Showing papers by "Lutz Schweikhard published in 2020"

Spectroscopy of short-lived radioactive molecules

Abstract: Molecular spectroscopy offers opportunities for the exploration of the fundamental laws of nature and the search for new particle physics beyond the standard model1-4. Radioactive molecules-in which one or more of the atoms possesses a radioactive nucleus-can contain heavy and deformed nuclei, offering high sensitivity for investigating parity- and time-reversal-violation effects5,6. Radium monofluoride, RaF, is of particular interest because it is predicted to have an electronic structure appropriate for laser cooling6, thus paving the way for its use in high-precision spectroscopic studies. Furthermore, the effects of symmetry-violating nuclear moments are strongly enhanced5,7-9 in molecules containing octupole-deformed radium isotopes10,11. However, the study of RaF has been impeded by the lack of stable isotopes of radium. Here we present an experimental approach to studying short-lived radioactive molecules, which allows us to measure molecules with lifetimes of just tens of milliseconds. Energetically low-lying electronic states were measured for different isotopically pure RaF molecules using collinear resonance ionisation at the ISOLDE ion-beam facility at CERN. Our results provide evidence of the existence of a suitable laser-cooling scheme for these molecules and represent a key step towards high-precision studies in these systems. Our findings will enable further studies of short-lived radioactive molecules for fundamental physics research.

First Glimpse of the N= 82 Shell Closure below Z= 50 from Masses of Neutron-Rich Cadmium Isotopes and Isomers

Abstract: 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 $^{132}\mathrm{Cd}$ offers the first value of the $N=82$, two-neutron shell gap below $Z=50$ and confirms the phenomenon of mutually enhanced magicity at $^{132}\mathrm{Sn}$. Using the recently implemented phase-imaging ion-cyclotron-resonance method, the ordering of the low-lying isomers in $^{129}\mathrm{Cd}$ and their energies are determined. The new experimental findings are used to test large-scale shell-model, mean-field, and beyond-mean-field calculations, as well as the ab initio valence-space in-medium similarity renormalization group.

Examining the N = 28 shell closure through high-precision mass measurements of Ar 46 – 48

Abstract: The strength of the $N=28$ magic number in neutron-rich argon isotopes is examined through high-precision mass measurements of $^{46\text{--}48}\mathrm{Ar}$, 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 $^{46}\mathrm{Ar}$ is the transition between the closed-shell $^{48}\mathrm{Ca}$ and collective $^{44}\mathrm{S}$.

The performance of the cryogenic buffer-gas stopping cell of SHIPTRAP

Abstract: Direct high-precision mass spectrometry of the heaviest elements with SHIPTRAP, at GSI in Darmstadt, Germany, requires high efficiency to deal with the low production rates of such exotic nuclides. A second-generation gas stopping cell, operating at cryogenic temperatures, was developed and recently integrated into the relocated system to boost the overall efficiency. Offline measurements using 223Ra and 225Ac recoil-ion sources placed inside the gas volume were performed to characterize the gas stopping cell with respect to purity and extraction efficiency. In addition, a first online test using the fusion-evaporation residue 254No was performed, resulting in a combined stopping and extraction efficiency of 33(5)%. An extraction time of 55(44) ms was achieved. The overall efficiency of SHIPTRAP for fusion-evaporation reaction products was increased by an order of magnitude to 6(1)%. This will pave the way for direct mass spectrometry of heavier and more exotic nuclei, eventually in the region of superheavy elements with proton numbers Z ⩾ 104 .

First steps in the development of the Multi Ion Reflection Apparatus for Collinear Laser Spectroscopy

Abstract: Collinear laser spectroscopy (CLS) has been combined with the multi-reflection time-of-flight (MR-ToF) technique. To this end, a photodetection system has been implemented at the drift region of a MR-ToF apparatus and a laser beam has been sent along the path of the ions that are stored between the two ion-optical mirrors. The main goal of the present proof-of-principle (PoP) experiments, is the confirmation of the expected increase in sensitivity compared to conventional fluorescence-based CLS due to the repeated probing of the trapped ion bunches. The novel method will be used for the precise measurement of nuclear ground- and isomeric-state properties of exotic nuclei with low production yields at radioactive ion-beam facilities. A significant sensitivity improvement of CLS is expected, depending on the half-life and mass of the nuclide of interest. The status of the PoP setup and future improvements are discussed.

Improved stability of multi-reflection time-of-flight mass spectrometers through passive and active voltage stabilization

Abstract: Multi-reflection time-of-flight (MR-ToF) spectrometers are devices in which ions are reflected between two electrostatic mirrors to prolong their flight path. The performance of the apparatus relies on the stability of the voltages that are used to define the mirror potentials. Especially the voltage of the mirror electrodes at the point where the ions reverse the direction of their axial motion needs to be stable in order to minimize fluctuations of their total flight time. Here we present a method to increase the short- and long-term stability of suitable voltage supplies for enhanced performance of the spectrometer.

Multiple-ion-ejection multi-reflection time-of-flight mass spectrometry for single-reference mass measurements with lapping ion species.

Abstract: Repeated switching of electric potentials within a single experimental cycle is introduced for a multi-reflection time-of-flight mass spectrometer (also known as an electrostatic ion beam trap) in order to eject different ion species after different storage times. The method is demonstrated with two cluster ions with considerably different mass-to-charge ratios (the A = 624 and 832 isotopologues of Pb3+ and Pb4+, respectively) for the specific case where the sequential ejections result in an identical number of revolution periods. Thus, the ions’ flight lengths are identical, and the resulting time-of-flight values allow single-reference mass determination. The requirements for the switching time window are studied in detail. For the present system and ion pair, the relative mass uncertainty is found to be 3 · 10−7 for short measurements (≈10 min) and 6 · 10−8 for longer ones (≈2 h).

Decay-rate power-law exponent as a link between dissociation energy and temperature

Abstract: The authors perform multi-reflection time-of-flight mass spectrometry of time-resolved photodissociation of indium clusters and observe a power-law exponent of the decay rate links cluster dissociation energy and ensemble temperature.

Photodissociation of mono- and di-anionic tin clusters

Abstract: Negatively charged tin clusters offer a broad range of decay products, as observed after electron-cluster interactions [S Konig, M Wolfram, S Bandelow, G Marx, L Schweikhard, Eur Phys J D 72, 153 (2018)] To get further insight into their decay pathways, size and charge-state selected clusters Snn- and Snn2- were photo-excited at the ClusterTrap setup by 532 nm Nd:YAG laser pulses For small mono-anionic precursor clusters containing up to n ≃ 45 atoms, Snn-7- and Snn-10- are observed as preferred ionic fragments For bigger clusters a transition to neutral monomer evaporation was found In the case of di-anionic precursors, preferred decay products are Sn10- and Snn-10-, indicating fission similar to the case of the group 14 neighbor element lead [S Konig, A Jankowski, G Marx, L Schweikhard, M Wolfram, Phys Rev Lett 120, 163001 (2018)] Furthermore, doubly charged fragment clusters such as Snn-72- are observed, originating from break-off of neutral heptamers Sn7, a behavior which has not been observed previously for di-anionic clusters