Investigating nuclear structure near $N=32 \mathrm{and} N=34:$ Precision mass measurements of neutron-rich Ca, Ti, and V isotopes

Nuclear mass measurements of isotopes are key to improving our understanding of nuclear structure across the chart of nuclides, in particular, for the determination of the appearance or disappearance of nuclear shell closures. We present high-precision mass measurements of neutron-rich Ca, Ti, and V isotopes performed at TRIUMF's Ion Trap for Atomic and Nuclear science (TITAN) and the Low Energy Beam and Ion Trap (LEBIT) facilities. These measurements were made using the TITAN multiple-reflection time-of-flight mass spectrometer (MR-ToF-MS) and the LEBIT 9.4T Penning trap mass spectrometer. In total, 13 masses were measured, 8 of which represent increases in precision over previous measurements. These measurements refine trends in the mass surface around $N=32$ and $N=34$, and support the disappearance of the $N=32$ shell closure with increasing proton number. Additionally, our data do not support the presence of a shell closure at $N=34$.

Figure 1

A sample mass spectrum taken at $A=58$ with the TITAN MR-ToF-MS. The identified ion species are labeled.

Figure 2

A ToF-ICR spectrum taken for ${}^{55}\mathrm{Ti}{}^{16}\mathrm{O}^{+}$ with the LEBIT Penning trap mass spectrometer. The fit result of an analytical function described in Ref. [38], shown in red, is used to extract the minimum in time of flight that occurs at ${\omega }_{\mathrm{rf}}={\omega }_c$.

Figure 3

A plot comparing mass values from Ref. [43] to experimental mass results from this paper. Gray bands represent error bars given on values in Ref. [43].

Figure 4

A graph of the $S_{2n}$ in the $Z=19–24$ isotope chains based on mass values from Ref. [43] (black symbols), and Ca, Ti, and V mass measurements from this paper (red symbols). For the cases of ${}^{55}\mathrm{Ti}$, ${}^{56}\mathrm{V}$, and ${}^{57}\mathrm{V}$ where both TITAN and LEBIT measured a mass value, the more precise mass value was used in the determination of the plotted $S_{2n}$ value.

Figure 5

A graph of the ${\delta }_{2n}$ for the $Z=19–24$ isotope chains. Values are offset by amounts as presented in the legend. Unfilled symbols represent values from this paper. The disappearance of a peak at $N=32$, signifying the disappearance of a shell closure at $N=32$, can be seen as the proton number increases.

Figure 6

A graph comparing ${\delta }_{2n}$ experimental values (solid lines) to theoretical calculations (dashed lines) from Ref. [45]. Values are offset by amounts as presented in the legend. Unfilled symbols represent values from this paper.