Abstract
Two different experimental approaches were combined to study the electric dipole strength in the doubly-magic nucleus 48Ca below the neutron threshold. Real-photon scattering experiments using bremsstrahlung up to 9.9 MeV and nearly mono-energetic linearly polarized photons with energies between 6.6 and 9.51 MeV provided strength distribution and parities, and an (α,α′γ) experiment at Eα=136MeV gave cross sections for an isoscalar probe. The unexpected difference observed in the dipole response is compared to calculations using the first-order random-phase approximation and points to an energy-dependent isospin character. A strong isoscalar state at 7.6 MeV was identified for the first time supporting a recent theoretical prediction.
Highlights
Two different experimental approaches were combined to study the electric dipole strength in the doubly-magic nucleus 48Ca below the neutron threshold
The unexpected difference observed in the dipole response is compared to calculations using the first-order random-phase approximation and points to an energy-dependent isospin character
Doubly-magic nuclei are exceptional cases for studying nuclearstructure properties. On one hand, such nuclei are traditionally considered as key-stones for testing theoretical approaches, such as, for example, the random-phase approximation (RPA) [1]
Summary
Two different experimental approaches were combined to study the electric dipole strength in the doubly-magic nucleus 48Ca below the neutron threshold. The J π = 1− state at 7.3 MeV, which has the strongest E1 transition probability in the (γ , γ ) experiment, was not excited by the α particles within an experimental sensitivity limit for the cross section of 0.15(4) mb/sr.
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