An electrostatic cryogenic storage ring, CSR, for beams of anions and cations with up to 300 keV kinetic energy per unit charge has been designed, constructed, and put into operation. With a circumference of 35 m, the ion-beam vacuum chambers and all beam optics are in a cryostat and cooled by a closed-cycle liquid helium system. At temperatures as low as (5.5 ± 1) K inside the ring, storage time constants of several minutes up to almost an hour were observed for atomic and molecular, anion and cation beams at an energy of 60 keV. The ion-beam intensity, energy-dependent closed-orbit shifts (dispersion), and the focusing properties of the machine were studied by a system of capacitive pickups. The Schottky-noise spectrum of the stored ions revealed a broadening of the momentum distribution on a time scale of 1000 s. Photodetachment of stored anions was used in the beam lifetime measurements. The detachment rate by anion collisions with residual-gas molecules was found to be extremely low. A residual-gas density below 140 cm(-3) is derived, equivalent to a room-temperature pressure below 10(-14) mbar. Fast atomic, molecular, and cluster ion beams stored for long periods of time in a cryogenic environment will allow experiments on collision- and radiation-induced fragmentation processes of ions in known internal quantum states with merged and crossed photon and particle beams.

We probe the N = 82 nuclear shell closure by mass measurements of neutron-rich cadmium isotopes with the ISOLTRAP spectrometer at ISOLDE-CERN. The new mass of 132 Cd offers the first value of the N = 82, two-neutron shell gap below Z = 50 and confirms the phenomenon of mutually enhanced magicity at 132 Sn. Using the recently implemented phase-imaging ion-cyclotronresonance method, the ordering of the low-lying isomers in 129 Cd and their energies are determined. The new experimental findings are used to test large-scale shell-model, mean-field and beyondmean-field calculations, as well as the ab initio valence-space in-medium similarity renormalization group.The so-called magic numbers of protons and neutrons are associated with large energy gaps in the effective single-particle spectrum of the nuclear mean field [1], revealing shell closures. As such, they are intimately connected to the nuclear interaction and represent essential benchmarks for nuclear models.Experiments with light radioactive beams have shown that shell closures at N = 8, 20 and 28 are substantially weakened when the number of protons in the nuclear system is reduced (see [2, 3] for a review). New, but weaker shell closures have also been found, e.g., N = 32 and 34 [4][5][6][7]. In the shell model, this evolution results from the interplay between the monopole part of the valencespace nucleon-nucleon interaction that determines the single-particle spectrum and multipole forces that induce correlations [8]. Starting from realistic nuclear forces, the study of closed-shell nuclei provides benchmarks for microscopic calculations of valence-space Hamiltonians, with their many-body contributions [9][10][11][12][13]. Despite extensive work, significantly less is known for heavier nuclei, in particular for the magic N = 82.The doubly magic nature of 132 Sn (with 50 protons and 82 neutrons) was reconfirmed recently [14,15]. But below Z = 50 the orbitals occupied by the Fermi-level protons change, as does the proton-neutron interaction, which drives shell evolution. This means that without data for nuclides with Z < 50 and N ≈ 82, any predictions for the N = 82 shell gap are rather uncertain. While decayspectroscopy [16][17][18], laser-spectroscopy [19] and massspectrometry [20,21] studies have been performed for the neutron-rich cadmium isotopes, the energies of the low-lying isomers in 129 Cd and the N = 82 two-neutron shell gap remain unknown.The A ≈ 130 r-process abundance peak has long been considered an indication of a persistent N = 82 shell gap in various models. However, recent studies of r-process nucleosynthesis have underlined the importance of fission recycling in certain scenarios, in which the A = 130 abundance peak is primarily determined by the fissionfragment distribution of r-process actinides [22,23].In this work, we present the first direct determination of the N = 82 shell gap for Z < 50 with mass measurements of exotic cadmium isotopes and isomers between 124 Cd and 132 Cd. We exploit all mass-measurement techniques of the ISOLTR...

We have studied the photodissociation of CH^{+} in the Cryogenic Storage Ring at ambient temperatures below 10 K. Owing to the extremely high vacuum of the cryogenic environment, we were able to store CH^{+} beams with a kinetic energy of ∼60  keV for several minutes. Using a pulsed laser, we observed Feshbach-type near-threshold photodissociation resonances for the rotational levels J=0-2 of CH^{+}, exclusively. In comparison to updated, state-of-the-art calculations, we find excellent agreement in the relative intensities of the resonances for a given J, and we can extract time-dependent level populations. Thus, we can monitor the spontaneous relaxation of CH^{+} to its lowest rotational states and demonstrate the preparation of an internally cold beam of molecular ions.

The strength of the N = 28 magic number in neutron-rich argon isotopes is examined through high-precision mass measurements of 46-48 Ar, performed with the ISOLTRAP mass spectrometer at ISOLDE/CERN. The new mass values are up to 90 times more precise than previous measurements. While they suggest the persistence of the N = 28 shell closure for argon, we show that this conclusion has to be nuanced in light of the wealth of spectroscopic data and theoretical investigations performed with the SDPF-U phenomenological shell model interaction. Our results are also compared with ab initio calculations using the valence space in-medium similarity renormalization group and the self-consistent Green's function approaches. Both calculations provide a very good account of mass systematics at and around Z = 18 and, generally, a consistent description of the physics in this region. This combined analysis indicates that 46 Ar is the transition between the closed-shell 48 Ca and collective 44 S.

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A high-precision measurement of the 131 Cs→ 131 Xe ground-to-groundstate electron-capture Q EC -value was performed using the ISOLTRAP mass spectrometer at ISOLDE/CERN. The novel PI-ICR technique allowed to reach a relative mass precision δm/m of 1.4 · 10 −9 . A mass resolving power m/∆m exceeding 1 · 10 7 was obtained in only 1 s trapping time. Allowed electron-capture transitions with sub-keV or lower decay energies are of high interest for the direct determination of the νe mass. The new measurement improves the uncertainty on the ground-to-ground-state Q EC -value by a factor 25 precluding the 131 Cs→ 131 Xe pair as a feasible candidate for the direct determination of the νe mass.