Penning-trap mass measurements of the neutron-rich K and Ca isotopes: Resurgence of the $N=28$ shell strength

We present Penning-trap mass measurements of neutron-rich ${}^{44,47-50}$K and ${}^{49,50}$Ca isotopes carried out at the TITAN facility at TRIUMF-ISAC. The ${}^{44}$K mass measurement was performed with a charge-bred 4$+$ ion utilizing the TITAN electron beam ion trap and agrees with the literature. The mass excesses obtained for ${}^{47}$K and ${}^{49,50}$Ca are more precise and agree with the values published in the 2003 Atomic Mass Evaluation (AME’03). The ${}^{48,49,50}$K mass excesses are more precise than the AME’03 values by more than 1 order of magnitude. For ${}^{48,49}$K, we find deviations of 7$\sigma$ and 10$\sigma$, respectively. The new ${}^{49}$K mass excess lowers significantly the two-neutron separation energy at the neutron number $N=30$ compared with the separation energy calculated from the AME’03 mass-excess values and thus increases the $N=28$ neutron-shell gap energy at $Z=19$ by approximately 1 MeV.

Figure 1

${}^{49}$K${}^{+}$ and ${}^{50}$K${}^{+}$ TOF-ICR resonance curves. The rf quadrupolar excitation times were 997 and 147 ms, respectively. The red solid curve is a fit of the theoretically expected line shape [35] to the data points.

Figure 2

Deviation of the measured mass excesses with respect to those published in AME'03. Our ${}^{44}$K mass excess of $-$35 778.7(1.6) keV measured with multiply charged K${}^{4+}$ agrees with the more precise ISOLTRAP value of $-$35 781.29(0.47) keV [38]. The black error bars represent the uncertainties of the AME'03 values. The red thick elongated lines lying on the zero-deviation-energy line represent the uncertainties of our measurements.

Figure 3

Two-neutron separation energy curves of neutron-rich S, Cl, Ar, K, Ca, Sc, Ti, and V isotopes. The red solid line shows the $S_{2n}$ values calculated with the results of our K and Ca mass measurements. The black dotted line, named AME'03*, represents those calculated from the AME'03 mass-excess values for which measured mass excesses of K [38], S [16, 17], and Cl [16] isotopes were included with a weighted average.

Figure 4

K and Ca neutron-shell gap energies as a function of the neutron number $N$. Our K measurements increase significantly the value of the neutron-shell gap energies at $N=28$, with respect to the shell gap energy calculated from the AME'03 mass-excess values.

Figure 5

Experimental neutron-shell gap energy as a function of the atomic number $Z$ at $N=28$ compared with the shell gap energy obtained from various mass models (see text).