Clustering aspects and the shell model

In this talk I shall discuss the clustering aspect and the shell model. I shall first discuss the $\alpha$-cluster aspects based on the shell model calculations. Then I shall discuss the spin zero ground state dominance in the presence of random interactions and a new type of cluster structure for fermions in a single-$j$ shell in the presence of only pairing interaction with the largest multiplicity.Comment: 10 pages and 3 figures. International symposium in Nara, to appear in Nuclear Physics

Damping of the double giant dipole resonance

A microscopic approach is proposed to the damping of the double giant dipole resonance ͑DGDR͒. The double-time Green's function method is used to derive a closed set of coupled equations for the propagation of two-phonon excitation through the field of incoherent nucleon pairs. The analytical expressions for the width and energy shift of the DGDR are obtained. The numerical calculations are performed for 90 Zr, 90 Sn, and 208 Pb for several characteristics of the DGDR at zero as well as nonzero temperatures T. The results are found in reasonable agreement with existing experimental systematics for the width and energy of the DGDR. As compared to the estimation within the harmonic picture, the anharmonicity leads to a noticeable enhancement of the integrated photoabsorption cross section ͑IPACS͒ over the DGDR region. The DGDR width is found to increase sharply with increasing T at Tр3 MeV, but goes to a saturation at T Ͼ3 MeV. The harmonic limit for the DGDR width is restored already at Tу1.5 MeV. It is shown that the IPACS of the DGDR can also be enhanced compared to its harmonic value if it is built on a hot GDR

Pairing effect on the giant dipole resonance width at low temperature

The width of the giant dipole resonance (GDR) at finite temperature T in Sn-120 is calculated within the Phonon Damping Model including the neutron thermal pairing gap determined from the modified BCS theory. It is shown that the effect of thermal pairing causes a smaller GDR width at T below 2 MeV as compared to the one obtained neglecting pairing. This improves significantly the agreement between theory and experiment including the most recent data point at T = 1 MeV.Comment: 8 pages, 5 figures to be published in Physical Review