The neutron-proton pairing interaction $\lambda$ between the last odd neutron and the last odd proton in the outermost neutron and proton shells of an odd-odd nucleus has been estimated from nuclear masses in the regions just beyond $Z=20\mathrm{}$, $N=20\mathrm{}$, and just beyond $Z=40\mathrm{}$, $N=50\mathrm{}$. Behavior of $\lambda$ in these two regions and in the heavy element region $Z>\mathrm{}82\mathrm{}$, $N>\mathrm{}126\mathrm{}$, as estimated by Ghoshal and Saxena, is discussed. It is found that the behavior of $\lambda$ may be understood in terms of a simple $\mathrm{jj}$-coupling shell model. According to this model, $\lambda$ arises from the spin-independent part of the two-body force and is proportional to $(2\mathrm{}{j}_1+1-2z)(2\mathrm{}{j}_2+1-2n)$, where $z$ is the odd number of protons in the outermost proton shell $j_1$, and $n$ is the odd number of neutrons in the outermost neutron shell $j_2$ of the odd-odd nucleus.

• Received 26 August 1960

DOI:https://doi.org/10.1103/PhysRev.121.595