The Nuclear Pairing Gap - How Low Can It Go?

The pairing gap for $^{53}$Ca obtained from new experimental data on the masses of $^{52-54}$Ca [F. Wienholtz et al., Nature {\bf 346}, 498 (2013)] has the smallest value yet observed. This is explained in the framework of the nuclear shell model with schematic and realistic Hamiltonians as being due to shell gaps around the low-$ j $ orbital $ 1p_{1/2} $ at $N=33$. I will also show comparisons of experiment and theory for the oxygen isotopes that have a small pairing gap at $N=15$ due to shell gaps around the low-$ j $ orbital $ 1s_{1/2} $. Minima in the pairing gaps for all nuclei are shown and discussed.