Isoscalar giant dipole resonance in Zr-90, Sn-116, and Pb-208

Abstract

Strength functions for isoscalar dipole excitations in Zr-90, Sn-116, and Pb-208 have been measured with inelastic scattering of 240 MeV alpha particles at small angles. The isoscalar E1 strength distribution in each nucleus is found to consist of a broad component at E-x similar to 114/A(1/3) MeV containing approximately 100% of the E1 EWSR and a narrower one at E-x similar to 72/A(1/3) MeV containing 15-28% of the total isoscalar E1 strength. The higher component is the compression mode E1 strength previously reported only in Pb-208, whereas the lower component may be a new mode not reported previously, but suggested by recent RPA-HF and relativistic mean field calculations.