Empirical residual neutron-proton interaction in odd-odd nuclei

TL;DR

This paper compares two methods of extracting neutron-proton interactions from nuclear binding energies, revealing systematic behaviors and irregular fluctuations that can guide improvements in nuclear mass models.

Contribution

It introduces and compares two approaches to quantify residual neutron-proton interactions, highlighting their systematic properties and discrepancies with theoretical models.

Findings

Empirical interactions are positive and weakly dependent on mass number.

Large fluctuations are observed in certain nuclei in theoretical models.

Comparison suggests potential for refining nuclear mass models.

Abstract

Two types of average neutron-proton interaction formulas are compared: In the first type, neutron-proton interactions for even-even and odd-$A$ nuclei extracted from experimental binding energies show a smooth behavior as a function of mass number $A$ and are dominated by the contribution from the symmetry energy. Whereas in the second type large systematic staggering is seen between even-$A$ and odd-$A$ nuclei. This deviation is understood in term of the additional neutron-proton interaction in odd-odd nuclei relative to the neighboring even-even and odd-$A$ systems. We explore three possible ways to extract this additional interaction from the binding energy difference of neighboring nuclei. The extracted interactions are positive in nearly all cases and show weak dependence on the mass number. The empirical interactions are also compared with theoretical values extracted from recent nuclear mass models where large unexpected fluctuations are seen in certain nuclei. The reproduction of the residual neutron-proton interaction and the correction of those irregular fluctuations can be a good criterion for the refinement of those mass models.