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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 K$\alpha$ 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.

$^{229}$Th with a low-lying nuclear isomeric state is an essential candidate for a nuclear clock as well as many other applications. Laser excitation of the isomeric state has been a long-standing goal. With relativistic $^{229}$Th ions in storage rings, high-power lasers with wavelengths in the visible range or longer can be used to achieve high excitation rates of $^{229}$Th isomers. This can be realized through direct resonant excitation, or excitation via an intermediate nuclear or electronic state, facilitated by the tunability of both the laser-beam and ion-bunch parameters. Unique opportunities are offered by highly charged $^{229}$Th ions due to the nuclear-state mixing. The significantly reduced isomeric-state lifetime corresponds to a much higher excitation rate for direct resonant excitation. Importantly, we propose electric dipole transitions changing both the electronic and nuclear states that are opened by the nuclear hyperfine mixing. We suggest using them for efficient isomer excitation in Li-like $^{229}$Th ions, via stimulated Raman adiabatic passage or single-laser excitation. We also propose schemes for probing the isomers, utilizing nuclear radiative decay or laser spectroscopy on electronic transitions, through which the isomeric-state energy can be determined with an orders-of-magnitude higher precision than the current value. The schemes proposed here for $^{229}$Th could also be adapted to low-energy nuclear states in other nuclei, such as $^{229}$Pa., Comment: 20+7 pages, 12 figures

The Gamma Factory (GF) is an ambitious proposal, currently explored within the CERN Physics Beyond Colliders program, for a source of photons with energies up to $\approx 400\,$MeV and photon fluxes (up to $\approx 10^{17}$ photons per second) exceeding those of the currently available gamma sources by orders of magnitude. The high-energy (secondary) photons are produced via resonant scattering of the primary laser photons by highly relativistic partially-stripped ions circulating in the accelerator. The secondary photons are emitted in a narrow cone and the energy of the beam can be monochromatized, eventually down to the $\approx1$ ppm level, via collimation, at the expense of the photon flux. This paper surveys the new opportunities that may be afforded by the GF in nuclear physics and related fields., Comment: 69 pages, 13 tables, 33 figures

The atomic physic collaboration SPARC is a part of the APPA pillar at the future Facility for Antiproton and Ion Research. It aims for atomic-physics research across virtually the full range of atomic matter. A research area of the atomic physics experiments is the study of the collision dynamics in strong electro-magnetic fields as well as the fundamental interactions between electrons and heavy nuclei at the HESR. Here we give a short overview about the central instruments for SPARC experiments at this storage ring., Comment: 4 pages, 1 figures

Storage rings have been employed over three decades in various kinds of nuclear and atomic physics experiments with highly charged ions. Storage ring operation and precision physics experiments benefit from the availability of beam cooling which is common to nearly all facilities. The basic aspects of the storage ring components and the operation of the ring in various ion-optical modes as well as the achievable beam conditions are described. Ion storage rings offer unparalleled capabilities for high precision experiments with stable and radioactive beams. The versatile techniques and methods for beam manipulations allow for preparing beams of highest quality at any energy of interest. The rings are therefore part of the experiment . Recent experiments conducted in a wide energy range and with various experimental installations are discussed. An overview of active and planned facilities, new experimental set-ups and proposed physics experiments completes this review., Comment: Progress in Particle and Nuclear Physics (in press)

The Isochronous Mass Spectrometry (IMS) is a powerful tool for mass measurements of exotic nuclei with half-lives as short as several tens of micro-seconds in storage rings. In order to improve the mass resolving power while preserving the acceptance of the storage ring, the IMS with two Time-Of-Flight (TOF) detectors has been implemented at the storage ring CSRe in Lanzhou, China. Additional velocity information beside the revolution time in the ring can be obtained for each of the stored ions by using the double TOF detector system. In this paper, we introduced a new method of using a 658 nm laser range finder and a short-pulsed ultra-violet laser to directly measure the distance and time delay difference between the two TOF detectors which were installed inside the $10^{-11}$ mbar vacuum chambers. The results showed that the distance between the two ultra-thin carbon foils of the two TOF detectors was ranging from 18032.5 mm to 18035.0 mm over a measurable area of 20$\times$20 mm$^2$. Given the measured distance, the time delay difference which comes with signal cable length difference between the two TOF detectors was measured to be $\Delta t_{delay1-2}=99$(26) ps. The new method has enabled us to use the speed of light in vacuum to calibrate the velocity of stored ions in the ring. The velocity resolution of the current double TOF detector system at CSRe was deduced to be $\sigma(v)/v=4.4\times 10^{-4}$ for laser light, mainly limited by the time resolution of the TOF detectors.

The accuracy of description of measured nuclear masses by presently used nuclear-mass models is studied. Twelve models of various kinds are considered, eleven of the global character and one local model specially adapted to description of heavy nuclei. To decrease the number of nuclei over which the accuracy is averaged, the global region (Z,N >= 8) is divided into four subregions, in which the accuracy is studied separately. Still, to reach the best precision, the accuracy is finally investigated without any averaging, for each nucleus separately. The results are presented in a form of colored maps, large enough to be easily and accurately read. Besides the accuracy of the studied models, also their predictive power is considered. It is found that the accuracy of description of mass strongly depends on a nuclear-mass model and on the region of nuclei to which the model is applied. The best accuracy is obtained by the recent two Chinese models WS3+ and WS4+. Generally, no clear, strong correlation between the accuracy of description of already known masses by a given model and its predictive power for new masses is observed. Still, such correlation is found for separate models and in separate regions of nuclei. More often for the macroscopic-microscopic models than for the other approaches., Comment: 70 pages

The LIBELLE experiment performed at the experimental storage ring (ESR) at the GSI Helmholtz Center for Heavy Ion Research in Darmstadt, Germany, has successfully determined the ground state hyperfine (HFS) splittings in hydrogen-like ($^{209}\rm{Bi}^{82+}$) and lithium-like ($^{209}\rm{Bi}^{80+}$) bismuth. The study of HFS transitions in highly charged ions enables precision tests of QED in extreme electric and magnetic fields otherwise not attainable in laboratory experiments. Besides the transition wavelengths the time resolved detection of fluorescence photons following the excitation of the ions by a pulsed laser system also allows to extract lifetimes of the upper HFS levels and g-factors of the bound 1s and 2s electrons for both charge states. While the lifetime of the upper HFS state in $^{209}\rm{Bi}^{82+}$ has already been measured in earlier experiments, an experimental value for lifetime of this state in $^{209}\rm{Bi}^{80+}$ is reported for the first time in this work., Comment: 14 pages, 6 figures

The quest for the origin of the chemical elements, which we find in our body, in our planet (Earth), in our star (Sun), or in our galaxy (Milky Way) could only be resolved with a thorough understanding of the nuclear physics properties of stable and unstable atomic nuclei. While the elements until iron are either created during the big bang or during fusion reactions in stars, most of the elements heavier than iron are produced via neutron-induced reactions. Therefore, neutron capture cross sections of stable and unstable isotopes are important. So far, time-of-flight or activation methods have been applied very successfully, but these methods reach their limits once the isotopes with half-lives shorter than a few months are of interest. A combination of a radioactive beam facility, an ion storage ring and a high flux reactor or a spallation source would allow a direct measurement of neutron-induced reactions over a wide energy range of isotopes with half-lives down to minutes. The idea is to measure neutron-induced reactions on radioactive ions in inverse kinematics. This means, the radioactive ions will pass through a neutron target. In order to efficiently use the rare nuclides as well as to enhance the luminosity, the exotic nuclides can be stored in an ion storage ring. The neutron target can be the core of a research reactor, where one of the central fuel elements is replaced by the evacuated beam pipe of the storage ring. Alternatively, a large moderator surrounding a spallation source can be intersected by the beam pipe of an ion storage ring. Using particle detectors and Schottky spectroscopy, most of the important neutron-induced reactions, such as (n,$\gamma$), (n,p), (n,$\alpha$), (n,2n), or (n,f), could be investigated., Comment: 14 pages, 11 figures

We discuss the possibility to build a neutron target for nuclear reaction studies in inverse kinematics utilizing a storage ring and radioactive ion beams. The proposed neutron target is a specially designed spallation target surrounded by a large moderator of heavy water (D$_2$O). We present the resulting neutron spectra and their properties as a target. We discuss possible realizations at different experimental facilities., Comment: 10 pages, 12 figures

The concept of isochronous mass spectrometry (IMS) applying two time-of-flight (TOF) detectors originated many years ago at GSI. However, the corresponding method for data analysis has never been discussed in detail. Recently, two TOF detectors have been installed at CSRe and the new working mode of the ring is under test. In this paper, a data analysis method for this mode is introduced and tested with a series of simulations. The results show that the new IMS method can significantly improve mass resolving power via the additional velocity information of stored ions. This improvement is especially important for nuclides with Lorentz factor $\gamma$-value far away from the transition point $\gamma _t$ of the storage ring CSRe., Comment: published in Chinese Physics C Vol. 39, No. 10 (2015) 106201

In this paper, we present direct mass measurements of neutron-rich $^{86}$Kr projectile fragments conducted at the HIRFL-CSR facility in Lanzhou by employing the Isochronous Mass Spectrometry (IMS) method. The new mass excesses of $^{52-54}$Sc nuclides are determined to be -40492(82), -38928(114), -34654(540) keV, which show a significant increase of binding energy compared to the reported ones in the Atomic Mass Evaluation 2012 (AME12). In particular, $^{53}$Sc and $^{54}$Sc are more bound by 0.8 MeV and 1.0 MeV, respectively. The behavior of the two neutron separation energy with neutron numbers indicates a strong sub-shell closure at neutron number $N$ = 32 in Sc isotopes., Comment: publised in Chinese Physics C Vol. 39, No. 10 (2015) 104001

Neutron capture cross sections of unstable isotopes are important for neutron-induced nucleosynthesis as well as for technological applications. A combination of a radioactive beam facility, an ion storage ring and a high flux reactor would allow a direct measurement of neutron induced reactions over a wide energy range on isotopes with half lives down to minutes. The idea is to measure neutron-induced reactions on radioactive ions in inverse kinematics. This means, the radioactive ions will pass through a neutron target. In order to efficiently use the rare nuclides as well as to enhance the luminosity, the exotic nuclides can be stored in an ion storage ring. The neutron target can be the core of a research reactor, where one of the central fuel elements is replaced by the evacuated beam pipe of the storage ring. Using particle detectors and Schottky spectroscopy, most of the important neutron-induced reactions, such as (n,$\gamma$), (n,p), (n,$\alpha$), (n,2n), or (n,f), could be investigated., Comment: 5 pages, 7 figures, Invited Talk given at the Fifteenth International Symposium on Capture Gamma-Ray Spectroscopy and Related Topics (CGS15), Dresden, Germany, 2014

This work presents a direct measurement of the $^{96}$Ru($p, \gamma$)$^{97}$Rh cross section via a novel technique using a storage ring, which opens opportunities for reaction measurements on unstable nuclei. A proof-of-principle experiment was performed at the storage ring ESR at GSI in Darmstadt, where circulating $^{96}$Ru ions interacted repeatedly with a hydrogen target. The $^{96}$Ru($p, \gamma$)$^{97}$Rh cross section between 9 and 11 MeV has been determined using two independent normalization methods. As key ingredients in Hauser-Feshbach calculations, the $\gamma$-ray strength function as well as the level density model can be pinned down with the measured ($p, \gamma$) cross section. Furthermore, the proton optical potential can be optimized after the uncertainties from the $\gamma$-ray strength function and the level density have been removed. As a result, a constrained $^{96}$Ru($p, \gamma$)$^{97}$Rh reaction rate over a wide temperature range is recommended for $p$-process network calculations., Comment: 10 pages, 7 figs, Accepted for publication at PRC

We performed a laser spectroscopic determination of the $2s$ hyperfine splitting (HFS) of Li-like $^{209}\text{Bi}^{80+}$ and repeated the measurement of the $1s$ HFS of H-like $^{209}\text{Bi}^{82+}$. Both ion species were subsequently stored in the Experimental Storage Ring at the GSI Helmholtzzentrum f\"ur Schwerionenforschung Darmstadt and cooled with an electron cooler at a velocity of $\approx 0.71\,c$. Pulsed laser excitation of the $M1$ hyperfine-transition was performed in anticollinear and collinear geometry for $\text{Bi}^{82+}$ and $\text{Bi}^{80+}$, respectively, and observed by fluorescence detection. We obtain $\Delta E^{(1s)}= 5086.3(11)\,\textrm{meV}$ for $\text{Bi}^{82+}$, different from the literature value, and $\Delta E^{(2s)}= 797.50(18)\,\textrm{meV}$ for $\text{Bi}^{80+}$. These values provide experimental evidence that a specific difference between the two splitting energies can be used to test QED calculations in the strongest static magnetic fields available in the laboratory independent of nuclear structure effects. The experimental result is in excellent agreement with the theoretical prediction and confirms the sum of the Dirac term and the relativistic interelectronic-interaction correction at a level of 0.5% confirming the importance of accounting for the Breit interaction., Comment: 5 pages, 2 figures

SPARC collaboration at FAIR pursues the worldwide unique research program by utilizing storage ring and trapping facilities for highly-charged heavy ions. The main focus is laid on the exploration of the physics at strong, ultra-short electromagnetic fields including the fundamental interactions between electrons and heavy nuclei as well as on the experiments at the border between nuclear and atomic physics. Very recently SPARC worked out a realization scheme for experiments with highly-charged heavy-ions at relativistic energies in the High-Energy Storage Ring HESR and at very low-energies at the CRYRING coupled to the present ESR. Both facilities provide unprecedented physics opportunities already at the very early stage of FAIR operation. The installation of CRYRING, dedicated Low-energy Storage Ring (LSR) for FLAIR, may even enable a much earlier realisation of the physics program of FLAIR with slow anti-protons., Comment: IX International Workshop on "APPLICATION OF LASERS AND STORAGE DEVICES IN ATOMIC NUCLEI RESEARCH", Recent Achievements and Future Prospects, May 13 - 16, 2013, Pozna\'n, Poland

Neutron capture cross sections of unstable isotopes are important for neutron induced nucleosynthesis as well as for technological applications. A combination of a radioactive beam facility, an ion storage ring and a high flux reactor would allow a direct measurement of neutron induced reactions over a wide energy range on isotopes with half lives down to minutes., Comment: Accepted for publication in Physical Review ST AB (2014)

The new regime of resonant nuclear photoexcitation rendered possible by x-ray free electron laser beams interacting with solid state targets is investigated theoretically. Our results unexpectedly show that secondary processes coupling nuclei to the atomic shell in the created cold high-density plasma can dominate direct photoexcitation. As an example we discuss the case of $^{93m}$Mo isomer depletion for which nuclear excitation by electron capture as secondary process is shown to be orders of magnitude more efficient than the direct laser-nucleus interaction. General arguments revisiting the role of the x-ray free electron laser in nuclear experiments involving solid-state targets are further deduced., Comment: 6 pages, 2 figures; v2 updated to published version; results unchanged

The linear polarization of the characteristic photon emission from few-electron ions is studied for its sensitivity with regard to the nuclear spin and magnetic moment of the ions. Special attention is paid, in particular, to the K$\alpha_1$ ($1s 2p_{3/2} ^{1,3}P_{1,2} \to 1s^2 ^1S_0$) decay of selected helium-like ions following the radiative electron capture into initially hydrogen-like species. Based on the density matrix theory, a unified description is developed that includes both, the many-electron and hyperfine interactions as well as the multipole-mixing effects arising from the expansion of the radiation field. It is shown that the polarization of the K$\alpha_1$ line can be significantly affected by the mutipole mixing between the leading $M2$ and hyperfine-induced $E1$ components of $1s2p ^3P_2, F_i \to 1s^2 ^1S_0, F_f$ transitions. This $E1$-$M2$ mixing strongly depends on the nuclear properties of the considered isotopes and can be addressed experimentally at existing heavy-ion storage rings.

Modulated in time orbital electron capture (EC) decays have been observed recently in stored H-like $^{140}$Pr$^{58+}$ and $^{142}$Pm$^{60+}$ ions. Although, the experimental results are extensively discussed in literature, a firm interpretation has still to be established. Periodic transitions between the hyperfine states could possible lead to the observed effect. Both selected nuclides decay to stable daughter nuclei via allowed Gamow-Teller transitions. Due to the conservation of total angular momentum, the allowed EC decay can only proceed from the hyperfine ground state of parent ions. In this work we argue that periodic transitions to the excited hyperfine state (sterile) in respect to the allowed EC decay ground state cannot explain the observed decay pattern.