Storage Ring ESR Research Articles

At the borderline between atomic and nuclear physics: two-body β-decay of highly charged ions

In this paper, we discuss the radioactive decay of highly charged ions. There are several motivations for performing this kind of research. One of them is that stellar nucleosynthesis proceeds at high temperatures and therefore the involved atoms are highly ionized. Highly charged ions also offer the possibility of addressing the decay of well-defined quantum-mechanical systems such as, for example, one-electron ions, where all the interactions with other electrons are excluded. These studies can be performed solely at ion storage rings or ion traps, where the high atomic charge states can be preserved for extended periods of time. Although we have focused on experiments conducted at the storage ring ESR of GSI, we have tried to describe the general requirements for such experiments.

Differential energy measurement between He- and Li-like uranium intra-shell transitions

We present the first clear identification and highly accurate measurement of the intra-shell transition 1s2p 3P2→1s2s 3S1 of He-like uranium performed via x-ray spectroscopy. The present experiment was conducted at the gas-jet target of the ESR storage ring in GSI (Darmstadt, Germany), where a Bragg spectrometer, with a bent germanium crystal, and a Ge(i) detector were mounted. Using the ESR deceleration capabilities, we performed a differential measurement between the 1s2p 3P2→1s2s 3S1 He-like U transition energy, at 4510 eV, and the 1s22p 2P3/2→1s22s 2S1/2 Li-like U transition energy, at 4460 eV. By a proper choice of the ion velocities, the x-ray energies from the He- and Li-like ions could be measured, in the laboratory frame, at the same photon energy. This allowed for a drastic reduction of experimental systematic uncertainties, principally due to the Doppler effect, and for a comparison with theory without the uncertainties arising from one-photon quantum electrodynamics predictions and nuclear size corrections.

Comparison of measurements and simulations of internal target effects in the ESR storage ring

A proper description of beam-target effects is necessary to predict the operation conditions for high beam luminosity and quality. In order to evaluate the beam dynamics in the storage ring, where cooling in combination with a dense target is applied, a number of numerical models have been developed. Systematic benchmarking experiments were carried out with different gases in the gas-jet target at the existing ESR storage ring at GSI. The influence of the internal target on the beam parameters was investigated. Comparison of the measurements with an analytical formula for the energy loss due to the target as well as with more sophisticated simulations with the BETACOOL code will be given.

Spectral distribution of the 2S →1 S two-photon transition in atoms and few-electron ions

The two-photon decay of the 2S state to the ground state in dressed atoms and one- or two-electron ions has been studied for several decades. Relativistic calculations have shown an Z-dependence of the spectral shape of this two-photon transition in one- or two-electron ions. We have measured the spectral distribution of the 1s2s \(^1\!S_0\) → 1s 2 \(^1\!S_0\) two-photon transition in He-like tin at the ESR storage ring using a new approach for such experiments. In this method, relativistic collisions of initially Li-like projectiles with a gaseous target were used to populate exclusively the first excited state, 1s2s, of He-like tin, which provided a clean two-photon spectrum. The measured two-photon spectral distribution was compared with fully relativistic calculations. The obtained results show very good agreement with the calculations for He-like tin.

First feasibility experiment for the EXL project with prototype detectors at the ESR storage ring

The investigation of light-ion reactions with exotic beams in inverse kinematics gives access to a comprehensive range of nuclear structure information in the region far off stability. The future FAIR facility at GSI will provide new opportunities to extend and advance these investigations. The present paper will focus on the results of the first feasibility experiment for the EXL project in which we used a stored 136Xe beam with E = 350 MeV/nucleon interacting with an internal hydrogen gas-jet target at the ESR storage ring of GSI. In this experiment we made use of at least one element of every detector part of the future experimental setup. Selected results from this measurement will be presented.

Time-modulation of entangled two-body weak decays with massive neutrinos

In recent experiments at the GSI, Darmstadt, time-modulated orbital Electron Capture (EC) decays of H-like 140Pr58+, 142Pm60+ and 122I52+ ions with one electron in the K-shell, coasting in the ESR storage ring with velocity β = 0.71, were observed. The EC-branches observed using time-resolved Schottky Mass Spectroscopy show exponential decay curves time-modulated with periods T = 7.06(8) s, T = 7.10(22) s, and T = 6.16(3) s and amplitudes a = 0.18(3), a = 0.23(4), and a = 0.16(2) for 140Pr, 142Pm, and 122I decays, respectively. The β+ branch of 142Pm shows no modulation with a < 0.03. As origin of the modulation, we propose quantum beats produced by massive neutrino mass-eigen states emitted in the two body weak decay. From the modulation frequency a value for the difference of the quadratic mass values m22 − m21 = 2.22(3) × 10−4 eV2 is deduced, which is 2.9 times larger than the value derived by the KamLAND antineutrino oscillation experiment. The origin of the small modulation amplitudes is discussed as the result of partial restoration of the interference terms which are expected to cancel for the usual assumed unitarity of the neutrino flavour mixing matrix.

Resonant recombination at ion storage rings: a conceptual alternative for isotope shift and hyperfine studies

Sharp resonant structures in the cross section of the atomic electron-ion collision process of dielectronic recombination are exploited to study isotope shifts and hyperfine interaction of heavy highly-charged ions. This novel approach provides a conceptual alternative to existing methods. In this contribution, we present a series of measurements, which we performed at the heavy ion storage ring ESR of the GSI Helmholtzzentrum fur Schwerionenforschung, Darmstadt, Germany. In a first experiment the stable isotopes A = 142 and A = 150 of three-electron neodymium ANd57 + were studied. Isotope shifts of dielectronic resonances associated with 2s − 2pj (j = 1/2, 3/2) transitions were extracted from the measured data. The evaluation of the energy shift was performed within a full QED framework and yielded a change in the mean-square charge radius of \(^{142,150}\delta \langle r^2 \rangle = -1.36(1)(3)\) fm2. At GSI, in addition to the investigation of stable isotopes unstable species can be artificially synthesized and studied. Radioisotopes produced in-flight from fragmentation of a 238U primary beam were injected into the ESR and were subsequently separated by their mass-to-charge ratio. This enabled us to perform first DR experiments with the exotic nuclei 237U89 + (Z = 92 )and 234Pa88 + (Z = 91).

Progress in the applicability of plasma X-ray lasers

Proposed as satellite-based weapons during the 1980s, X-ray lasing was for a long time only achieved with enormous amounts of pump energy in either nuclear explosions or at kilojoule-class laser installations. During the last few years a tremendous development was achieved, most visible in the realisation of the FEL lasers at DESY and SLAC. As important for a wider applicability is the enormous reduction in pump energy for laser pumped plasma X-ray lasers, which now brings such devices into the range of applications for diagnostics and spectroscopy even in smaller laboratories. Main developments were the transient excitation scheme and the optimized pumping concepts. This paper concentrates on developments at the GSI Helmholtzcenter at Darmstadt aiming towards reliable X-ray laser sources in the range from 50 to several 100 eV. The main driving forces for the laser development at GSI are the possible application for the spectroscopy of Li-like ions in the storage ring ESR and the future storage ring NESR at FAIR, and the interest in novel plasma diagnostics.

Spectral Shape of the Two-Photon Decay of the2 S01State in He-Like Tin

The spectral distribution of the 1s2s {1}S{0}-->1s{2} 1S0 two-photon decay of He-like tin was measured using a novel approach at the gas-jet target of the ESR storage ring. Relativistic collisions of Li-like projectiles with low-density gaseous matter have been exploited to selectively populate the desired 1s2s state. Compared to conventional techniques, this approach results in a substantial gain in statistical and systematic accuracy, which allowed us to achieve for the first time a sensitivity to relativistic effects on the two-photon decay spectral shape as well as to discriminate the measured spectrum for Sn from theoretical shapes for different elements along the He-isoelectronic sequence.

Time-modulation of entangled two-body weak decays with massive neutrinos

In recent experiments at the GSI, Darmstadt, time-modulated orbital Electron Capture (EC) decays of H-like 140Pr 58+, 142Pm 60+, and 122I 52+ ions with one electron in the K-shell, coasting in the ESR storage ring with velocity β = 0.71 , were observed. The EC-branches show exponential decay curves time-modulated with period T = 7.06 ( 8 ) s and amplitude a = 0.18 ( 3 ) for 140Pr decays, T = 7.10 ( 22 ) s and a = 0.23 ( 4 ) for 142Pm decays, and T = 6.04 ( 6 ) s and a = 0.19 ( 3 ) for 122I (preliminary) decays in the laboratory frame. The simultaneously measured β + branch of 142Pm shows no modulation with a < 0.03 . We discuss here as origin of the modulation quantum beats produced by the superposition of massive neutrino, mass eigenstates emitted in the entangled two body weak decay. From the modulation frequency a value for the difference of the quadratic mass values m 2 2 − m 1 2 = 2.22 ( 3 ) × 1 0 − 4 eV 2 is deduced, which is 2.9 times larger than the value derived by the KamLAND antineutrino oscillation experiment. The origin of the small modulation amplitudes is discussed as the result of a partial restoration of the interference terms which are expected to cancel for the usual assumed unitarity of the neutrino flavor mixing matrix.

Isotope shifts in dielectronic recombination: From stable to in-flight-produced nuclei

The study of isotope shifts and hyperfine effects in dielectronic recombination (DR) resonance spectra strikes a conceptually new path for investigations of nuclear properties such as charge radius, spin, magnetic moment of nuclei or lifetimes of long-lived excited nuclear states. A series of DR experiments with heavy three-electron ions (Li-like) was performed at the heavy-ion storage ring ESR of the GSI Helmholtzzentrum für Schwerionenforschung, Darmstadt, Germany. In a pilot experiment the two stable isotopes A=142 and A=150 of neodymium ANd57+ were investigated. From the displacement of DR resonances the isotope shifts δE142,150(2s – 2Pl/2) = 40.2(3)(6) meV and δE142,150(2s – 2p3/2) = 42.3(12)(20) meV for 2s – 2Pj transitions of the Li-like ions have been obtained. An evaluation within a full QED framework yielded a change in the mean-square charge radius of 142,150δ(r2) = −1.36(1)(3) fm2. At GSI, in addition to stable isotopes, in-flight synthesized radioisotopes can be studied as well. The production of radioisotopes of interest, the subsequent separation in the storage ring ESR and first DR experiments with the exotic nuclei 237U89+ and 234Pa88+ (Z=91) are presented. The paper is concluded with a brief outlook at future DR experiments with heavy radioisotopes at the ESR.

Observation of enhancement in K-shell radiative recombination of U92+ ions with cooling electrons

The observation of the enhancement of radiative recombination (RR) into the K-shell for U92+ ions interacting with cooling electrons in x-ray RR experiment performed at the ESR storage ring is reported. From the measured intensities of K-RR x-rays for the electron relative energies in the range 0-1000 meV a clear indication of the recombination enhancement, with respect of the fully relativistic RR calculations, was found for the K-shell for the zero relative electron energy. This results add new aspects to existing interpretations of the enhancement effect emphasizing a role of recombination into high Rydberg states.

HITRAP – a facility for experiments on heavy highly charged ions and on antiprotons

HITRAP is a facility for very slow highly-charged heavy ions at GSI. HITRAP uses the GSI relativistic ion beams, the Experimental Storage Ring ESR for electron cooling and deceleration to 4 MeV/u, and consists of a combination of an interdigital H-mode (IH) structure with a radiofrequency quadrupole structure for further deceleration to 6 keV/u, and a Penning trap for accumulation and cooling to low temperatures. Finally, ion beams with low emittance will be delivered to a large variety of atomic and nuclear physics experiments. Presently, HITRAP is in the commissioning phase. The deceleration of heavy-ion beam from the ESR storage ring to an energy of 500 keV/u with the IH structure has been demonstrated and studied in detail. The commissioning of the RFQ structure and the cooler trap is ongoing.

Time-Modulation of Orbital Electron Capture Decays by Mixing of Massive Neutrinos

We report on the observation of time-modulated orbital EC decays of H-like 140Pr 58+, 142Pm 60+, and 122I 52+ (preliminary) ions with only one electron in the K-shell coasting in the ESR storage ring of GSI with a velocity β = 0.71 and a spread Δ v / v ∼ 5 × 10 − 7 . The decays were observed with time resolved single ion Schottky Mass Spectroscopy by observation of the time of change of the precisely measured revolution frequency of the mother into the daughter ion which is proportional to the mass change or Q-value of the decay. We observed in the EC-branches exponential decay curves time-modulated with periods T = 7.06 ( 8 ) s and amplitude a = 0.18 ( 3 ) for 140Pr decays, T = 7.10 ( 22 ) s and a = 0.23 ( 4 ) for 142Pm decays, and T = 6.04 ( 6 ) s and a = 0.19 ( 3 ) for 122I decays (preliminary) in the laboratory frame. The simultaneously measured β + branch of 142Pm shows no modulation with a < 0.03 . An explanation by mixing of massive electron neutrinos has been suggested, according to which the observed modulation frequency yields a value for the quadratic mass difference: m 2 2 − m 1 2 = 2.22 ( 3 ) × 10 − 4 eV 2 . This value is 2.9 times larger than the value derived by the KamLAND antineutrino oscillation experiment.

Spin and rotation

The theoretical aspects of spin-rotation coupling are presented. The approach is based on the general covariance principle. It is shown that the gyrogravitational ratio of the bare spin-1/2 and the spin-1 particles is equal to unity. That is why spin couples with rotation as an ordinary angular momentum. This result is the rigorous substantiation of the cranking model in which spin-rotation coupling is introduced by means of a unitary rotation. To observe the phenomenon, the experiment with hydrogen-like ions in a storage ring is suggested. The degenerated magnetic sub-states of such ion are split if it circulates in a storage ring along a helical trajectory. It is found that the splitting of the 12S1/2, F = 1/2 hyperfine state of the 140Pr58+ and 142Pm60+ ions in the storage ring ESR in Darmstadt is about 4.5 MHz. We argue that such splitting can be experimentally determined by means of the laser-induced quantum beat spectroscopy or the ionic interferometry.

Two-photon decay in highly charged heavy ions: Spectral shape of the 2E1 (2 1S0 → 1 1S0) in He-like tin

We have measured the two-photon 2E1 transition from the singlet 1s2s 1S0 to the 1s21S0 ground state of helium-like tin. The experiment was performed by using initially Li-like tin ions in collision with a nitrogen gas target at the ESR storage ring at the energy of 300 MeV/u. The two-photon emission arises from the decay of the exclusively formed 1s2s states following the ionization of one K-shell electron from the Li-like ions. The obtained spectral shape of the 1s2s1S0 → 1s21S0 (2E1) transition was compared with the one calculated within a relativistic approach. The spectral shape is displayed and its sensitivity with regard to details in the theoretical description of the 2E1 decay is discussed.

Doppler-tuned Bragg spectroscopy of excited levels in He-like uranium: A discussion of the uncertainty contributions

We present the uncertainty discussion of a recent experiment performed at the GSI storage ring ESR for the accurate energy measurement of the He-like uranium 1s2p3P2→ 1s2s3S1 intra-shell transition. For this propose we used a Johann-type Bragg spectrometer that enables to obtain a relative energy measurement between the He-like uranium transition, about 4.51 keV, and a calibration x-ray source. As reference, we used the Kα fluorescence lines of zinc and the Li-like uranium 1s22p2P3/2 → 1 s22s 2S1/2 intra-shell transition from fast ions stored in the ESR. A comparison of the two different references, i.e., stationary and moving x-ray source, and a discussion of the experimental uncertainties is presented.

Mass and lifetime measurements of exotic nuclei in storage rings

Mass and lifetime measurements lead to the discovery and understanding of basic properties of matter. The isotopic nature of the chemical elements, nuclear binding, and the location and strength of nuclear shells are the most outstanding examples leading to the development of the first nuclear models. More recent are the discoveries of new structures of nuclides far from the valley of stability. A new generation of direct mass measurements which allows the exploration of extended areas of the nuclear mass surface with high accuracy has been opened up with the combination of the Experimental Storage Ring ESR and the FRragment Separator FRS at GSI Darmstadt. In-flight separated nuclei are stored in the ring. Their masses are directly determined from the revolution frequency. Dependent on the half-life two complementary methods are applied. Schottky Mass Spectrometry SMS relies on the measurement of the revolution frequency of electron cooled stored ions. The cooling time determines the lower half-life limit to the order of seconds. For Isochronous Mass Spectrometry IMS the ring is operated in an isochronous ion-optical mode. The revolution frequency of the individual ions coasting in the ring is measured using a time-of-flight method. Nuclides with lifetimes down to microseconds become accessible. With SMS masses of several hundreds nuclides have been measured simultaneously with an accuracy in the 2 x 10(-7)-range. This high accuracy and the ability to study large areas of the mass surface are ideal tools to discover new nuclear structure properties and to guide improvements for theoretical mass models. In addition, nuclear half-lives of stored bare and highly charged ions have been measured. This new experimental development is a significant progress since nuclear decay characteristics are mostly known for neutral atoms. For bare and highly charged ions new nuclear decay modes become possible, such as bound-state beta decay. Dramatic changes in the nuclear lifetime have been observed in highly charged ions compared to neutral atoms due to blocking of nuclear decay channels caused by the modified atomic interaction. High ionization degrees prevail in hot stellar matter and thus these experiments have great relevance for the understanding of the synthesis of elements in the universe and astrophysical scenarios in general.

Storage rings for radioactive ion beams

Storage rings for radioactive heavy ions can be applied for a wide range of experiments in atomic and nuclear physics. The rare isotope beams are produced in flight via fragmentation or fission of high-intensity primary ions and they circulate in the storage ring at moderately relativistic energies (typically between 0.1GeV/u up to 1GeV/u). Due to their production mechanism they are usually highly charged or even fully stripped. The circulating radioactive heavy ion beams can be used to measure nuclear properties such as masses and decay times, which, in turn, can depend strongly on the ionic charge state. The storage rings must have large acceptances and dynamic apertures. The subsequent application of stochastic precooling of the secondary ions which are injected with large transverse and longitudinal emittances, and electron cooling to reach very high phase space densities has turned out to be a helpful tool for experiments with short-lived ions having lifetimes down to a few seconds. Some of these experiments have already been performed at the experimental storage ring ESR at GSI. The storage ring complex of the FAIR project is intended to enhance significantly the range of experimental possibilities. It is planned to extend the scope of experimental possibilities to collisions with electron or antiproton beams.

First Experiments Aiming for Precise Lamb Shift Measurements on Hydrogen-Like Heavy Ions with Low Temperature Calorimeters

The precise determination of the Lamb shift in hydrogen-like heavy ions provides a sensitive test of quantum electrodynamics in very strong Coulomb fields, not accessible otherwise. To increase the accuracy of the Lamb shift measurement on stored heavy ions at the ESR storage ring at GSI, a calorimetric low temperature detector for hard X-rays was developed. A prototype array consisting of 8 pixels with silicon thermistors and Sn or Pb absorbers was applied in first experiments with stored 238U91+ and 207Pb81+ ions interacting with an internal gas-jet target. In both experiments the Lyman-α lines were, for the first time by using calorimetric low temperature detectors, clearly identified in clean, almost background-free spectra. A total detection efficiency of 1×10−7 for the Lyman-α lines was reached and an energy resolution of 115 eV was obtained under the present experimental conditions, which corresponds to a considerable improvement of about half an order of magnitude if compared to conventional Ge semiconductor detectors. Finally preliminary results of the data analysis and future perspectives are discussed.