Dipole Resonance In Hot Nuclei Research Articles

Shear-viscosity to entropy-density ratio from giant dipole resonances in hot nuclei

The Green-Kubo relation and fluctuation-dissipation theorem are employed to calculate the shear viscosity $\ensuremath{\eta}$ of a finite hot nucleus directly from the width and energy of the giant dipole resonance (GDR) of this nucleus. The ratio $\ensuremath{\eta}/s$ of shear viscosity $\ensuremath{\eta}$ to entropy density $s$ is extracted from the experimental systematics of the GDR in copper, tin, and lead isotopes at finite temperature $T$. These empirical results are then compared with the predictions by several independent models as well as with almost model-independent estimations. Based on these results, it is concluded that the ratio $\ensuremath{\eta}/s$ in medium and heavy nuclei decreases with increasing temperature $T$ to reach $(1.3\text{--}4)\ifmmode\times\else\texttimes\fi{}\ensuremath{\hbar}/(4\ensuremath{\pi}{k}_{B})$ at $T=5\phantom{\rule{0.28em}{0ex}}\mathrm{MeV}$.

Double giant dipole resonance in hot nuclei

Signals from Double Dipole Giant Resonances (DGDR) in hot nuclei have been searched in a γ-γ coincidence experiment using the HECTOR array at the Laboratori Nazionali di Legnaro. The experimental single γ-ray spectrum and the projection of the γ-γ matrix have been compared with a standard Monte Carlo Statistical Model code including only the single GDR excitation. These calculations have been used as background to determine the extra-yield associated with the DGDR de-excitation. Results have been compared with a previous experiment confirming the presence of the DGDR excitation in fusion-evaporation reactions.

Double giant dipole resonance in hot nuclei

Signals from Double Dipole Giant Resonances (DGDR) in hot nuclei have been searched in a γ-γ coincidence experiment using the HECTOR array at the Laboratori Nazionali di Legnaro. The experimental single γ-ray spectrum and the projection of the γ-γ matrix have been compared with a standard Monte Carlo Statistical Model code including only the single GDR excitation. These calculations have been used as background to determine the extra-yield associated with the DGDR de-excitation. Results have been compared with a previous experiment confirming the presence of the DGDR excitation in fusion-evaporation reactions.

Double giant dipole resonance in hot nuclei

Signals from Double Dipole Giant Resonances (DGDR) in hot nuclei have been searched in a γ-γ coincidence experiment using the HECTOR array at the Laboratori Nazionali di Legnaro. The experimental single γ-ray spectrum and the projection of the γ-γ matrix have been compared with a standard Monte Carlo Statistical Model code including only the single GDR excitation. These calculations have been used as background to determine the extra-yield associated with the DGDR de-excitation. Results have been compared with a previous experiment confirming the presence of the DGDR excitation in fusion-evaporation reactions.

Summary talk: Giant resonances at finite temperature

This summary talk on giant resonances at finite temperature focuses on changes in nuclear properties with excitation energy in both deformed and spherical nuclei. There is a discussion of new exclusive experiments which measure giant dipole gamma rays in coincidence with other reaction products as well as a review of the current theoretical understanding of the temperature dependence of the width of giant dipole resonances in hot nuclei.

Thermal damping width of the giant dipole resonance in hot nuclei

An approach describing the thermal damping width of the giant dipole resonance (GDR) in hot nuclei is presented. The GDR is generated by the ph excitations within the finite-temperature random-phase approximation (FTRPA), while its damping at finite temperature arises from irreversible coupling of ph configurations to the thermal pp and hh ones beyond the FTRPA. A semimicroscopic unification of the quantal spreading and thermal damping widths is undertaken within the framework of motional damping. The numerical calculations are performed, using a schematic model with equally degenerate equidistant shells for a hot nucleus of mass A=112 carrying no angular momentum. The results show that the total width of the GDR increases strongly as a function of the excitation energy up to E{sup {asterisk}}{approximately}120{endash}130 MeV, where it reaches a saturation value. The limiting temperature for the GDR in very hot nuclei is discussed. {copyright} {ital 1997} {ital The American Physical Society}

On the width of giant dipole resonance in hot nuclei

An approach describing the width of the giant dipole resonance in hot nuclei is proposed, unifying the microscopic thermal damping of multiquasiparticle states over the compound nucleus state and the quantal damping. The numerical calculations in the schematic model with equally degenerate equidistant shells for a hot nucleus of mass A = 110 carrying no angular momentum are performed which provide a good agreement with the behavior of the recent experimental data.

Isovector response function of hot nuclear matter with Skyrme interactions.

We investigate the role of the effective nucleon-nucleon interaction in the description of giant dipole resonances in hot nuclei. For this purpose we calculate the response function of hot nuclear matter to a small isovector external perturbation using various effective Skyrme interactions. We find that for Skyrme forces with an effective mass close to unity an undamped zero sound mode occurs at zero temperature. This mode gives rise in finite nuclei (calculated via the Steinwedel-Jenssen model) to a resonance whose energy agrees with the observed value. We find that zero sound disappears at a temperature of a few MeV, leaving only a broad peak in the dipole strength. For Skyrme forces with a small value of the effective mass (0.4), there is no zero sound at zero temperature but only a weak peak located too high in energy. The strength distribution in this case is nearly independent of temperature and shows small collective effects. The relevance of these results for the saturation of photon multiplicities observed in recent experiments is pointed out.

Increase in width of the giant dipole resonance in hot nuclei: Shape change or collisional damping?

The strength function and the angular distribution of the high energy \ensuremath{\gamma} rays emitted by the giant dipole resonance (GDR) in hot rotating ${}^{109,110}$Sn nuclei have been measured at temperature $T\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}1.8\mathrm{MeV}$ and at four values of the angular momentum $I$. The GDR width is $\ensuremath{\approx}2$ times larger than at $T\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}0$ and increases by $\ensuremath{\approx}20%$ as $I$ goes from 40 to $54\ensuremath{\Elzxh}$. The ${a}_{2}({E}_{\ensuremath{\gamma}})$ increases by a factor of $\ensuremath{\approx}2$. Based on these two facts and on the comparison with theory we conclude that large deformations driven by $I$ and combined with shape and orientation fluctuations are responsible for the measured increases. Collisional damping is constant and practically equal to the $T\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}0$ case.

Evidence for different time scales controlling thermal fluctuations in hot nuclei.

A comparison between the predictions of a theoretical model describing the giant dipole resonance in hot nuclei, which includes the coupling to time-dependent thermal fluctuations of the nuclear surface, and experimental data on the cross section and the angular distribution associated with the dipole decay of {sup 92}Mo{sup *} is presented. Effects of different time scales for fluctuuations in the deformation and orientation degrees of freedom are observed.

The giant dipole resonance in hot nuclei

A consistent picture of the giant dipole resonance in hot nuclei is presented, in terms of general arguments of sum rules, and of the interplay with the fluctuations of the nuclear surface. The same concepts are useful to understand the optical response of metal clusters.

Limiting temperature for the existence of collective motion in hot nuclei.

Experimental evidence testifying to the existence of a limiting temperature for the observation of gamma decay from the giant dipole resonance in hot nuclei is interpreted as indicating a remarkable constancy with temperature of the width with which the giant resonance couples to the compound nucleus.

Time-dependent fluctuations and the giant dipole resonance in hot nuclei: Solvable models

A recent macroscopic theory of time-dependent shape fluctuations in hot nuclei and their effects on the giant dipole resonance is investigated in the context of solvable models with one quadrupole shape degree of freedom. Using the framework of the Landau theory of shape transitions, both the quadrupole shape and the giant dipole degrees of freedom are described by a coupled set of stochastic equations. Two solvable models for which the dipole correlation function is found in closed form are discussed; one for a spherical nucleus and one for a deformed nucleus. The adiabatic and sudden limits of the models are examined. The latter limit is shown to produce a phenomenon known as motional narrowing. For the more general cases we introduce Monte Carlo techniques and test them against the solvable models.

Time-dependent shape fluctuations and the giant dipole resonance in hot nuclei: Realistic calculations

The effects of time-dependent shape fluctuations on the giant dipole resonance (GDR) in hot rotating nuclei are investigated. Using the framework of the Landau theory of shape transitions we develop a realistic macroscopic stochastic model to describe the quadrupole time-dependent shape fluctuations and their coupling to the dipole degrees of freedom. In the adiabatic limit the theory reduces to a previous adiabatic theory of static fluctuations in which the GDR cross section is calculated by averaging over the equilibrium distribution with the unitary invariant metric. Nonadiabatic effects are investigated in this model and found to cause structural changes in the resonance cross section and motional narrowing. Comparisons with experimental data are made and deviations from the adiabatic calculations can be explained. In these cases it is possible to determine from the data the damping of the quadrupole motion at finite temperature.

Excitation energy dependence of the giant dipole resonance in hot nuclei

Abstract High energy γ-rays were measured with a large BaF2 detector in coincidence with fusion residues produced in the 92 Mo+ 40 Ar reactions at E A=21 and 26 MeV . The obtained γ-ray spectra were compared with statistical calculations for γ-ray transitions from highly excited nuclei. Contributions of bremsstrahlung in the early stages of the reaction were subtracted. The evolution of the GDR γ-ray yields and the shape of the γ-ray spectra are well reproduced by calculations in which the width of the GDR increases with the excitation energy.

Angular momentum splitting and spin dependence of the giant dipole resonance in hot nuclei

High energy γ-rays from the decay of the electric giant dipole resonance (GDR) in the reaction 130 Te( 34 S,xn) 164− x Er at E lab = 156 MeV have been measured as a function of the γ-ray multiplicity using the Darmstadt-Heidelberg crystal ball. From the analysis of the γ-angular distributions the γ-rays with E γ ≥ 4 MeV were found to be consistent with pure dipole radiation. Moreover, in the region of the GDR pronounced anisotropies in the γ-angular distributions are observed, proving an angular momentum splitting of the giant resonance into the spin components with Δ I = 0 and Δ I = ± 1. A consistent description of both the angular distributions and energy spectra can only be obtained (i) by assuming the nuclei in the spin- and excitation energy region studied ( I ≥30 h ̵ , E ∗ ≈ 50 MeV ) to have an oblate deformation with the spin axis pointing in the direction of the symmetry axis ( γ = -60°) and (ii) by allowing the centroid energy E R to decrease with increasing spin.

Landau theory of shapes, shape fluctuations and giant dipole resonances in hot nuclei

Universal features of evolution of the equilibrium nuclear shapes with temperature and angular momentum are predicted by the Landau theory of nuclear shape transitions. The general dependence of the nuclear free energy on the deformation given by this theory also provides a unified description of thermal fluctuations of all quadrupole degrees of freedom. Using this unified theory we calculate the giant dipole absorption by hot rotating nuclei and investigate its systematics as a function of nuclear spin and temperature. Direct comparison with experimental data is presented.