)

Fig. 3. Left panel (top left) measured and fitted results of crystal 4#; (bottom left) focal spots of three crystals; right panel, measured results for all SBCAs (Image by IHEP)

Fig. 4 Evolution of focal spots on the detector during energy scanning (Image by IHEP)

The working team (from left to right: Dr. ZHANG Yujun, Dr. GUO Zhiying, Mr. GAN Xiaolong, Dr. XU Wei, Dr. ZHANG Jiuchang, Dr. DIAO Qianshun, Dr. JIN Shuoxue) (Image by IHEP)

Source:  http://english.ihep.cas.cn/nw/han/y22/202211/t20221104_322820.html