Empirical pairing gaps, shell effects, and di-neutron spatial correlation in neutron-rich nuclei.

The empirical pairing gaps derived from four different odd–even mass staggering formulas are compared. By performing single- j shell and multi-shell seniority model calculations as well as by using the standard HFB approach with Skyrme force we show that the simplest three-point formula Δ C ( 3 ) ( N ) = 1 2 [ B ( N , Z ) + B ( N − 2 , Z ) − 2 B ( N − 1 , Z ) ] can provide a good measure of the neutron pairing gap in even- N nuclei. It removes to a large extent the contribution from the nuclear mean field as well as contributions from shell structure details. It is also less contaminated by the Wigner effect for nuclei around N = Z . We also show that the strength of Δ C ( 3 ) ( N ) can serve as a good indication of the two-particle spatial correlation in the nucleus of concern and that the weakening of Δ C ( 3 ) ( N ) in some neutron-rich nuclei indicates that the di-neutron correlation itself is weak in these nuclei. [ABSTRACT FROM AUTHOR]