Search

The saddle-to-scission dynamics of the induced fission process is explored using a microscopic finite-temperature model based on time-dependent nuclear density functional theory (TDDFT), that allows to follow the evolution of local temperature along fission trajectories. Starting from a temperature that corresponds to the experimental excitation energy of the compound system, the model propagates the nucleons along isentropic paths toward scission. For the four illustrative cases of induced fission of $^{240}$Pu, $^{234}$U, $^{244}$Cm, and $^{250}$Cf, characteristic fission trajectories are considered, and the partition of the total energy into various kinetic and potential energy contributions at scission is analyzed, with special emphasis on the energy dissipated along the fission path and the prescission kinetic energy. The model is also applied to the dynamics of neck formation and rupture, characterized by the formation of few-nucleon clusters in the low-density region between the nascent fragments., Comment: 31 pages, 10 figures

Effects of rotation and valence nucleons in molecular linear α- chain nuclei are analyzed using a three dimensional lattice cranking model based on covariant density functional theory. The structure of the mirror nuclei ^16C and ^16Ne is investigated as a function of rotational frequency. The valence nucleons, with respect to the 3α linear chain core of ^12C, at low frequency occupy the π molecular orbital. With increasing rotational frequency these nucleons transition from the π orbital to the σ molecular orbital, thus stabilizing the 3α linear chain structure. It is predicted that the valence protons in 16Ne change occupation from the π to the σ molecular orbital at ħω ≈ 1.3 MeV, a lower rotational frequency compared to ħω ≈ 1.7 MeV for the valence neutrons in 16C. The same effects of valence protons are found in ^20Mg, compared to the four valence neutrons in ^20O. The model is also used to examine the effect of alignment of valence nucleons on the relative positions and size of the three α clusters in the mirror nuclei ^16C and ^16Ne.

The dynamics of low-energy induced fission is explored using a consistent microscopic framework that combines the time-dependent generator coordinate method (TDGCM) and time-dependent nuclear density functional theory (TDDFT). While the former presents a fully quantum mechanical approach that describes the entire fission process as an adiabatic evolution of collective degrees of freedom, the latter models the dissipative dynamics of the final stage of fission by propagating the nucleons independently toward scission and beyond. By combining the two methods, based on the same nuclear energy density functional and pairing interaction, we perform an illustrative calculation of the charge distribution of yields and total kinetic energy for induced fission of $^{240}$Pu. For the saddle-to-scission phase a set of initial points for the TDDFT evolution is selected along an iso-energy curve beyond the outer fission barrier on the deformation energy surface, and the TDGCM is used to calculate the probability that the collective wave function reaches these points at different times. Fission observables are computed with both methods and compared with available data. The relative merits of including quantum fluctuations (TDGCM) and the one-body dissipation mechanism (TDDFT) are discussed., Comment: 7 pages,6 figures

Efficient and accurate algorithms for the calculation of the multipole response of deformed atomic nuclei are very important tools in nuclear structure, especially for large scale calculations. In this paper we present an implementation of the algorithm based on the expansion of the response function in terms of the Chebyshev polynomials in conjunction with the kernel polynomial method for a very efficient calculation of the quasiparticle random phase approximation response function. Several test calculations are performed in order to asses the applicability and feasibility of this algorithm, already used successfully in the field of computational solid state physics, in various nuclear structure calculations.

A microscopic calculation of half-lives for both the α and 2α decays of ^{212}Po and ^{224}Ra is performed, using a self-consistent framework based on energy density functionals. A relativistic density functional and a separable pairing interaction of finite range are used to compute axially symmetric deformation energy surfaces as functions of quadrupole, octupole, and hexadecapole collective coordinates. Dynamical least-action paths are determined, that trace the α and 2α emission from the equilibrium deformation to the point of scission. The calculated half-lives for the α decay of ^{212}Po and ^{224}Ra are in good agreement with data. A new decay mode, the symmetric 2α emission, is predicted with half-lives of the order of those observed for cluster emission.

The finite-temperature linear response theory based on the finite-temperature relativistic Hartree-Bogoliubov (FT-RHB) model is developed in the charge-exchange channel to study the temperature evolution of spin-isospin excitations. Calculations are performed self-consistently with relativistic point-coupling interactions DD-PC1 and DD-PCX. In the charge-exchange channel, the pairing interaction can be split into isovector ($T = 1$) and isoscalar ($T = 0$) parts. For the isovector component, the same separable form of the Gogny D1S pairing interaction is used both for the ground-state calculation as well as for the residual interaction, while the strength of the isoscalar pairing in the residual interaction is determined by comparison with experimental data on Gamow-Teller resonance (GTR) and Isobaric analog resonance (IAR) centroid energy differences in even-even tin isotopes. The temperature effects are introduced by treating Bogoliubov quasiparticles within a grand-canonical ensemble. Thus, unlike the conventional formulation of the quasiparticle random-phase approximation (QRPA) based on the Bardeen-Cooper-Schrieffer (BCS) basis, our model is formulated within the Hartree-Fock-Bogoliubov (HFB) quasiparticle basis. Implementing a relativistic point-coupling interaction and a separable pairing force allows for the reduction of complicated two-body residual interaction matrix elements, which considerably decreases the dimension of the problem in the coordinate space. The main advantage of this method is to avoid the diagonalization of a large QRPA matrix, especially at finite temperature where the size of configuration space is significantly increased. The implementation of the linear response code is used to study the temperature evolution of IAR, GTR, and spin-dipole resonance (SDR) in even-even tin isotopes in the temperature range $T = 0 - 1.5$ MeV., 20 pages, 9 figures, submitted for publication

Lifetime measurements of excited states of the light N = 52 isotones 88 Kr , 86 Se , and 84 Ge have been performed, using the recoil distance Doppler shift method and VAMOS and AGATA spectrometers for particle identification and gamma spectroscopy, respectively. The reduced electric quadrupole transition probabilities B ( E 2 ; 2 + → 0 + ) and B ( E 2 ; 4 + → 2 + ) were obtained for the first time for the hard-to-reach 84 Ge . While the B ( E 2 ; 2 + → 0 + ) values of 88 Kr , 86 Se saturate the maximum quadrupole collectivity offered by the natural valence ( 3 s , 2 d , 1 g 7 / 2 , 1 h 11 / 2 ) space of an inert 78 Ni core, the value obtained for 84 Ge largely exceeds it, suggesting that shape coexistence phenomena, previously reported at N ≲ 49 , extend beyond N = 50 . The onset of collectivity at Z = 32 is understood as due to a pseudo-SU(3) organization of the proton single-particle sequence reflecting a clear manifestation of pseudospin symmetry. It is realized that the latter provides actually reliable guidance for understanding the observed proton and neutron single particle structure in the whole medium-mass region, from Ni to Sn, pointing towards the important role of the isovector-vector ρ field in shell-structure evolution.

The structure of low-energy collective states in the neutron-rich nucleus 44S is analyzed using a microscopic collective Hamiltonian model based on energy density functionals (EDFs). The calculated triaxial energy map, low-energy spectrum and corresponding probability distributions indicate a coexistence of prolate and oblate shapes in this nucleus.

A microscopic analysis, based on nuclear energy density functionals, is presented for shape phase transitions in Nd isotopes. Low‐lying excitation spectra and transition probabilities are calculated starting from a five‐dimensional Hamiltonian, with parameters determined by constrained relativistic mean‐field calculations for triaxial shapes. The results reproduce available data, and show that there is an abrupt change of structure at N = 90, that corresponds to a first‐order quantum phase transition between spherical and axially deformed shapes.

An implementation of the relativistic random phase approximation with the proper treatment of the continuum has been developed for the relativistic point coupling model and applied to investigate collective excitations in spherical nuclei. The results are compared with the spectral implementation of the same model. In heavy nuclei, where the escape width is negligible, we find an excellent agreement between both methods in the region of giant resonance and some discrepancies in the region of low‐lying pygmy resonance. The differences are more pronounced in light nuclei due to the larger values of the escape widths.

The framework of relativistic energy density functionals is extended to include correlations related to the restoration of broken symmetries and to fluctuations of collective variables. The generator coordinate method is used to perform configuration mixing of angular‐momentum and particle‐number projected relativistic wave functions. This approach enables a quantitative description of the evolution of shell‐structure, deformation and shape coexistence phenomena in nuclei with soft potential energy surfaces, and singular properties of excitation spectra and transition rates at critical points of quantum shape phase transitions.

A systematic, fully self‐consistent calculation of β‐decay rates is presented, based on a microscopic theoretical framework. Analysis is performed on a large number of nuclei from the valley of β stability towards the neutron drip‐line. Nuclear ground state is determined using the Relativistic Hartree‐Bogoliubov (RHB) model with density‐dependent meson‐nucleon coupling constants. Transition rates are calculated within the proton‐neutron relativistic quasiparticle RPA (pn‐RQRPA) using the same interaction that was used in the RHB equations.

The standard relativistic mean-field density functionals based on non-linear meson exchange terms are extended to include density dependent meson-nucleon coupling constants. Special care is taken for the density dependence in the isovector channel. This provides not only an improved description of the equation of state for neutron matter and asymmetric nuclear matter but also for isovector properties of finite nuclei far from stability such as the neutron skin thickness. In particular it improves nuclear binding energies considerably as compared earlier applications of relativistic density functional theory to nuclear mass tables. An average root mean square deviation of 900 keV is found.