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Register a new userPrecision mass measurements of neutron-rich scandium isotopes refine the evolution of $N=32$ and $N=34$ shell closures
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We report high-precision mass measurements of $^{50-55}$Sc isotopes performed at the LEBIT facility at NSCL and at the TITAN facility at TRIUMF. Our results provide a substantial reduction of their uncertainties and indicate significant deviations, up to 0.7 MeV, from the previously recommended mass values for $^{53-55}$Sc. The results of this work provide an important update to the description of emerging closed-shell phenomena at neutron numbers $N=32$ and $N=34$ above proton-magic $Z=20$. In particular, they finally enable a complete and precise characterization of the trends in ground state binding energies along the $N=32$ isotone, confirming that the empirical neutron shell gap energies peak at the doubly-magic $^{52}$Ca. Moreover, our data, combined with other recent measurements, does not support the existence of closed neutron shell in $^{55}$Sc at $N=34$. The results were compared to predictions from both emph{ab initio} and phenomenological nuclear theories, which all had success describing $N=32$ neutron shell gap energies but were highly disparate in the description of the $N=34$ isotone.
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A precision mass investigation of the neutron-rich titanium isotopes $^{51-55}$Ti was performed at TRIUMFs 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 confirm the existence of a weak shell effect at $N=32$, establishing the abrupt onset of this shell closure. Our data were compared with state-of-the-art textit{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 TITANs Penning trap mass spectrometer.
The region near Z=28, N=40 is a subject of great interest for nuclear structure studies due to spectroscopic signatures in $^{68}$Ni suggesting a subshell closure at N=40. Trends in nuclear masses and their derivatives provide a complementary approach to shell structure investigations via separation energies. Penning trap mass spectrometry has provided precise measurements for a number of nuclei in this region, however a complete picture of the mass surfaces has so far been limited by the large uncertainty remaining for nuclei with N > 40 along the iron and cobalt chains. Here we present the first Penning trap measurements of $^{68,69}$Co, performed at the Low-Energy Beam and Ion Trap facility at the National Superconducting Cyclotron Laboratory. In addition, we perform ab initio calculations of ground state and two-neutron separation energies of cobalt isotopes with the valence-space in-medium similarity renormalization group approach based on a particular set of two- and three-nucleon forces which predict saturation in infinite matter. We discuss the importance of these measurements and calculations for understanding the evolution of nuclear structure near $^{68}$Ni.
We present Penning-trap mass measurements of neutron-rich 44,47-50K and 49,50Ca isotopes carried out at the TITAN facility at TRIUMF-ISAC. The 44K mass measurement was performed with a charge-bred 4+ ion utilizing the TITAN EBIT, and agrees with the literature. The mass excesses obtained for 47K and 49,50Ca are more precise and agree with the values published in the 2003 Atomic Mass Evaluation (AME03). The 48,49,50K mass excesses are more precise than the AME03 values by more than one order of magnitude. For 48,49K, we find deviations by 7 sigma and 10 sigma, respectively. The new 49K mass excess lowers significantly the two-neutron separation energy at the neutron number N=30 compared with the separation energy calculated from the AME03 mass-excess values, and thus, increases the N=28 neutron-shell gap energy at Z=19 by approximately 1 MeV.
The neutron-rich, even-even 122,124,126Pd isotopes has been studied via in-beam gamma-ray spectroscopy at the RIKEN Radioactive Isotope Beam Factory. Excited states at 499(9), 590(11), and 686(17) keV were found in the three isotopes, which we assign to the respective 2+ -> 0+ decays. In addition, a candidate for the 4+ state at 1164(20) keV was observed in 122Pd. The resulting Ex(2+) systematics are essentially similar to those of the Xe (Z=54) isotopic chain and theoretical prediction by IBM-2, suggesting no serious shell quenching in the Pd isotopes in the vicinity of N=82.
We present the results of precision mass measurements of neutron-rich cadmium isotopes. These nuclei approach the $N=82$ closed neutron shell and are important to nuclear structure as they lie near doubly-magic $^{132}$Sn on the chart of nuclides. Of particular note is the clear identification of the ground state mass in $^{127}$Cd along with the isomeric state. We show that the ground state identified in a previous mass measurement which dominates the mass value in the Atomic Mass Evaluation is an isomeric state. In addition to $^{127/m}$Cd, we present other cadmium masses measured ($^{125/m}$Cd and $^{126}$Cd) in a recent TITAN experiment at TRIUMF. Finally, we compare our measurements to new emph{ab initio} shell-model calculations and comment on the state of the field in the $N=82$ region.
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