Storage Ring ESR Research Articles

Applications of position sensitive germanium detectors for X-ray spectroscopy of highly charged heavy ions

The spectroscopy of atomic transitions in the hard X-ray regime above 15 keV utilizing position-sensitive solid state detectors is discussed. Special emphasis is given to the current detector developments for X-ray spectroscopy of heavy ions at the ESR storage ring where applications for precision spectroscopy as well as for polarization studies are of particular interest. For both cases, the advantages and new possibilities which are opened up by position and energy resolving solid state detectors are illustrated by the presentation of first experiments.

X-ray transitions studied for decelerated bare and H-like uranium ions at the ESR electron cooler

Here we report on X-ray spectra induced by spontaneous capture of free electrons into decelerated bare- and hydrogen-like uranium ions which we measured recently at the cooler section of the ESR storage ring. The most intense lines observed in spectra can be attributed to direct transition of electrons into the K shell of the projectile ions and to characteristic L→K (Lyα) transitions. Radiative recombination lines into the K shell of bare and H-like uranium can be exploited for measuring the two-electron contribution to the ground state binding energy in helium-like uranium. The goal is to probe for high-Z ions bound-state QED corrections which are of the order of α2. Besides the dominant characteristic L→K transitions, the strongly reduced Bremsstrahlung (due to the low cooler voltage applied to the decelerated ions) allowed us to observe for the very first time RR transitions into the L shell as well as the balmer radiation located at the low-energy part of the spectra.

Structure and Dynamics of High-Z Ions Studied at the ESR Storage Ring

The ESR storage ring offers unique conditions for precision investigations in the realm of atomic structure and collisions research at highest nuclear charges. For atomic structure studies, a very recent experiment will be discussed aiming on a precise determination of the electron–electron QED contribution to the groundstate ionization potential in He-like uranium. For the regime of collision dynamics, the close interplay between dynamics and atomic structure is outlined. Here, for the case of the 2p3/2→1s1/2 transition in H-like uranium, a strong interference between the E1 and M2 transition amplitudes has been identified which gives rise to a remarkable modification of the angular distribution of emitted photons.

Recent Results from ESR

The ESR storage ring has been designed for high resolution and high precision experiments with highly charged ions, both stable and radioactive species. Stochastic and electron cooling provide fast and powerful cooling for the preparation of cold ion beams. For small ion beam intensities beams of unprecedented quality can be obtained by electron cooling. The internal gas jet target allows operation with a variety of gases. The highly charged ions can be decelerated to reduce the resolution limitation originating from Doppler effects. The decelerated ions are either used for internal experiments or extracted to a fixed target area with an ultraslow extraction mode. For the determination of the masses of radioactive nuclei two methods have been developed. Schottky mass spectrometry of cooled ions with nuclear lifetimes of the order of seconds or longer allows mass measurements by determination of the revolution frequency from the Schottky noise with a mass resolution of 1 ppm. Tuning of the focusing structure of the storage ring to the transition energy of 344 MeV/u is applied to measure the masses of short–lived nuclei by a time of flight method.

Storage Ring Complex for Rare Isotope Beams in the GSI Future Project

The operation of the experimental storage ring ESR with beams of highly charged heavy ions has been studied at GSI over more than a decade. Beam cooling, either stochastic or by electrons, and internal gas jet target have been applied routinely. The conceptual design of a new storage ring complex at GSI is based mainly on experimental data for equilibrium beam parameters and cooling times measured at that ring. The generation of intense rare isotope beams is one of the pillars for the proposed GSI Future Project (http://www.gsi.de/GSI-future/project) which makes use of the existing UNILAC/SIS18 as injector for a new 100 Tm-synchrotron, SIS100. Especially nuclear physics experiments with stored and cooled rare isotope beams at essentially increased beam intensities will deliver important information on properties and structure of nuclei far from stability.

Dielectronic Recombination of Very Heavy Lithiumlike Ions

An overview of measurements of dielectronic recombination (DR) with the heaviest lithiumlike ions is presented. The experiments have been carried out at the heavy ion storage ring ESR at GSI utilizing the electron cooler as an electron target. In particular, results for low energy dielectronic recombination of Au76+ and U89+ are discussed. In the energy range of 0 to 400 eV which can be covered with our present set-up dielectronic resonances associated with Δn = 0 core excitations from 2s1/2 → 2p1/2,3/2 were found. The experimental results can only be explained within a fully relativistic treatment and are sensitive to QED contributions of the order α2 as well as to the finite size of the atomic nucleus.

High Rydberg resonances in dielectronic recombination of pb(79+).

Dielectronic recombination resonances of Pb (79+) associated with 2s(1/2)-->2p(1/2) excitations were measured at the heavy-ion storage ring ESR at GSI. The fine structure of the energetically lowest resonance manifold Pb (78+)(1s(2)2p(1/2)20l(j)) at around 18 eV could partially be resolved, and rate coefficients on an absolute scale were obtained. A comparison of the experimental data with results of a fully relativistic theoretical approach shows that high-angular-momentum components up to j=31/2 significantly contribute to the total resonance strength demonstrating the necessity to revise the widespread notion of negligible high-angular-momentum contributions at least for very highly charged ions.

Mass and lifetime measurements at the storage ring ESR

We present results from two methods of direct mass measurements of relativistic exotic nuclei stored in the storage ring ESR. Schottky Mass Spectrometry (SMS) with electron-cooled ions is well suited for long-lived nuclei ( T 1/2⩾10 s), and the Isochronous mass measurements (IMS) for hot fragments ( T 1/2⩾1 μs). In addition, SMS provides information on nuclear lifetime for bare and highly ionized atoms.

A microstrip germanium detector for position-sensitive X-ray spectroscopy

For the current X-ray spectroscopy program at the ESR storage ring (GSI-Darmstadt) a position sensitive germanium detector system has been completed. The position sensitive structure of the detector has been realized on an area of 47/spl times/23.4 mm/sup 2/ by an array of 200 strips (200 /spl mu/m wide and 23.4 mm long) separated by 35 /spl mu/m wide grooves etched through a boron implanted contact. The thickness of the detector inclusive of a 0.6 mm thick Li-diffused rear contact amounts to 4.1 mm. Each strip has been joined to a preamplifier placed outside the cryostat with printed leads inside the flexible Kapton foil. The energy resolution of the strips has been about 1.8 keV [FWHM] for 60 keV photons. Coincidences between neighbouring strips were measured whereby a time resolution of 70 ns [FWHM] was obtained. First preliminary results obtained with the detector mounted at the transmission X-ray spectrometer FOCAL demonstrated that an energy resolution better than 100 eV is achievable together with high detection efficiency. Along with a new kind of X-ray spectrometer this detector may play a key role for the next step of high precision X-ray experiments, aiming for a precise test of quantum electrodynamics in the heaviest one-electron systems such as a hydrogenlike uranium.

Multiple Coulomb ordered strings of ions in a storage ring.

We explain that the anomalous frequency shifts of very close masses obtained in the high precision mass measurement experiments in the ESR storage ring result from the locking of Coulomb interacting strings of ions. Here two concentric strings which run horizontally close to each other are captured into a single string if their thermal clouds overlap and give up their identity.

Commissioning of a hydrogen cluster target at the experimental storage ring

The internal jet target of the ESR storage ring is an extensively used experimental instrument where nitrogen, argon and even heavy noble gases such as xenon can routinely be used with densities ranging from 1×10 12 up to 1×10 14 p/cm 3 . In order to broaden further on the spectrum of possible experiments at the jet target and in particular to meet the requirements of the deceleration technique, the jet target has now been optimized with respect to molecular hydrogen. By the installation of a liquid nitrogen cooling system, a density increase for molecular hydrogen by two orders of magnitude has been accomplished.

Near-threshold photoionization of hydrogenlike uranium studied in ion-atom collisions via the time-reversed process.

Radiative electron capture, the time-reversed photoionization process occurring in ion-atom collisions, provides presently the only access to photoionization studies for very highly charged ions. By applying the deceleration mode of the ESR storage ring, we studied this process in low-energy collisions of bare uranium ions with low- Z target atoms. This technique allows us to extend the current information about photoionization to much lower energies than those accessible for neutral heavy elements in the direct reaction channel. The results prove that for high- Z systems, higher-order multipole contributions and magnetic corrections persist even at energies close to the threshold.

Radiative Electron Capture Studied for Bare, Decelerated Uranium Ions

By applying the deceleration technique for bare uranium ions at the ESR storage ring we studied the Radiative Electron Capture process in collisions with low-Z target atoms. This allowed us to extend the current information about the time-reversed photoionization process of highly-charged heavy ions to much lower energies than those accessible for neutral heavy elements in the direct reaction channel. The angular differential data obtained prove theoretical predictions that at high-Z higher-order multipole contributions and magnetic corrections play an important role even at energies close to the threshold.

Subshell Differential Cross Sections for Electron Transfer in Collisions of U90+ Ions with Gaseous Targets

Balmer spectra produced in collisions between He-like uranium projectiles and gaseous targets (N2, Ar, Kr, and Xe) were measured at the jet-target area of the ESR storage ring. The X-ray radiation were recorded in coincidence with the down charged projectiles having captured one electron from the target atom. By means of spectra simulation, all the significant Balmer transition lines were identified. Further, the j-differential cross sections for electron capture into projectile subshells were deduced from the well resolved spectra. For the heavier targets, where NRC contributes considerably, systematic deviations between experiment and the continuum distorted wave approximation are observed

1s lamb shift in hydrogenlike uranium measured on cooled, decelerated ion beams.

The Lyman- alpha transitions of hydrogenlike uranium associated with electron capture were measured in collisions of stored bare U (92+) ions with gaseous targets at the storage ring ESR. By applying the deceleration technique, the experiment was performed at slow collision energies in order to reduce the uncertainties associated with Doppler corrections. From the measured centroid energies, a ground state Lamb shift of 468 eV+/-13 eV is deduced which gives the most precise test of quantum electrodynamics for a single electron system in the strong field regime. In particular, the technique applied paves the way towards the 1 eV precision regime.

Schottky mass measurements of stored and cooled neutron-deficient projectile fragments in the element range of [formula omitted

A novel method for direct, high precision mass measurements of relativistic exotic nuclei has been successfully applied in the storage ring ESR at GSI. The nuclei of interest were produced by projectile fragmentation of 930 MeV/u bismuth ions, separated in-flight by the fragment separator FRS, stored and cooled in the ESR. The mass values have been deduced from the revolution frequencies of the coasting cooled ions. We have measured 104 new mass values with a precision of about 100 keV and a resolving power of 3.5×105 for the neutron-deficient isotopes of the elements 57≤Z≤84. This paper presents the experimental method, the mass evaluation and a table of the experimental mass values.

First isochronous mass spectrometry at the experimental storage ring ESR

Short-lived exotic nuclei can be produced and separated with the high-energy secondary nuclear beam facility FRS at GSI. These nuclides can be injected and stored in the storage ring ESR. The lower lifetime limit of the presently existing methods for mass measurements on these nuclides at the ESR is about a few seconds. We have developed and investigated an isochronous operational mode of the ESR, that makes mass measurements of nuclides with lifetimes down to a few μs feasible. It has been commissioned in experiments using long-lived nuclides with known masses. A mass resolving power of about 150 000 has been achieved in a first pilot experiment. A suitable detector system has been implemented and commissioned at the ESR that allows measurements of the revolution period of single particles within a few turns in the ESR.

Low-temperature X-ray detectors for precise Lamb shift measurements on hydrogen-like heavy ions

The precise determination of the Lamb shift in heavy hydrogen-like ions provides a sensitive test of quantum electrodynamics in very strong Coulomb fields, not accessible otherwise. For the investigation of the Lyman- α transitions in 208 Pb 81+ or 238 U 91+ with sufficient accuracy a high resolving calorimetric detector for hard X-rays ( E⩽100 keV) is presently developed. The detector modules consist of arrays of silicon thermistors and of X-ray absorbers made of high Z material to optimize the absorption efficiency. The detectors are housed in a specially designed 3 He/ 4 He dilution refrigerator with a side arm which fits to the geometry of the internal target of the storage ring ESR at GSI Darmstadt. The detector performance presently achieved is already close to fulfill the demands of the Lamb shift experiment. For a prototype detector pixel with a 0.3 mm 2×66 μm Sn absorber an energy resolution of Δ E FWHM=75 eV is obtained for 60 keV X-rays.

Stochastic cooling at the ESR

Stochastic precooling at the ESR storage ring of GSI will be used mainly for experiments with stored radioactive fragment beams. They arrive from the fragment separator with momentum spreads and emittances for which electron cooling is too slow. The installation of components at the ESR is now complete and first commissioning experiments have been performed. Both longitudinal and transverse stochastic cooling have been demonstrated. The paper gives a short account of the system architecture, and of the response of quarter-wave plates and superelectrodes at intermediate energies. The preparation of fragment beams suitable for subsequent electron cooling is discussed for the case that a mixture of different ion species is present in the cooler ring. Results of commissioning and future prospects are presented.

X ray laser spectroscopy at the ESR: a proposed novel tool for the investigation of exotic isotopes

Novel X-ray laser sources driven by short-pulse high power lasers can provide intense beams of radiation at fixed energy. In combination with the Doppler-tuning possible at the ESR storage ring, this can be utilized for precision spectroscopy of highly charged ions. The sensitivity should be sufficient to apply this technique to lithium-like radioactive ions.