A precision mass investigation of the neutron-rich titanium isotopes 51−55 Ti was performed at TRIUMF's Ion Trap for Atomic and Nuclear science (TITAN). The range of the measurements covers the N = 32 shell closure and the overall uncertainties of the 52−55 Ti mass values were significantly reduced. Our results conclusively establish the existence of weak shell effect at N = 32, narrowing down the abrupt onset of this shell closure. Our data were compared with state-of-the-art ab initio shell model calculations which, despite very successfully describing where the N = 32 shell gap is strong, overpredict its strength and extent in titanium and heavier isotones. These measurements also represent the first scientific results of TITAN using the newly commissioned Multiple-Reflection Time-of-Flight Mass Spectrometer (MR-TOF-MS), substantiated by independent measurements from TITAN's Penning trap mass spectrometer.Atomic nuclei are highly complex quantum objects made of protons and neutrons. Despite the arduous efforts needed to disentangle specific effects from their many-body nature, the fine understanding of their structures provides key information to our knowledge of fundamental nuclear forces. One notable quantum behavior of bound nuclear matter is the formation of shell-like structures for each fermion group [1], as electrons do in atoms. Unlike for atomic shells, however, nuclear shells are known to vanish or move altogether as the number of protons or neutrons in the system changes [2]. Particular attention has been given to the emergence of strong shell effects among nuclides with 32 neutrons, pictured in a shell model framework as a full valence ν2p 3/2 orbital. Across most of the known nuclear chart, this orbital is energetically close to ν1f 5/2 , which prevents the appearance of shell signatures in energy observables. However, the excitation energies of the lowest 2 + states show a relative, but systematic, local increase below proton number Z = 24 [3]. This effect, characteristic of shell closures, has been attributed in shell model calculations to the weakening of attractive proton-neutron interactions between the ν1f 5/2 and π1f 7/2 orbitals as the latter empties, making the neutrons in the former orbital less bound [4,5]. Ab initio calculations are also extending their reach over this sector of the nuclear chart, yet no systematic investigation of the N = 32 isotones has been produced so far.

Photoionization (PI) of multiply and highly charged ions was studied using an electron beam ion trap and synchrotron radiation at the BESSY II electron storage ring. The versatile new method introduced here extends the range of ions accessible for PI investigations beyond current limitations by providing a dense target of ions in arbitrary, i. e., both low and high, charge states. Data on near-threshold PI of N 3+ and Ar 8+ ions, species of astrophysical and fundamental interest, show high resolution and accuracy allowing various theoretical models to be distinguished, and highlight shortcomings of available PI calculations. We compare our experimental data with our new fully relativistic PI calculations within a multiconfiguration Dirac-Fock approach and with other advanced calculations and find generally good agreement; however, detailed examination reveals significant deviations, especially at the threshold region of Ar 8+ .

Using the Penning trap mass spectrometer TITAN, we performed the first direct mass measurements of (20,21)Mg, isotopes that are the most proton-rich members of the A = 20 and A = 21 isospin multiplets. These measurements were possible through the use of a unique ion-guide laser ion source, a development that suppressed isobaric contamination by 6 orders of magnitude. Compared to the latest atomic mass evaluation, we find that the mass of (21)Mg is in good agreement but that the mass of (20)Mg deviates by 3 σ. These measurements reduce the uncertainties in the masses of (20,21)Mg by 15 and 22 times, respectively, resulting in a significant departure from the expected behavior of the isobaric multiplet mass equation in both the A = 20 and A = 21 multiplets. This presents a challenge to shell model calculations using either the isospin nonconserving universal sd USDA and USDB Hamiltonians or isospin nonconserving interactions based on chiral two- and three-nucleon forces.

Photoionization (PI) of Fe14+ in the range from 450 to 1100 eV was measured at the BESSY II storage ring using an electron beam ion trap achieving high target-ion area densities of 10(10)  cm(-2). Photoabsorption by this ion is observed in astrophysical spectra and plasmas, but until now cross sections and resonance energies could only be provided by calculations. We reach a resolving power E/ΔE of at least 6500, outstanding in the present energy range, which enables benchmarking and improving the most advanced theories for PI of ions in high charge states.