Two-quasiproton states in 168Er studied by the [formula omitted] reaction

Abstract

The 169 Tm( t , α) 168 Er reaction has been studied using 17 MeV polarized tritons from the Los Alamos National Laboratory tandem Van de Graaff accelerator. The α-spectra were analyzed with a Q3D magnetic spectrometer. The overall energy resolution was typically ~ 15 keV (FHWM) and angular distributions of cross sections and analyzing powers were obtained for levels up to ~ 2.7 MeV. The fact that spins and parities for all levels up to ⩾ 2 MeV were previously known from an extensive series of (n, γ) studies made it possible to determine specific two-quasiproton structures for many bands from the present results. The K π = 2 + γ-vibrational band was found to have a large 3 2 + [411] p + 1 2 +[411] p admixture, consistent with the predicted microscopic composition of this phonon, but no 5 2 [413] p − 1 2 + [411] p component was observed. The K π = 0 4 + band at 1833 keV has ∼ 25% of the 1 2 + [411] p − 1 2 +[411] p two-quasiproton strength. This is in excellent agreement with the Soloviev model but is inconsistent with the interacting boson model, in which the K π = 0 4 + band is composed almost completely of multiphonon configurations that should not be populated in a single-nucleon transfer reaction. The K π = 4 −, 7 2 −[523] p + 1 2 + [411] p two-quasiproton and the K π = 4 −, 7 2 +[633] n + 1 2 −[521] n two-quasineutron states are mixed strongly with each other, but the two K π = 3 − bands composed of antiparallel couplings of the same particles are not. A good qualitative explanation of this mixing pattern is provided in terms of the effective neutron-proton interaction.