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.

We present precision Penning trap mass measurements of neutron-rich calcium and potassium isotopes in the vicinity of neutron number N=32. Using the TITAN system, the mass of ⁵¹K was measured for the first time, and the precision of the ^51,52Ca mass values were improved significantly. The new mass values show a dramatic increase of the binding energy compared to those reported in the atomic mass evaluation. In particular, ⁵²Ca is more bound by 1.74 MeV, and the behavior with neutron number deviates substantially from the tabulated values. An increased binding was predicted recently based on calculations that include three-nucleon (3N) forces. We present a comparison to improved calculations, which agree remarkably with the evolution of masses with neutron number, making neutron-rich calcium isotopes an exciting region to probe 3N force

We report a direct measurement of the Q ββ -value of the neutrinoless double-β-decay candidate 48 Ca at the TITAN Penning-trap mass spectrometer, with the result that Q ββ = 4267.98(32) keV. We measured the masses of both the mother and daughter nuclides, and in the latter case found a 1 keV deviation from the literature value. In addition to the Q ββ -value, we also present results of a new calculation of the neutrinoless double-β-decay nuclear matrix element of 48 Ca. Using diagrammatic many-body perturbation theory to second order to account for physics outside the valence space, we constructed an effective shell-model double-β-decay operator, which increased the nuclear matrix element by about 75% compared with that produced by the bare operator. The new Q ββ -value and matrix element strengthen the case for a 48 Ca double-β-decay experiment.The discovery of neutrino oscillations represents the first evidence for new physics beyond the Standard Model [1,2]. The oscillations conclusively demonstrate that neutrinos have mass, that flavor eigenstates are mixtures of mass eigenstates, and that neutrino physics is more complicated than we had thought. The observation of neutrinoless double-β (0νββ) decay, extremely rare if it exists, would at once fill multiple gaps in our understanding of the neutrino's nature and would represent a major breakthrough for particle physics. Since this lepton-number-violating process can occur only if the neutrino is its own antiparticle, its discovery would unambiguously confirm the neutrino as a Majorana particle, while a measured lifetime would provide a value for the neutrino mass scale [3]. In order to extract that value from the 0νββ-decay half-life, however, two quantities must be accurately determined: a phase-space factor, which depends on the Q ββ -value of the decay, and a nuclear matrix element, which is not observable and therefore must be obtained from nuclear structure theory.The twelve nuclides that have been observed to undergo two-neutrino double-β (2νββ) decay [4,5] are the basis for a number of large-scale experimental 0νββdecay searches currently underway. Of these nuclides, 48 Ca possesses the largest Q ββ -value of 4.3 MeV [6], giv-ing it several distinct experimental advantages. Because the Q ββ -value lies well above the energy of naturally occurring background, a good signal-to-noise ratio is ensured, while the large phase-space factor enhances the 0νββ-decay rate. The low isotopic abundance of 48 Ca, however, requires enrichment. 48 Ca is currently being measured at NEMO-III [7] and studied at CANDLES [8] and CARVEL [9]. The Q ββ -value provides vital input for the simulation of signal and background, the analysis of current data, and the design of future detectors. In order for the uncertainty of the Q ββ -value to be negligible in these studies, the required precision has to be better than the intrinsic resolution of the detector.The deep implications of massive neutrinos have led to a concentrated effort to calculate the nuclear matrix element for 0νββ-...

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.