No abstract

The damping of the giant dipole resonance (GDR) is studied in 90 Zr, 120 Sn, and 208 Pb as a function of temperature T . The results show that the coupling of GDR to the pp and hh states is responsible for the increase of the GDR width with increasing T up to 3 MeV and its saturation at higher T . The quantal width, caused by the coupling to ph excitations, decreases slowly with increasing T . An overall agreement is found between the theoretical evaluations and the recent experimental data in heavy-ion fusion reactions and inelastic a scattering. At very high T a behavior similar to the transition from zero to ordinary sounds is observed. [S0031-9007(98) The giant dipole resonance (GDR), built on compound nuclear states, has been studied intensively during the last 15 years [1]. The measurements show that the energy of the GDR is rather stable with varying temperature. Its width increases rapidly with increasing excitation energy E ‫ء‬ (or temperature T) up to around 130 MeV in Sn isotopes [2][3][4]. At higher E ‫ء‬ the width increases slowly and even saturates [3,[5][6][7]. The ground-state GDR (g.s. GDR) acquires an escape width G " via a g or particle emission and a spreading (or quantal) width G # by coupling to more complicated configurations. The extension of the microscopic approaches in [8,9] to T fi 0 shows only a little change of G # [10,11]. Shape fluctuations are taken into account to describe the increase in the width at T fi 0. The new experimental methods involving compound nuclear reactions [12] and inelastic a scattering [13] allow separating the effects due to thermal fluctuations and due to angular momentum in the study of hot GDR. The recent calculations in [14], including the thermal shape fluctuations and the evaporation width [15], agree nicely with the data of [13] for the GDR width in 120 Sn and 208 Pb atThe effects of angular momentum J are important only at rather high J $ 35h at T Ӎ 1.5 1.8 MeV and only in a lighter nucleus 106 Sn [16]. Recently, we have demonstrated in [17] that the coupling of the GDR excitation to the pp and hh configurations, which appear at T fi 0, leads to the thermal damping of the GDR. In this Letter, elaborating further this concept, we will perform a systematic study of the width of the hot GDR in 90 Zr, 120 Sn, and 208 Pb in a large range of temperature up to at least T ϳ 6 MeV.The coupling of collective oscillations (phonons) to the field of ph, pp, and hh pairs can be studied using the model Hamiltonian:The first term on the right-hand side (RHS) of Eq. (1) describes the field of independent single particles a y s and a s . The second term stands for the phonon field ͕Q y q , Q q ͖. The last term describes the coupling between the first two fields. E s e s 2 e F , where e s is the single-particle energy and e F -the Fermi surface energy. We will simply refer to the energy E s as single-particle energy. The phonon energy is denoted as v q .We introduce the double-time Green's functions [18], which describe (a) the propagation of a free particle (or hole):...

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

Nuclear pairing properties are studied within an approach that includes the quasiparticle-number fluctuation (QNF) and coupling to the quasiparticle-pair vibrations at finite temperature and angular momentum. The formalism is developed to describe non-collective rotations about the symmetry axis. The numerical calculations are performed within a doubly-folded equidistant multilevel model as well as several realistic nuclei. The results obtained for the pairing gap, total energy and heat capacity show that the QNF smoothes out the sharp SN phase transition and leads to the appearance of a thermally assisted pairing gap in rotating nuclei at finite temperature. The corrections due to the dynamic coupling to SCQRPA vibrations and particle-number projection are analyzed. The effect of backbending of the momentum of inertia as a function of squared angular velocity is also discussed.Comment: 30 pages and 9 figures. Accepted in Phys. Rev.

No abstract

No abstract

No abstract

Particle-number projection is applied to the modified BCS (MBCS) theory. The resulting particle-numberprojected MBCS theory, taking into account the effects due to fluctuations of particle and quasiparticle numbers at finite temperature, is tested within the exactly solvable multilevel model for pairing as well as the realistic 120 Sn nucleus. The signature of the pseudogap in the crossover region above the critical temperature of superfluid-normal phase transition is discussed in terms of the pairing spectral function.