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.
The elusive β − p + decay was observed in 11 Be by directly measuring the emitted protons and their energy distribution for the first time with the prototype Active Target Time Projection Chamber (pAT-TPC) in an experiment performed at ISAC-TRIUMF. The measured β − p + branching ratio is orders of magnitude larger than any previous theoretical model predicted. This can be explained by the presence of a narrow resonance in 11 B above the proton separation energy.
In the Letter, we presented the first direct observation of the elusive β −-delayed proton emission (β − p þ) in 11 Be using a time projection chamber. There is an error in the extracted logðftÞ, which should read 2.8(4). The corrected logðftÞ value does not affect the conclusions presented in the Letter, since it was only used to assert the allowed character of the transition, a conclusion that is not changed. We would like to point out that, assuming a pure Gamow-Teller (GT) transition, it yields BðGTÞ ¼ 5.5 þ8.3 −3.3 , thus, including the BðGTÞ < 3 limit for a single neutron decay within one sigma. It should be noted that, due to the small energy window, the uncertainty in the resonance energy amounts to nearly 75% of the error budget for the logðftÞ value. Thus, a more precise measurement of the resonance energy could bring the B(GT) to within theoretical limits without significantly affecting the measured branching ratio.
We performed the first direct mass measurements of neutron-rich vanadium 52−55 V isotopes passing the N = 32 neutron shell closure with TRIUMF's Ion Trap for Atomic and Nuclear science (TITAN). The new direct measurements confirm all previous indirect results. Through a reduced uncertainty of the mass of 55 V we confirm the quenching of the N = 32 neutron shell closure in vanadium. We discuss the evolution of the N = 32 neutron shell closure between K and Cr and show similar signatures in the half-life surface when studied along the isotopic chains.
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