Thermal shape fluctuation effects in the description of hot nuclei

Abstract

The behavior of several nuclear properties with temperature is analyzed within the framework of the Finite Temperature Hartree-Fock-Bogoliubov (FTHFB) theory with the Gogny force and large configuration spaces. Thermal shape fluctuations in the quadrupole degree of freedom, around the mean field solution, are taken into account with the Landau prescription. As representative examples the nuclei 164 Er, 152 Dy and 192 Hg are studied. Numerical results for the superfluid to normal and deformed to spherical shape transitions are presented. We found a substantial effect of the fluctuations on the average value of several observables. In particular, we get a decrease in the critical temperature (Tc) for the shape transition as compared with the plain FTHFB prediction as well as a washing out of the shape transition signatures. The new values of Tc are closer to the ones found in Strutinsky calculations and with the Pairing Plus Quadrupole model Hamiltonian. IntroductionSince the advent of the new generation of 4π gamma ray detectors and the improved accuracy in the channel selection new possibilities have opened up in the study of nuclear structure. Besides this, the availability of faster computers has made possible to perform realistic theoretical investigations with large configuration spaces. The high excitation energy is specially interesting since new features may take place. For example, in the quasicontinuum, the high level density gives rise to the unexpected phenomenon of the damping of the rotational motion. In the limit of high excitation energies (or temperature T ) quantum effects become less relevant or may even disappear. Thus one expects that in a heated nucleus physical effects like superfluidity or shape deformations are washed out when T increases. This expectation can be easily understood in terms of the shell model since, by increasing T , one promotes particles from levels below the Fermi surface to levels above it. In the case of pairing correlations, blocking levels amounts to destroying Cooper pairs. In the case of shape deformation, by depopulating the deformation driving levels (intruders) one gets on the average less deformation. Experimental information about nuclear shape changes can be obtained by means of the Giant Dipole Resonance (GDR) built on excited states. Exclusive experiments studying the GDR strength at a given excitation energy (or T ) of the nucleus have been carried out in refs. [1,2,3,4]. The understanding of these phenomena is relevant because it affects important features like the fission barriers and the stability of the nucleus itself. For a recent review on hot nuclei see ref. [5].The shape transitions have been object of many studies, most of them with schematic models, separable forces, and small configuration spaces [6,7,8,9,10]. The theoretical approaches used in the calculations are based on the mean field approximation, mainly the Finite Temperature Hartree-Fock-Bogoliubov theory (FTHFB). The mean field approximations predict sharp shape transitions, wherea...

Fission of compound nuclei and the developments of microscopic fission theory

“…在有限温度Hartree-Fock-Bogoliubov (HFB)计算中, 随 着温度增加可以得到复合核形状逐渐变为球形 [14] . 随 着温度增加, 对关联会突然消失而产生相变, 对应于 比热的突变 [14] . 因为有限量子体系不应有相变, 所以 实际上对关联的消失应该是比较平缓的.…”

Mean field and beyond description of nuclear structure with the Gogny force: A review

“…In the same reference, level densities are also evaluated using the FT-HFB formalism. Thermal fluctuations and their effect of washing out the abrupt phase transitions observed at the mean field level are analyzed in [119] in a variety of systems. Finally, the evolution of the fission barrier heights with temperature is studied in the case of 240 Pu in Ref [120] where the decrease with temperature of the fission barrier heights is observed.…”

The (n,2n) reaction for the lightest stable erbium isotope Er162 from reaction threshold up to 19 MeV

“…The last implemented microscopic level density model (microscopic model 3) predicts simultaneously the single-particle levels and collective properties without contribution of any phenomenological potential effect. More specifically, for deformed nuclei their transition to spherical shape, as the excitation energy increases, is taken into account by temperature-dependent Hartree-Fock-Bogolyubov calculations [50]. This model was recommended by Hilaire et al [47].…”