Recent Developments in the Description of Electric Giant Resonances

Abstract

The experimental situation concerning electric isovector dipole and iso· scalar monopole strengths is reviewed. When describing the giant resonance region, emphasis is put on selfconsistent RPA theories. The role of the effective interaction is examined. The photoabsorption cross-section in the region 40"-' 140 MeV is also discussed as well as evaluations of the dipole enhancement factor. § l. lntrod uction It is well known that atomic nuclei show collective properties. One important aspect of collective motion is the concentration in energy of excitation strength. This is best illustrated by the nuclear photoeffect which has the following two main characteristics: i) The photoabsorption cross-section shows, all over the periodic table, a broad peak that takes a large part of the integrated photo cross-section. ii) The variation of the peak energy is a smooth function of the mass number A. Two main routes can be followed in order to describe the strength function S (E) S(E) =I:: J(nJQJO)J 2 o(E-E,.);(1) " m Eq. (1) Q is the excitation operator, En are excitation energies corresponding to the n-th excited state In) and JO) is the ground state. In the more detailed route the strength function S(E) is calculated at each energy E.D. 2 > A more global approach can be followed, in which only some energy moments mk (or sum-rules) of the strength function S(E) are computed,s>-5 > the k-th

Energy weighted sums for collective excitations in nuclear Fermi-liquid

“…Using the Landau-Vlasov kinetic theory, we have studied the linear response function and the EWS m k for the isoscalar and isovector excitations in heavy nuclei and the nuclear (97) and ( 99) for two values of the isovector amplitude F ′ 1 = 1.2 (solid line) and F ′ 1 = 0.58 (dashed line) at the constant value of F 1 = −0.64; points are the experimental data from [20].…”

“…The origin of this effect is the same as in the case of the nuclear stiffness coefficients C Q,ad and C Q,sc , see Eqs. (15) and (20).…”