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Description
The VOXES (VOn Hamos X-ray spectrometer using HAPG for Extended Sources) Bragg spectrometer is a high-resolution X-ray spectrometer developed at the INFN National Laboratories of Frascati (LNF). It is based on cylindrically bent mosaic crystals operating in the Von Hamos configuration and was specifically designed to overcome the intrinsic efficiency limitations of traditional Bragg spectroscopy when dealing with extended and diffuse X-ray sources. By using Highly Annealed Pyrolytic Graphite (HAPG) crystals with typical mosaicity of 0.007–0.01°, optimized beam optics, and a compact geometry (source–crystal distance ~50–100 cm, crystal radius ~100–200 mm), VOXES achieves energy resolutions of about 2–5 eV (σ) in the 5–10 keV range, corresponding to resolving powers E/ΔE up to ~2000–3000. Absolute energy precision at the level of ~0.1–0.2 eV can be obtained after calibration, while maintaining high collection efficiency even for effective source sizes of a few mm in the dispersive plane, and up to a few cm in the vertical one. The spectrometer provides simultaneous multi-line detection over energy windows of ~100–2000 eV, depending on geometry and detector, and demonstrates detection efficiencies orders of magnitude higher than traditional perfect-crystal Bragg systems when measuring extended or low-intensity sources.
Since its first prototype, VOXES has undergone continuous development, including improvements in ray-tracing-based optimization, efficiency characterization, and calibration procedures. Recent upgrades introduced enhanced automation, motorized positioning with micrometric precision, and the integration of an energy-dispersive fluorescence monitoring line using a silicon PIN-diode detector, enabling simultaneous wavelength- and energy-dispersive measurements and real-time flux normalization. Dedicated sample environments, including liquid-sample holders and transmission geometries, allow laboratory X-ray emission and (in future) absorption studies with count rates ranging from a few Hz up to several kHz, depending on configuration and source intensity.
VOXES has been successfully employed in multiple research programs. In fundamental and nuclear physics, it has been developed as a precision instrument for high-resolution spectroscopy of exotic-atom X-ray transitions and related measurements. Within the TASTE (INFN-CNTT) and MITIQO project (Regione Lazio), VOXES demonstrated high-resolution X-ray emission spectroscopy of transition metals in liquid solutions, enabling determination of oxidation states and chemical environments with energy sensitivities of a few tenths of an eV.
Today, VOXES integrates multiple detectors, including MYTHEN2 (50 µm pitch strip detector), Timepix3 (55 µm pixel, event-driven readout with 1.56 ns time resolution), a CCD detector, and a silicon PIN diode, allowing flexible spectroscopic 1D and 2D measurements. A large collection of HAPG and Silicon crystals enables coverage of a wide photon-energy range from approximately 4 to 40 keV. With its modular design, high resolving power, and ability to operate with extended and weak sources, VOXES represents a versatile laboratory-scale spectroscopic facility bridging fundamental physics, materials science, and applied X-ray research.