We present an experimental study of the Rayleigh scattering of highly linearly polarized hard x rays. For this study, photons produced by a synchrotron source with an energy of 175 keV are scattered by a thin gold foil target. While the linear polarization in terms of degree of polarization and its orientation of the incident photon beam are well known, the outgoing scattered photons are analyzed by means of Compton polarimetry. For this, a polarimeter detector is positioned at different observation angles with respect to the incident photon beam. For the first time the polarization of elastically scattered photons outside of the polarization plane of the incident radiation is investigated, enabling novel access to a so-far untested component of the polarization transfer. The experimental findings are compared to predictions of two theoretical models. It is found that, when leaving the polarization plane of the incident photon beam, a strong deviation from the form factor approximation is evident, while calculations of the Rayleigh scattering in the S-matrix approach agree with the experimental results. Furthermore, this study is a foundation for future experiments dedicated to a polarization-resolved analysis of Delbr & uuml;ck scattering.

We present a theoretical study of Compton scattering of X- and γ-rays by a K-shell electron. Special attention is paid to the double-differential cross section and polarization of the scattered photons for linearly polarized incident photons. To investigate these observables, we employ the scattering matrix (S-matrix) approach based on relativistic Green's functions. The S-matrix results are moreover compared with predictions of the free-electron and impulse approximations, allowing us to assess the role of electron binding effects. Detailed calculations are carried out for hydrogen-like Ne^9+ and Pb^81+ targets over a wide range of incident photon energies and scattering angles. The calculations reveal kinematic regimes in which the impulse approximation agrees reasonably well with the S-matrix results. We also explore the polarization of scattered photons for slightly depolarized incident radiation, including the highly sensitive case of scattering at 90^∘.

Metallic Magnetic Calorimeters (MMCs) are a promising new tool for high precision X-ray spectroscopy. However, the complexity of the detector response and the need for scalable processing pipelines pose significant challenges for their widespread adoption. In this work, we explore the application of Machine Learning (ML) methods to address these challenges and enhance the performance of MMCs. We demonstrate how ML can be used for pulse classification and artifact rejection, as well as for pulse shape analysis and feature extraction. By leveraging unsupervised learning techniques for label auto-discovery and supervised learning for classification and regression tasks, we show that ML can provide robust and scalable solutions for MMC signal processing. Our results indicate that ML-based approaches can achieve comparable performance to traditional methods while offering greater adaptability and efficiency, paving the way for the next generation of high-precision X-ray spectroscopy with MMCs.

Polarized heavy ions in storage rings are seen as a valuable tool for a wide range of research, from the study of spin effects in relativistic atomic collisions to the tests of the Standard Model. For forthcoming experiments, several important challenges need to be addressed to work efficiently with such ions. Apart from the production and preservation of ion polarization in storage rings, its measurement is an extremely important issue. In this contribution, we employ the radiative recombination (RR) of polarized electrons into the ground state of initially hydrogen-like, finally helium-like, ions as a probe process for beam diagnostics. Our theoretical study clearly demonstrates that the RR cross section, integrated over photon emission angles, is highly sensitive to both the degree and the direction of ion polarization. Since the (integrated) cross-section measurements are well established, the proposed method offers promising prospects for ion spin tomography at storage rings.

Vacuum fluctuations give rise to effective nonlinear interactions between electromagnetic fields. These generically modify the characteristics of light traversing a strong-field region. X-ray free-electron lasers (XFELs) constitute a particularly promising probe, due to their brilliance, the possibility of precise control and favorable frequency scaling. However, the nonlinear vacuum response is very small even when probing a tightly focused high-intensity laser field with XFEL radiation and direct measurement of light-by-light scattering of real photons and the associated fundamental physics constants of the quantum vacuum has not been possible to date. Achieving a sufficiently good signal-to-background separation is key to a successful quantum vacuum experiment. To master this challenge, a dark-field detection concept has recently been proposed. Here we present the results of a proof-of-principle experiment validating this approach by demonstrating that using real-world x-ray optics the background signal can be suppressed sufficiently to measure the weak nonlinear response of the vacuum.

Using novel metallic magnetic calorimeter detectors at the CRYRING@ESR, we recorded X-ray spectra of stored and electron cooled helium-like uranium (U^90+) with an unmatched spectral resolution of close to 90 eV. This allowed for an accurate determination of the energies of all four components of the Kα transitions in U^90+. We find good agreement with state-of-the-art bound-state QED calculations for the strong-field regime. Our results do not support any systematic deviation between experiment and theory in helium-like systems, the presence of which was subject of intense debates in recent years.

State-of-the-art 2D sensitive semiconductor detectors developed within the SPARC collaboration can be utilized as dedicated Compton polarimeters in the hard X-ray regime. We report on the technique of Compton polarimetry utilizing such a detector and present a method to determine the linear polarization of an analyzed hard X-ray beam by means of Monte-Carlo-simulated data sets.

Beim nuklearen Zweiphotonenzerfall sendet ein angeregter Atomkern simultan zwei Photonen aus. Dieser sehr seltene Zerfallsmodus kann einerseits Informationen über eine Reihe von fundamentalen Eigenschaften des Atomkerns geben. Dazu zählt seine Polarisierbarkeit, je nachdem in welchem Anregungszustand er sich befindet. Andererseits kann er aber auch helfen, die nuklearen Grundlagen des doppelten Betazerfalls besser zu verstehen.

Vacuum fluctuations give rise to effective nonlinear interactions between electromagnetic fields. These generically modify the characteristics of light traversing a strong-field region. X-ray free-electron lasers constitute a particularly promising probe, due to their brilliance, the possibility of precise control and favourable frequency scaling. However, the nonlinear vacuum response is very small even when probing a tightly focused high-intensity laser field with XFEL radiation and direct measurement of light-by-light scattering of real photons and the associated fundamental physics constants of the quantum vacuum has not been possible to date. Achieving a sufficiently good signal-to-background separation is key to a successful quantum vacuum experiment. To master this challenge, a darkfield detection concept has recently been proposed. Here we present the results of a proof-of-principle experiment validating this approach at the High Energy Density scientific instrument of the European X-Ray Free Electron Laser.

We have performed systematic measurements of the resistive cooling behavior of bunches of highly charged ions in a Penning trap after injection from an external source. In particular, we have been able to measure the exponential cooling rate of the axial center -of -mass motion and experimentally show its linear dependence on the ion number within the bunch, as expected from theory. The common center -of -mass energy of the ions is reduced by more than four orders of magnitude on the time scale of milliseconds while the single -ion cooling occurs on the scale of seconds, representing a highly effective way to remove the dominating part of the ion kinetic energy.

Quantum field theory predicts a nonlinear response of the vacuum to strong electromagnetic fields of macroscopic extent. This fundamental tenet has remained experimentally challenging and is yet to be tested in the laboratory. A particularly distinct signature of the resulting optical activity of the quantum vacuum is vacuum birefringence. This offers an excellent opportunity for a precision test of nonlinear quantum electrodynamics in an uncharted parameter regime. Recently, the operation of the high-intensity laser ReLaX provided by the Helmholtz International Beamline for Extreme Fields (HIBEF) has been inaugurated at the High Energy Density (HED) scientific instrument of the European XFEL. We make the case that this worldwide unique combination of an x-ray free-electron laser and an ultra-intense near-infrared laser together with recent advances in high-precision x-ray polarimetry, refinements of prospective discovery scenarios, and progress in their accurate theoretical modelling have set the stage for performing an actual discovery experiment of quantum vacuum nonlinearity.

We present a non-destructive electronic detector for stored charged particles in a Penning trap that uses a symmetric electrode arrangement for signal pickup and a resonator without tap. This system has advantageous off-resonance features which are demonstrated by means of detection and cooling measurements with highly charged ions in a cryogenic Penning trap. In particular, it allows for particle detection across a wide range of frequencies that is not concomitant with cooling and offers a novel way to tune a trap with the help of non-destructive measurements on large particle ensembles.

We have developed and operated an electronic system for the non-destructive detection and cooling of charged-particle bunches that are captured and confined in a Penning trap, together with methods for the evaluation of corresponding measurements that allow for a detailed characterization of each individual particle bunch. Once calibrated, from a single measurement of the particles’ induced electronic signal as a function of time directly upon capture, the setup and method allow for a fast determination of the initial and final absolute particle energies, of the cooling rate, and of the absolute number of particles in the bunch. We demonstrate this with highly charged ions (Ne8+) that are injected into the Penning trap of the HILITE experiment.

In this report, we compare two filter algorithms for extracting timing information using novel metallic magnetic calorimeter detectors, applied to the precision X-ray spectroscopy of highly charged ions in a storage ring. Accurate timing information is crucial when exploiting coincidence conditions for background suppression to obtain clean spectra. For X-rays emitted by charge-changing interactions between ions and a target, this is a well-established technique when relying on conventional semiconductor detectors that offer a good temporal resolution. However, until recently, such a coincidence scheme had never been realized with metallic magnetic calorimeters, which typically feature much longer signal rise times. In this report, we present optimized timing filter algorithms for this type of detector. Their application to experimental data recently obtained at the electron cooler of CRYRING@ESR at GSI, Darmstadt is discussed.

We perform a study on Rayleigh scattering of highly linearly polarized hard x rays on a thin Au foil target. In the study the angular distribution of the scattered radiation is analyzed in a relativistic regime both within and out of the plane of polarization of the incident beam. Within this experiment we scatter a synchrotron beam with a photon energy of 175 keV on a high-Z target foil, namely, gold. Our findings correlate well with state-of-the-art calculations of the scattering process performed in the framework of quantum electrodynamics and may have considerable impact on future experiments regarding a polarization-resolved analysis of Delbruck scattering. Furthermore, we show that the angular distribution of Rayleigh scattering can be used for a highly sensitive determination of the degree and orientation of the linear polarization of the incident hard-x-ray beam, if we rely on the theoretical framework.

This contribution is based on our input to the NuPECC LRP on per-spectives of precision experiments at heavy-ion storage rings in the realm of nuclear structure, atomic-and astrophysics. A focus here is on experi-ments with secondary beams of heavy ions, which can either be stable or long-lived nuclei in specific, high atomic charge states, or unstable nuclides.

We report on a new experimental approach for the Doppler correction of X-rays emitted by heavy ions, using novel metallic magnetic calorimeter detectors which uniquely combine a high spectral resolution with a broad bandwidth acceptance. The measurement was carried out at the electron cooler of CRYRING@ESR at GSI, Darmstadt, Germany. The X-ray emission associated with the radiative recombination of cooler electrons and stored hydrogen-like uranium ions was investigated using two novel microcalorimeter detectors positioned under 0∘ and 180∘ with respect to the ion beam axis. This new experimental setup allowed the investigation of the region of the N, M → L transitions in helium-like uranium with a spectral resolution unmatched by previous studies using conventional semiconductor X-ray detectors. When assuming that the rest-frame energy of at least a few of the recorded transitions is well-known from theory or experiments, a precise measurement of the Doppler shifted line positions in the laboratory system can be used to determine the ion beam velocity using only spectral information. The spectral resolution achievable with microcalorimeter detectors should, for the first time, allow intrinsic Doppler correction to be performed for the precision X-ray spectroscopy of stored heavy ions. A comparison with data from a previous experiment at the ESR electron cooler, as well as the conventional method of conducting Doppler correction using electron cooler parameters, will be discussed.

We report on an experiment conducted at the ESR storage ring aiming at the study of the X-ray emission of Xe^54+ ions colliding with Xe atoms at a beam energy of 50 MeV/u. The radiation resulting from the ion–atom interaction was observed using a high-resolution spectrometer based on metallic–magnetic calorimeter technology. In order to benchmark the capabilities of these detectors for high-precision atomic physics experiments, we identified several transitions from H-like and He-like xenon and determined their energies. Furthermore, the 1s-Lamb shift in Xe^53+ was estimated using the measured line energies. The results are compared with previous experimental studies and theoretical predictions.

Atomic physics of highly charged ions can be studied by high precision x-ray spectroscopy. Collision experiments at storage ring facilities are one of the most promising approaches for this, particularly when the unique capabilities of metallic magnetic calorimeters (MMC) are exploited. To obtain clear spectra from these devices, accurate time of flight measurements are crucial for background reduction. Previously a so called k sigma trigger - which is a leading edge trigger with an adaptive threshold - has been used. In this contribution we present a further background reduction by a factor of three by applying the constant fraction discrimination (CFD) trigger in the MMC data evaluation.

Experimental determination of the cross sections of proton capture on radioactive nuclei is extremely difficult. Therefore, it is of substantial interest for the understanding of the production of the p-nuclei. For the first time, a direct measurement of proton-capture cross sections on stored, radioactive ions became possible in an energy range of interest for nuclear astrophysics. The experiment was performed at the Experimental Storage Ring (ESR) at GSI by making use of a sensitive method to measure (p,γ) and (p,n) reactions in inverse kinematics. These reaction channels are of high relevance for the nucleosyn-thesis processes in supernovae, which are among the most violent explosions in the universe and are not yet well understood. The cross section of the 118Te(p,γ) reaction has been measured at energies of 6 MeV/u and 7 MeV/u. The heavy ions interacted with a hydrogen gas jet target. The radiative recombination process of the fully stripped 118Te ions and electrons from the hydrogen target was used as a luminosity monitor. An overview of the experimental method and preliminary results from the ongoing analysis will be presented.