Giant Dipole Resonance Width Research Articles

Microscopic description of the hot giant dipole resonance's shape evolution

A systematic description of the evolution of the giant dipole resonance (GDR) at non-zero temperature T is presented within the Phonon Damping Model (PDM). The damping of the GDR is caused by coupling to all ph, pp and hh configurations at T ≠ 0, where the coupling to pp and hh configurations is responsible for the increase and saturation of the GDR width as a function of T. The results of numerical calculations for the GDR width, the strength function and the integrated yield of γ rays in 120Sn and 208Pb at 0 ≤ T ≤ 6 MeV are obtained in a reasonable agreement with the most recent experimental data.

Giant Dipole Resonance in Highly Excited Nuclei: Does the Width Saturate?

We examine the behavior of the width of the giant dipole resonance (GDR) at high excitation energies and show, based on new detailed measurements together with a reanalysis of previous experimental results, that the GDR width in Sn and nearby mass compound nuclei continues to increase up to final-state temperatures $T\ensuremath{\sim}3.2\mathrm{MeV}$. These temperatures correspond to the highest energies at which the GDR width can be extracted reliably from existing data.

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.

Giant dipole resonances in hot rotating nuclei: Overview and recent advances

The giant dipole resonance (GDR) built on highly excited states is an important experimental tool in understanding the properties of hot nuclei. A new generation of experiments allows a more detailed study of the dependence of the GDR on temperature and spin. We review the macroscopic theory of the GDR at finite temperature, which is based on a mean-field theory and an exact treatment of thermal shape fluctuations. The systematics of the GDR width as a function of temperature, spin and mass is discussed and compared with experimental data.

Systematics of the nuclear giant dipole resonance

We study the systematics of the giant dipole resonance width Γ in hot rotating nuclei as a function of temperature T, spin J and mass A. We compare available experimental results with theoretical calculations that include thermal shape fluctuations in nuclei ranging from A = 45 to A = 208. Using the appropriate scaled variables, we find a simple phenomenological function Γ( T, J, A) which approximates the global behavior of the giant dipole resonance width in the liquid drop model. We reanalyze recent experimental and theoretical results for the resonance width in Sn isotopes and 208Pb.

Shape evolution of the hot giant dipole resonance

A systematic description of the evolution of the giant dipole resonance (GDR) at non-zero temperature T is given within the framework of two versions PDM-1 and PDM-2 of the Phonon Damping Model (PDM). The PDM interprets the damping of the GDR as a result of coupling to all ph, pp and hh configurations at T ≠ 0, where the coupling to pp and hh configurations is decisively important for an adequate description of the increase and saturation of the GDR width as a function of T. The numerical calculations have been performed for the GDR width, the strength function and the integrated yield of the γ rays in 120Sn and 208Pb at 0 ⩽ T ⩽ 6 MeV. The results obtained are found in a reasonable agreement with the most recent experimental data for all these three characteristics including the saturation of the yields within the GDR region and in the region above it. Predictions have been made for the GDR shape in both nuclei at T up to 6 MeV and for the integrated yield of γ rays in 208Pb.

Damping of hot giant dipole resonance due to complex configuration mixing

An approach is presented to study the width of the giant dipole resonance (GDR) at non-zero temperature T, which includes all forward-going processes up to two-phonon ones. Calculations are performed in 120 Sn and 208 Pb. An overall agreement between theory and experiment is found for the GDR width and shape. The total width of the GDR due to coupling of the GDR phonon to all ph, pp and hh configurations increases sharply as T increases up to T∼3 MeV and saturates at T∼4–6 MeV. The quantal width Γ Q due to coupling to ph configurations decreases with increasing T. It is almost independent of T if the contribution of two-phonon processes at T≠ 0 is omitted.

Scaling Properties of the Giant Dipole Resonance Width in Hot Rotating Nuclei

We study the systematics of the giant dipole resonance width $\Gamma$ in hot rotating nuclei as a function of temperature $T$, spin $J$ and mass $A$. We compare available experimental results with theoretical calculations that include thermal shape fluctuations in nuclei ranging from A=45 to A=208. Using the appropriate scaled variables, we find a simple phenomenological function $\Gamma(A,T,J)$ which approximates the global behavior of the giant dipole resonance width in the liquid drop model. We reanalyze recent experimental and theoretical results for the resonance width in Sn isotopes and $^{208}$Pb.

Temperature dependence of quantal and thermal dampings of the hot giant dipole resonance

A systematic study of the damping of the giant dipole resonance (GDR) in 90Zr, 120Sn and 208Pb as a function of temperature T is performed. The double-time Green function technique is employed to determine the single-particle and GDR dampings. The single-particle energies, obtained in the Woods-Saxon potential for these nuclei, are used in the calculations. The results show that the coupling of collective vibration to the pp and hh excitations, which causes the thermal damping width, is responsible for the enlargement of the total width with increasing temperature up to T ≈ 3MeV and its saturation at higher temperatures. The quantal width, which arises from the coupling of the collective mode to the ph excitations decreases slowly with increasing temperature. The effect of single-particle damping on the GDR width is small. The results are found in an overall agreement with the experimental data for the GDR width, obtained in the inelastic α scattering and heavy-ion fusion reactions at excitation energies E* ⩽ 450 MeV. At high excitation energies (E* > 400 MeV) a behavior similar to the transition from zero to ordinary sounds is observed.

Quantal and Thermal Dampings of Giant Dipole Resonances in90Zr,120Sn, and208Pb

The damping of the giant dipole resonance (GDR) is studied in ${}^{90}\mathrm{Zr}$, ${}^{120}\mathrm{Sn}$, and ${}^{208}\mathrm{Pb}$ as a function of temperature $T$. The results show that the coupling of GDR to the $\mathrm{pp}$ and $\mathrm{hh}$ 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 $\mathrm{ph}$ 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 $\ensuremath{\alpha}$ scattering. At very high $T$ a behavior similar to the transition from zero to ordinary sounds is observed.

Angular momentum dependence of the GDR width in Sn nuclei at fixed excitation energy

High-energy γ-rays from the decay of the giant-dipole resonance (GDR) in the hot 106Sn compound nucleus and its daughters were measured in coincidence with heavy recoiling evaporation residues. The compound nucleus was formed at excitation energy E ∗ = 80 MeV using the reaction 56Ni+ 48Ti at a bombarding energy of 260 MeV. The analysis yields the GDR width for two different intervals of angular momentum 〈 J〉 = 24 and 36ℏ. The present data, combined with previous data at higher angular momentum permit a study of the angular momentum dependence of the GDR width for 10 ⩽ J ⩽ 60ℏ at approximately fixed temperature. The width of the GDR is found to be roughly constant for J < 35ℏ, increasing rapidly for higher angular momenta. The data are found to be in good agreement with theoretical calculations within an adiabatic model describing thermal fluctuations of the nuclear shape. The model also reproduces the much weaker angular momentum dependence of the GDR width in the heavier nucleus 176W.

Temperature dependence of the giant dipole resonance width in 208Pb

The evolution of the giant dipole resonance (GDR) in hot 208Pb nuclei was studied by measuring high energy γ-rays from the decay of the resonance built on excited states. Nuclei in the excitation energy range of 40–110 MeV were populated by inelastic scattering of 40 MeV/nucleon α-particles. The GDR width was observed to increase systematically from 4 MeV at the ground state to ∼8 MeV at the highest excitation energy.

Probing the giant dipole resonance in the compound nucleus 156Er

The giant dipole resonance (GDR) from the decay of excited 156Er nuclei, populated in the reaction 241 MeV 64Ni + 92Zr, has been studied employing a high-resolution 4π γ-ray spectrometer. High-energy γ-ray spectra have been measured in coincidence with both the spectrometer inner ball and the Ge detectors selecting discrete γ-transitions in the residual nuclei 155,154Er. The analysis of the high-energy γ-ray spectra shows that the GDR can be reproduced by a single-lorentzian fit using standard parameters for strength, energy and width, only when a fold k > 10 is required in the inner ball. On the contrary, with lower-fold conditions (k ≤ 10) unrealistic GDR parameters are obtained, mainly due to the presence of light target contaminants. The GDR width determined for k > 10 compares well with systematics and with predictions of adiabatic calculations. The line-shape analysis of the γ-ray spectrum associated with a single evaporation residue (154Er) yields GDR parameters similar to those obtained for the high-fold selected spectra, with the exception of a slightly larger width. No signature is therefore observed for a pre-equilibrium component in the decay of 156Er populated in a nearly symmetric reaction. Nevertheless, differences are evidenced in the comparison of the present spectrum from the 64Ni + 92Zr reaction with earlier results from the 12C + 144Sm asymmetric system.

The Giant Dipole Resonance in hot Sn nuclei

Abstract We have studied the Giant Dipole Resonance (GDR) built on the excited states of 112Sn formed in the fusion of 19F with 93Nb. The excitation energy of the compound nucleus 112Sn was 130 and 152 MeV. High-energy γ-rays were detected in a 7-element BaF2 cluster and 4 NaI(Tl) scintillators. Coincidences with the evaporation residues detected in two parallel-plate avalanchecounters ensured that the γ-rays were associated with fusion events. In order to study the characteristics of the GDR, we have performed analyses of the γ-ray spectra within the statistical model, using a continuous evolution of the GDR centroid energy with the mass and the temperature of the emitting nucleus, and a nuclear temperature dependent level density parameter. The theoretical prescriptions concerning the evolution of the GDR width with excitation energy have been tested. We show that, in order to reproduce the γ-spectra, the increase of the width must be more rapid than the two theoretical predictions. The width has to reach large values, of the order of 15 to 20 MeV above 100 MeV of excitation energy. These values are greater than the saturation value of about 12 MeV which has been published following the analyses using constant parameters. On this condition, the shape of the spectra can be reproduced with or without a saturation of the width. It is therefore not possible to say if the increase of the width is due to angular momentum effects or to collisional effects.

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.

Thermal fluctuation and collisional damping effects in the GDR observables in hot rotating 176W nuclei

The strength function and the angular distribution of the high energy γ-rays emitted by the giant dipole resonance (GDR) in hot rotating 176W nuclei have been measured at temperature T ≈ 1.4 MeV and at 4 values of the angular momentum I in the interval I = 35–55, ħ. The measured GDR width is found not to change when the angular momentum is varied. An increase by a factor of ≈ 2 is measured for the a 2( E γ ) in the same I interval. The comparison of the data to model predictions shows that the collisional damping width is independent of temperature and rotational frequency and that thermal shape and orientation fluctuations are responsible for the measured widths and angular anisotropies.

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.

Influence of the compound-nucleus lifetime on the observed width of hot giant dipole resonances

We discuss the fact that the total width of the photo-absorbtion peak must contain twice the evaporation width of the hot nucleus. Indeed, the observed γ-rays are related with transitions between two levels of the compound nucleus which have a width due to the finite lifetime of the system. This novel physical effect must be taken into account in the discussion of the properties of the photo-absorbtion strength. We discuss how it modifies the predicted total width of giant resonances at moderate excitation energy and we stress that it may play a role in the observed suppression of the GDR at high excitation energy.

Saturation of the width of the giant dipole resonance at high excitation energies

Gamma-ray energy spectra have been measured for the 16O + 118Sn reaction at incident beam energies of 200 and 280 MeV. Comparing the experimentally obtained spectra with statistical-model calculations shows that the width of the isovector giant dipole resonance saturates beyond excitation energies of 165 MeV in the compound nucleus.

Width and Strength of the hot Giant Dipole Resonance: the role of the life time of the compound nucleus and the Transition from order to chaos.

In this article we investigate the role of the particle evaporation on the properties of the Giant Dipole Resonance built on excited states. We discuss two phenomena. The first one is based on the idea that the finite life time of the compound nucleus gives a finite width for any of the compound nucleus levels. Consequently, the γ-ray spectrum coming from the transitions between such two levels must exhibit a broadening characterized by a width equal to twice the compound nucleus width. This leads to the prediction of a fast increase of the width of the observed GDR transitions which is compatible with the data and may partly explain the observed saturation of the photon yield. The second new physical effect is based on the fact that each individual particle emission induces a strong fluctuation of the dipole moment of the nucleus. Therefore, the description of the dipole collective variable is akin to the problem of a brownian motion in a harmonic potential. One can conclude that when the time between two particle emissions becomes comparable to the period of the harmonic oscillation, the transition between order and chaos is reached and the collective vibration is suppressed. From this simple argument a quenching factor can be deduced and compared with existing suppression factors.