Isospin properties of electric dipole excitations in 48 Ca

Two different experimental approaches were combined to study the electric dipole strength in the doubly-magic nucleus 48 Ca 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 <math altimg="si1.gif" xmlns="http://www.w3.org/1998/Math/MathML"><mo stretchy="false">(</mo><mi>α</mi><mo>,</mo><msup><mrow><mi>α</mi></mrow><mrow><mo>′</mo></mrow></msup><mi>γ</mi><mo stretchy="false">)</mo></math> experiment at <math altimg="si2.gif" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mrow><mi>E</mi></mrow><mrow><mi>α</mi></mrow></msub><mo>=</mo><mn>136</mn><mspace width="0.25em"/><mtext>MeV</mtext></math> 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