High-Precision X-Ray Measurements 2023

High-Precision X-ray Measurements 2023 is a Special Issue of the journal Condensed Matter enclosing the scientific content of the 2023 High-Precision X-ray Measurements (HPXRM) conference. The conference is a biennial event that brings together experts from all over the world to discuss the latest developments in the field of X-ray precision detection, covering a wide range of topics, including X-ray sources, detectors, optics, and applications in various fields such as materials science, biology, and medicine. The conference also provides a forum for researchers to exchange ideas and collaborate on new projects. This Special Issue provides an overview of current state-of-the-art developments in X-ray measurements and highlights the most exciting developments presented.

This Special Issue’s contributions start with a detailed report on the PANDORA facility [1,2], an INFN project designed to study the $\beta$-decay lifetimes of isotopes in plasma environments that mimic stellar conditions. The paper focuses on a multi-detector system featuring a full-field pinhole CCD for high-resolution X-ray imaging and spectroscopy, enabling spatially resolved measurements of plasma density and temperature. The system’s capabilities in transient, stable, and turbulent plasma regimes are demonstrated, alongside the development of advanced plasma emission models and fast X-ray shutters for time-resolved analysis [3]. In [4], a combined spectroscopy system utilizing a Gas Electron Multiplier (GEM) and Timepix3 (TPX3) technology (developed by the Medipix collaboration [5]) is introduced for laser plasma experiments. The system’s ability to measure X-rays and gamma rays across a wide energy range is demonstrated, showcasing its suitability for high-precision plasma diagnostics [6].

Innovative approaches in data analysis are explored in [7], where differentiable programming is applied to improve the calibration of spectroscopic experiments (differentiable programming is used with great success in Particle Physics; see, e.g., [8,9,10]). This method enhances energy resolution and reduces uncertainty, demonstrating robustness and efficiency through synthetic data and Kernel Density Estimation. This computational advancement exemplifies how modern programming techniques can optimize experimental accuracy without extensive manual adjustments. Further applications of this method for underground detectors can be found in [11].

A focus on quantum mechanics and foundational physics is evident in several studies. The use of a broad-energy germanium detector (BEGe) prototype for low-energy threshold measurements in an underground LNGS laboratory is detailed in [12]. The setup is optimized to detect signals below 10 keV, enhancing sensitivity to potential new physics phenomena, demonstrating significant improvements in detection thresholds. An overview of the underground fundamental tests of quantum mechanics is provided in [13,14,15,16]. A novel approach to X-ray detection using lithium fluoride (LiF) films (see, e.g., [17,18,19,20] for some applications of this technique) is discussed in [21], presenting the characterization of LiF films through fluorescence and Raman micro-spectroscopy and demonstrating their potential in extreme ultraviolet and X-ray detection. Additionally, the improvement of time resolution in LaBr₃:Ce detectors with SiPM array readouts is explored in [22]. The development of hybrid ganging techniques and customized electronics addresses timing challenges, enhancing the detectors’ performance for low-energy X-ray applications in various fields, ranging from medical imaging to gamma-ray astronomy.

In [23,24], the potential of precise X-ray measurements in kaonic atoms is discussed. The SIDDHARTA-2 experiment (see, e.g., [25,26] for an overview of the experimental apparatus and measurements so far) is characterized through measurements of kaonic helium, which is essential for determining kaonic deuterium transitions (previous measurements were performed using E570 [27] and SIDDHARTA [28]). The importance of helium-4 as a calibration target, due to its high X-ray yield, is emphasized, revealing enhancements in the experimental setup that improve resolution and support challenging measurements of kaonic atoms. Advancements in detector technology allow for the determination of sub-threshold kaon-nucleon amplitudes, underscoring the need for future precise measurements to investigate nuclear states and quasi-bound states.

Contributions related to space missions and astrophysics are presented in [29,30], highlighting the role of the Italian Space Agency in supporting X-ray technologies for space exploration. The papers discuss advancements achieved over the past two decades, focusing on key collaborations that have provided insights into the universe’s origins and evolution. An overview of the Cryogenic Anticoincidence Detector (CryoAC) for the NewAthena [31] mission is provided, illustrating how the CryoAC, a key component of the X-IFU instrument, ensures sensitivity by minimizing background noise and enhancing detection capabilities. The mission’s collaboration with other large observatories and facilities is highlighted, emphasizing its role in future astrophysical discoveries.

The importance of X-ray fluorescence (XRF) techniques in cultural heritage is discussed in [32,33], where methods for evaluating quantitative data in copper-based artefacts are compared. The studies explore the performance of handheld XRF devices [34] and describe the design and use of portable XRF devices specifically tailored for analyzing heritage materials. These devices incorporate focusing optics and high-voltage X-ray tubes to enhance elemental analysis capabilities, emphasizing the need for a thorough understanding of X-ray spectrometry principles to achieve optimal results.

The VOXES X-ray emission spectrometer (XES), located at the INFN Frascati, is a mosaic crystal-based Von Hamos spectrometer [35,36] that offers a significant improvement in energy resolution. It is particularly suitable for studying metals as it can provide insight into their electronic structure and chemical environment. The system’s capabilities are demonstrated in [37], where the XES spectra of CuNiZn alloys are analyzed, revealing the potential of VOXES for high-precision X-ray measurements in fields ranging from materials science to the in situ monitoring of edible liquids.

Finally, an implementation of the MuDirac simulation in Geant4 is presented in [38]. This preliminary approach aims to improve the simulation of the muonic atom cascade process, which is crucial for muonic atom X-ray emission spectroscopy ($\mu$-XES), a novel method developed in past decades in several facilities, e.g., PSI [39], JPARC [40], and ISIS [41]. The study highlights the increasing demand for this technique in material characterization, driven by advancements in hardware and simulation capabilities.

The contributions to this Special Issue collectively provide a comprehensive overview of the latest advancements in X-ray detection and spectroscopy. Readers are invited to explore the individual papers to gain insights into cutting-edge research and innovative technologies that continue to shape the field of X-ray precision measurements.