  The pairing gap for $^{53}$Ca obtained from new experimental data on the
masses of $^{52-54}$Ca 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}
$. Minima in the pairing gaps for all nuclei are shown and discussed
