Identification of the pygmy dipole resonance in a well deformed nucleus by combined isoscalar and isovector probes

Abstract

The low-energy electric dipole response in the axially deformed 154Sm nucleus was investigated for the first time using complementary probes. The isoscalar (IS) response was extracted from an ( α, α ′ γ ) coincidence experiment and the isovector (IV) response from polarization observables measured with the (p→,p→′) reaction at 0∘. Both the IS and IV responses exhibit a resonance-like structure at excitation energies 5.5-6 MeV identified as the pygmy dipole resonance. Its evolution with deformation is investigated by comparison to the spherical isotope 144Sm. The low-energy IV strength in 154Sm departs from the K -splitting picture that characterizes the IV giant dipole resonance in this nucleus. This finding is further supported by the absence of significant IS strength above 7 MeV.