Isoscalar and isovector splitting of pygmy dipole structures.
The electric-dipole response of 140Ce is investigated using the fully consistent relativistic quasiparticle random phase approximation. By analyzing the isospin structure of the E1 response, it is shown that the low-energy (pygmy) strength separates into two segments with different isospin character. The more pronounced pygmy structure at lower energy is composed of predominantly isoscalar states with surface-peaked transition densities. At somewhat higher energy the calculated E1 strength is primarily of isovector character, as expected for the low-energy tail of the giant dipole resonance. The results are in qualitative agreement with those obtained in recent (gamma, gamma') and (alpha, alpha'gamma) experiments, and provide a simple explanation for the splitting of low-energy E1 strength into two groups of states with different isospin structure and radial dependence of the corresponding transition densities.
Paar
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Physics Department, Faculty of Science, University of Zagreb, Croatia. npaar@phy.hr
Niu
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Vretenar
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Meng
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