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^-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 electron beam ion trap 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 (AME'03). The 48^,49^,50K mass excesses are more precise than the AME'03 values by more than 1 order of magnitude. For 48 ^,49K, we find deviations of 7σ and 10σ, 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 AME'03 mass-excess values and thus increases the N=28 neutron-shell gap energy at Z=19 by approximately 1 MeV.