Marc Oliver Herdrich
Abstract: For decelerated bare lead ions at a low beam energy of 10 MeV/u, the x-ray emission associated with radiative recombination (RR) at threshold energies has been studied at the electron cooler of CRYRING@ESR at GSI, Darmstadt. In our experiment, we observed the full x-ray emission pattern by utilizing dedicated x-ray detection chambers installed at 0∘ and 180∘ observation geometry. Most remarkably, no line distortion effects due to delayed emission are present in the well-defined x-ray spectra, spanning a wide range of x-ray energies (from about 5 to 100 keV), which enables us to identify fine-structure resolved Lyman, Balmer, and Paschen x-ray lines along with the RR transitions into the K, L, and M shells of the ions. For comparison with theory, an elaborate theoretical model is established taking into account the initial population distribution via RR for all atomic levels up to Rydberg states with principal quantum number n=165 in combination with time-dependent feeding transitions. Within the statistical accuracy, the experimental data are in very good agreement with the results of rigorous relativistic predictions. Most notably, this comparison sheds light on the contribution of prompt and delayed x-ray emission (up to 70 ns) to the observed x-ray spectra, originating in particular from yrast transitions into inner shells.
Abstract: We study the electron-loss-to-continuum (ELC) cusp experimentally and theoretically by comparing the ionization of U89+ projectiles in collisions with N-2 and Xe targets, at a beam energy of 75.91 MeV/u. The coincidence measurement between the singly ionized projectile and the energy of the emitted electron is used to compare the shape of the ELC cusp at weak and strong perturbations. A significant energy shift for the centroid of the electron cusp is observed for the heavy target of Xe as compared to the light target of N-2. Our results provide a stringent test for fully relativistic calculations of double-differential cross sections performed in the first-order approximation and in the continuum-distorted-wave approach.
Abstract: The electron-capture process was studied for Xe54+ colliding with H2 molecules at the internal gas target of the Experimental Storage Ring (ESR) at GSI, Darmstadt. Cross-section values for electron capture into excited projectile states were deduced from the observed emission cross section of Lyman radiation, being emitted by the hydrogenlike ions subsequent to the capture of a target electron. The ion beam energy range was varied between 5.5 and 30.9 MeV/u by applying the deceleration mode of the ESR. Thus, electron-capture data were recorded at the intermediate and, in particular, the low-collision-energy regime, well below the beam energy necessary to produce bare xenon ions. The obtained data are found to be in reasonable qualitative agreement with theoretical approaches, while a commonly applied empirical formula significantly overestimates the experimental findings.
Abstract: For U89+ projectiles colliding at a beam energy of 75.91 MeV/u with a N2 target, we present a coincidence measurement between the cusp electrons emitted under an angle of 0° with respect to the projectile beam and the photons emitted under a polar angle of 90°. This radiative-electron-capture-to-continuum cusp directly probes the theory of electron-nucleus bremsstrahlung up to the high-energy endpoint in inverse kinematics. In the present study, significant improvement with respect to the experimental accuracy has been achieved, resulting in a finer agreement between experimental and theoretical results.
Abstract: Recently, the contribution of the generalized Breit interaction to electron impact ionization was identified for the first time in a high‐Z system, namely, hydrogen‐like uranium. This study employed a measurement of the relative population of the j = 1/2 and j = 3/2 states of the L shell by projectile excitation in collision of U91+ with hydrogen and nitrogen targets. However, for a rigorous test of ion–atom collision theory, also the absolute excitation cross sections are of great importance. In the present work, we report on our efforts to extend the previous study to a determination of the absolute projectile excitation cross sections by normalization to the well‐known radiative electron capture process.
Abstract: A micro-calorimeter X-ray detector of the maXs-30 type was used to record the X-ray radiation from Fe ions, being produced in the S-EBIT-I electron beam ion trap at the site of GSI. The resulting spectra demonstrate the superior energy resolving power of micro-calorimeter detectors compared with conventional semiconductor detectors. The experiment serves as another proof of principle for the application of calorimeters as dedicated high-resolution X-ray spectrometers at an ion facility. Together with the development of an improved analysis algorithm for online readout, these results present a step towards the use of maXs-type detectors as standard instrumentation at GSI/FAIR.
Abstract: An experiment addressing electron capture (EC) decay of hydrogen-like ¹⁴²Pm⁶⁰⁺ ions has been conducted at the experimental storage ring (ESR) at GSI. The decay appears to be purely exponential and no modulations were observed. Decay times for about 9000 individual EC decays have been measured by applying the single-ion decay spectroscopy method. Both visually and automatically analysed data can be described by a single exponential decay with decay constants of 0.0126(7) s⁻¹ for automatic analysis and 0.0141(7) s⁻¹ for manual analysis. If a modulation superimposed on the exponential decay curve is assumed, the best fit gives a modulation amplitude of merely 0.019(15), which is compatible with zero and by 4.9 standard deviations smaller than in the original observation which had an amplitude of 0.23(4).
Abstract: Abstract On the basis of a double-side segmented Si(Li) crystal a new Compton polarimeter was developed within the SPARC collaboration. The new detector is equipped with a cryogenic first stage of the preamplifiers to improve the energy resolution compared to previous detectors with preamplifiers operating at room temperature. We present first results from a commissioning measurement of the new instrument at the ESR storage ring of GSI in Darmstadt, Germany and contrast it with the performance of an precursor polarimeter system.
Abstract: Abstract In experiments with highly charged, fast heavy ions the Radiative Recombination (RR) and Radiative Electron Capture (REC) processes have significant cross sections in an energy range of up to a few GeV / u . They are some of the most important charge changing processes in collisions of heavy ions with atoms and electrons, leading to the emission of a photon along with the formation of the ground and excited atomic states. Hence, for the understanding and planning of experiments, in particular for X-ray spectroscopy studies, at accelerator ring facilities, such as FAIR, it is crucial to have a good knowledge of these cross sections and the associated radiation characteristics. In the frame of this work a fast calculator, named RECAL, for the RR and REC process is presented and its capabilities are demonstrated with the analysis of a recently conducted experiment at the Experimental Storage Ring (ESR) at the GSI Helmholtz Center for Heavy Ion Research in Darmstadt, Germany. A method is presented to determine unknown X-ray emission cross sections via normalization of the recorded spectra to REC cross sections calculated by RECAL.
Friedrich-Schiller-Universität Jena, Physikalisch-Astronomische Fakultät (2016)
Abstract: The planned FAIR-complex on the site of the GSI Helmholtz-Center for Heavy-Ion Research establishes a broad bandwidth of new experimental opportunities especially in the area of heavy-ion physics. New efforts to not only use its high-energy storagering HESR for proton-antiproton collisions, but also to open it up for experiments with relativistic heavy ions, are of great importance for the regime of relativistic collisions. They extend the options for atomic-physical studies into so far unreached areas of energy. This allows collision experiments of intensive, well-defined ion beams with virtually the full range of both energy and charge states with a variable gas-target. Electrons and photons released in those interactions lead the way to detailed observations and analysis of atomic structures and processes within the collision system. The planning of future experiments requires preferably pragmatic and precise methods of describing the cross-sections of the most important interaction-processes that lead to the emission of electrons and photons in ion-atom-colissions. In the frame of this work a basic overview of relevant interaction processes of collisions in the new energy range made available beyond 500 MeV/u is summarized. Furthermore the theoretical description of their emission characteristics is collected from already existing work, and used to calculate the energy and angle differential cross-sections and polarisation behaviours for a few processes in a wide range of parameters. The data sets are condensed into a database and compared to the results of other work, to test their quality. In the second part of this work the aquired data is used to plan a possible experiment at the HESR. For one, this demonstrates the practical usability of the database for future experiments. But also, the proposed experiment could be conducted in the initial phase of the storage-ring’s operation. The functionality of the facility could be checked and the effect of negative-polarized x-rays emitted by the radiative electron capture process, which - because of insufficient experimental capabilities - was not detectable yet, could be measured for the first time. Beyond the sole optimization of the experiment’s parameters using the database, several simulations were executed. The efficiency of a possible detector was studied, as well as the detectability of the effect itself under the precalculated experimental conditions. Secondly an analysis of the fraction of the radiation background was performed, that looked at the electrons which are also emitted and their interaction products with the experiment setup. The newly gained insight shows that a measurement of the negative polarization effect at the new storage-ring seems possible, but new problems and challenges arise from the fact that the emitted particles carry much higher energies. For example, binary encounter electrons can reach kinetic energies in the MeV-regime, which may lead to the emission of high energy secondary Bremsstrahlung. This has to be considered when designing the new target-chamber and detectors, and it is crucial for the planning of experiments to come.
Abstract: The radiative capture of free electrons (radiative recombination) and bound electrons (radiative electron capture) are among the most important charge changing processes for fast, highly-charged ions. While total cross sections can be obtained by an approximate formula with reasonable accuracy, the estimation of angular distributions and polarization properties of the emitted radiation requires a fully relativistic treatment that is numerical expensive. Therefore we recently started the development of a fast calculator for these radiation characteristics. The program is based on a grid of rigorously calculated data points for free- electron capture into bare ions, between which interpolation is performed to obtain radiation characteristics for specific collision systems. Also capture into few-electron systems is taken into account in an approximate way. We present first results from this development.
Abstract: Beam lifetimes of stored U^(28+) ions with kinetic energies of 30 and 50 MeV/u, respectively, were measured in the experimental storage ring of the GSI accelerator facility. By using the internal gas target station of the experimental storage ring, it was possible to obtain total projectile electron loss cross sections for collisions with several gaseous targets ranging from hydrogen to krypton from the beam lifetime data. The resulting experimental cross sections are compared to predictions by two theoretical approaches, namely the CTMC method and a combination of the DEPOSIT code and the RICODE program.
Friedrich-Schiller-Universität Jena, Physikalisch-Astronomische Fakultät (2013)
Abstract: For many experiments at accelerator facilities high luminosities are necessary, which are only achievable with highest ion beam intensities. Some of the experiments planned for the FAIR project need beam intensities up to a few 10^11 heavy ions in order to observe effects having extremely low reaction cross-sections. Furthermore, applications like the ion-driven fusion require high-intensity beams with beam currents up to 200 Ampere in total. Low charged particles have to be used, because space charge effects limit the maximum expected intensity and phase space volume of the ion beams. However, in typical beam energy regimes above 1 MeV/u, these particles are far from their equilibrium state, resulting in charge changing events during interactions with the residual gas of the accelerator tubes occurring more frequently. In ring accelerators these effects lead to the loss of ions, which for high intensities and high repetition-rates can result in dynamic processes leading to a sudden loss of the whole beam. To minimize the impact of such charge changing effects, a good understanding and characterization of the underlying processes is crucial. The theoretical description of dynamical processes in many electron systems is challenging and can only be done in an approximate way. Therefore an experimental validation of the theoretical predications within a broad parameter range is needed. For this purpose, beam lifetime experiments with two typical uranium charge states, namely U^28+ and U^73+, at three beam energies (30,50 and 150 MeV/u) have been carried out at the ESR storage-ring of the GSI Helmholtz Center for Heavy Ion Research, to determine their ionization cross-section in interactions with several different target gases.