UC Berkeley

Abstract.A segmented Si-telescope and HPGe array, STARS-LIBERACE, was used to study the 156Gd(p,tγ$$p,t\gamma$$)154Gd direct reaction by particle- γ$$ \gamma$$ coincidence spectroscopy. New cross sections with a 25MeV proton beam are reported and compared to previous (p,t) and (t,p) studies. Furthermore, additional evidence for coexisting Kπ=01+,21+$$ K^{\pi}=0_{1}^{+},2_{1}^{+}$$ and 02+$$0_{2}^{+}$$, 22+$$2_{2}^{+}$$ configurations at N = 90 is presented. Direct and indirect population patterns of the low-lying states are also explored. Review of the new and existing evidence favors an interpretation based on a configuration-dependent pairing interaction. The weakening of monopole pairing strength and an increase in quadrupole pairing strength could bring 2p-2h 0+$$0^{+}$$ states below 2Δ\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$2\Delta$\end{document}. This may account for a large number of the low-lying 0+$$0^{+}$$ states observed in two-nucleon transfer reactions. A hypothesis for the origin of the 02+$$0_{2}^{+}$$ and 03+$$ 0_{3}^{+}$$ states is provided.