NpNnscheme and the saturation of collectivity in theA≊170 and 230 regions

It is shown that the well known phenomenon of the saturation in the B(E2;$0_1^{+}$→$2_1^{+}$), as well as the ${\mathit{E}}_{21}^{+}$ values near midshell in the even rare-earth and actinide nuclei, can be reproduced in the ${\mathit{N}}_{\mathit{p}}$ ${\mathit{N}}_{\mathit{n}}$ scheme through a very simple parametrization in terms of the maximum number of valence protons and neutrons available in the major shells under consideration. This parametrization leads to a product (${\mathit{N}}_{\mathit{p}}$ ${\mathit{N}}_{\mathit{n}}$ ${)}_{\mathit{e}\mathit{f}\mathit{f}}$ which is found to have a more universal character as a structure variable than the usual ${\mathit{N}}_{\mathit{p}}$ ${\mathit{N}}_{\mathit{n}}$.