Abstract: We present a measurement of K‐shell transitions in H‐like gold (Au78+) using specially developed transmission type crystal spectrometers combined with Ge(i) microstrip detectors. The experiment has been carried out at the Experimental Storage Ring at GSI in Darmstadt. This is a first high‐resolution wavelength‐dispersive measurement of a K‐shell transition in a high‐Z H‐like ion, thus representing an important milestone in this field. Ideas on possible future improvements are discussed as well.
Abstract: Accurate spectroscopy of highly-charged high-Z ions in a storage ring is demonstrated to be feasible by the use of specially adapted crystal optics. The method has been applied for the measurement of the is Lamb shift in hydrogen-like gold (Au78+) in a storage ring through spectroscopy of the Lyman x-rays. This measurement represents the first result obtained for a high-Z element using high-resolution wavelength-dispersive spectroscopy in the hard x-ray regime, paving the way for sensitivity to higher-order QED effects.
Abstract: In this work, we present a pilot experiment in the experimental storage ring (ESR) at GSI devoted to impact parameter sensitive studies of inner shell atomic processes for low-energy (heavy-) ion-atom collisions. The experiment was performed with bare and He-like xenon ions (Xe54+, Xe52+) colliding with neutral xenon gas atoms, resulting in a symmetric collision system. This choice of the projectile charge states was made in order to compare the effect of a filled K-shell with the empty one. The projectile and target X-rays have been measured at different observation angles for all impact parameters as well as for the impact parameter range of ∼35–70 fm.
Abstract: We report on the first elastic hard x-ray scattering experiment where the linear polarization characteristics of both the incident and the scattered radiation were observed. Rayleigh scattering was investigated in a relativistic regime by using a high- Z target material, namely gold, and a photon energy of 175 keV. Although the incident synchrotron radiation was nearly 100% linearly polarized, at a scattering angle of θ=90° we observed a strong depolarization for the scattered photons with a degree of linear polarization of +27% ± 12% only. This finding agrees with second-order quantum electrodynamics calculations of Rayleigh scattering, when taking into account a small polarization impurity of the incident photon beam which was determined to be close to 98%. The latter value was obtained independently from the elastic scattering by analyzing photons that were Compton-scattered in the target. Moreover, our results indicate that when relying on state-of-the-art theory, Rayleigh scattering could provide a very accurate method to diagnose polarization impurities in a broad region of hard x-ray energies.
Abstract: The process of electron-loss to the continuum (ELC) has been studied for the collision systems U28+ -> H2 at a collision energy of 50 MeV/u, U28+ -> N2 at 30 MeV/u, and U28+ -> Xe at 50 MeV/u. The energy distributions of cusp electrons emitted at an angle of 0∘ with respect to the projectile beam were measured using a magnetic forward-angle electron spectrometer. For these collision systems far from equilibrium charge state, a significantly asymmetric cusp shape is observed. The experimental results are compared to calculations based on first-order perturbation theory, which predict an almost symmetric cusp shape. Some possible reasons for this discrepancy are discussed.
Friedrich-Schiller-Universität Jena, Physikalisch-Astronomische Fakultät (2016)
Abstract: This thesis investigates experimentally the elastic scattering of hard x-rays. Combining the novel technologies of a third-generation synchrotron radiation source and a Si(Li) strip detector which acts as a highly efficient x-ray Compton polarimeter allows to measure the linear polarization of the elastically scattered photons for a highly linearly polarized incident beam. Here, such a polarization transfer is considered for the first time in the hard x-ray regime. With a photon energy of 175 keV and gold as scatterer, a highly relativistic regime is chosen where Rayleigh scattering is the only significant elastic scattering contribution. In addition to the polarization of the elastically scattered photons, also the angular distribution is measured. The data are compared to fully relativistic second-order QED calculations. Both observables are well described by these predictions whereas the form factor approximation fails. The simultaneous measurement of angular distribution and polarization allows to identify spurious agreement of the form factor theory in only one observable. At scattering angles around 90°, the assumption that the incident beam is completely linearly polarized is not sufficient to explain the data. The measured linear polarization of the Compton-scattered photons is used to obtain an independent estimate for the incident beam polarization of about 98% which leads to an agreement between experiment and theory at all measured data points. The significant change introduced by this depolarization of 2% indicates a strong sensitivity on the polarization of the incident beam. In the present experiment, this sensitivity limits the precision, but on the other hand, it allows a precise reconstruction of the incident beam polarization when the theory is established. Here, such a reconstruction is performed and the result agrees with the 98% from the Compton polarization, but with a slightly lower uncertainty and with less statistics.
Abstract: In this work, we report on an experiment that investigated the elastic scattering of linearly polarized 175 keV photons on a gold target. A combined measurement of the angular distribution and the linear polarization of the scattered photons was performed using standard germanium detectors and a double-sided Si(Li) strip polarimeter. Since the data analysis is still in progress, we will show results in forthcoming papers and present here how the polarimeter was used to identify a lack of shielding during the experiment.
Abstract: The experimental investigation of quantum-electrodydamic contributions to the binding energies of inner shells of highly charged heavy ions requires an accurate spectroscopy in the region of hard x-rays suitable at a limited source strength. For this purpose the focusing compensated asymmetric Laue crystal optics has been developed and a twin-spectrometer assembly has been built and commissioned at the experimental storage ring of the GSI Helmholtzzentrum Darmstadt. We characterize the crystal optics and demonstrate the usefulness of the instrumentation for accurate spectroscopy of both stationary and fast moving x-ray sources. The experimental procedures discussed here may also be applied for other spectroscopic studies where a transition from conventional germanium x-ray detectors to crystal spectrometers seems too demanding because of low source intensity.
Abstract: The response of a double-sided segmented Si(Li) detector system has been investigated. The detector has been irradiated with a collimated, highly linearly polarized beam of 53.2 keV photons from the synchrotron radiation source PETRA III at DESY. The detector was mounted on a platform that could be moved with μm precision thus allowing for a defined beam position on the detector surface. In this paper, the effects of the isolation gaps (gap width = 50 μm) between adjacent segments (strips) were studied, in particular with respect to the effect of charge sharing. The fraction of such charge sharing events increases from about 5% (beam hits center of a strip) to over 50% when the beam is focused just on a gap. The fraction of reconstructed Compton scattering events, which is interesting for Compton polarimetry, amounts to about 3% with the beam impinging at a strip center and 2.8% on average. It can therefore be concluded that events related to charge sharing do not critically degrade the performance of the detector as a Compton polarimter.
Abstract: Hard x-ray polarimetry of radiation emitted in collisions of heavy ions, electrons or photons with matter provides detailed information on the collision dynamics as well as of the atomic structure in the presence of extreme field strengths. Moreover, it also opens a route for polarization diagnosis of spin-polarized ion and electron beams which, for example, might be useful in future parity non-conservation studies. Owing to recent progress in the development of highly segmented solid-state detectors, a novel type of polarimeter for the hard x-ray regime has become available. Applied as Compton polarimeters, two-dimensional position-sensitive x-ray detectors now allow for precise and efficient measurements of x-ray linear polarization properties. In this report recent polarimetry studies using such detector systems are reviewed.
Abstract: For the collision system U88+ -> N2 at a collision energy of 90 MeV/u, the energy distribution of electrons being nonradiatively captured from the target into the projectile continuum has been measured under an angle of 0∘ with respect to the projectile beam axis. This measurement of the electron-capture-to-continuum cusp with the highest effective projectile charge Z_eff,p=88 at a near-relativistic collision velocity of β≈0.41 is shown to be characterized by a strong asymmetry in the cusp shape. By comparing the data to measurements of the radiative-electron-capture-to-continuum cusp for the same collision system, the opposite asymmetry of the cusp is traced back to the varying underlying mechanisms. The experimental results are compared with the two theoretical calculations available for this process, one of them in the semirelativistic impulse approximation and the other in the nonrelativistic continuum-distorted-wave approach. A corresponding fully relativistic treatment may be motivated by the presented experimental data.
Abstract: The electron loss to the continuum has been studied for the collision system U88+ + N2→U89+ + [N2]∗ + e− at the low-relativistic projectile energy of 90 MeV/u. Using a magnetic electron spectrometer, the energy distribution of cusp electrons emitted at an angle of 0∘ with respect to the projectile beam was measured in coincidence with the up-charged projectile. At the experimental collision energy ionization of the berylliumlike U88+ projectile proceeds predominantly from the L shell, but a contribution from the K shell could also be identified experimentally. The measurement is shown to be in accordance with fully relativistic Dirac calculations applying first-order perturbation theory. Furthermore, the underlying continuum electron distribution in the projectile frame is illustrated.
Abstract: The radiative electron capture of a target electron into the projectile continuum has been studied for the collision system U88+ + N2 → U88+ + [N+2]∗ + e− + γ at 90 MeV/u. Using a magnetic electron spectrometer, the energy distribution of cusp electrons emitted under an angle of 0∘ with respect to the projectile beam and with a velocity close to the projectile velocity has been measured in coincidence with the emitted photons under various observation angles. The experimental results provide a stringent test for the corresponding process in inverse kinematics, namely, the theory of electron-nucleus bremsstrahlung at the high-energy endpoint. For comparison this process is calculated using fully relativistic Dirac wave functions and using semirelativistic Sommerfeld-Maue wave functions.
Abstract: The future x-ray spectroscopy and polarimetry experiment program of the SPARC collaboration at GSI and FAIR relies strongly on the availability of two-dimensional position-sensitive, energy- and time-dispersive thick semiconductor detector systems, including the appropriate signal processing electronics. To meet these demands, the development of a compact and scalable data acquisition system that has higher rate acceptance compared to commercial VME electronics by employing digital pulse processing electronics was started.
Abstract: In this work, the digital readout of semiconductor detectors in combination with digital filters was investigated. Both non-segmented high-purity germanium and segmented planar lithium-drifted silicon detectors were used. In each case, photons from a stationary americium ((241)^Am) gamma source were detected. The resulting preamplifier output pulses were digitized at a fixed sampling frequency and stored entirely. Digital filters were applied to the stored waveforms to extract time and energy information. The performance of different digital filters was compared. The optimum energy resolution obtained was comparable with the value resulting from an analogue readout system based on standard nuclear instrumentation module and versatile module Europe bus electronics.
Abstract: In recent years for the fundamental theory of quantum electrodynamics, considerable progress in the evaluation of higher order corrections has been achieved—not only for hydrogen—but also for helium-like systems—up to very heavy nuclei. We were aiming at a more precise determination of the lifetime of the metastable 2 3^P_0 state in He-like uranium which has a calculated value of 57.3 ps [1, 2]. From the lifetime it is possible to derive the energy of the state. In October 2011 we were able to perform a first test experiment at GSI, Darmstadt to study the feasibility of a new experimental detection technique. This advanced set-up consists of two state-of-the-art energy-, time- and position-sensitive germanium detectors  in combination with collimators in a Soller-slit like assembly. A beam of U^(91+) -ions at an energy of 290 MeV u^(−1) is passed through a thin nickel foil in the interaction chamber. From the decrease in intensity as a function of the target distance one may extract a decay curve from which the lifetime can be derived. The advantages of this new set-up, in comparison to former experiments [4, 5] will be discussed and the results of a preliminary data analysis will be presented.