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The multipole response of nuclei at temperatures T = 0 − 2 MeV is studied using a self-consistent finite-temperature RPA (random phase approximation) based on relativistic energy density functionals. Illustrative calculations are performed for the isoscalar monopole and isovector dipole modes and, in particular, the evolution of low-energy excitations with temperature is analyzed, including the modification of pygmy structures. Both for the monopole and dipole modes, in the temperature range T = 1 −2 MeV additional transition strength appears at low energies because of thermal unblocking of single-particle orbitals close to the Fermi level. A concentration of dipole strength around 10 MeV excitation energy is predicted in 60,62 Ni, where no low-energy excitations occur at zero temperature. The principal effect of finite temperature on low-energy strength that is already present at zero temperature, e.g. in 68 Ni and 132 Sn, is the spreading of this structure to even lower energy and the appearance of states that correspond to thermally unblocked transitions.

The problem of the damping of the Giant Dipole Resonance (GDR) at finite temperature at T>2 MeV is discussed here. The experimental results are based on fusion evaporation reactions. The most recent results on the mass region A = 132 ( Ce isotopes) obtained in exclusive measurements are compared with the existing results on the A = 110–120 region ( Sn isotopes). The comparison with the theoretical predictions based on thermal shape fluctuations is also discussed. The GDR width is found to increase also in the region T>2 MeV and this is accounted by the combined effect of the increase of the compound nucleus width (smaller lifetime) and to the increase of the average deformation of the nucleus.

Level density parameters of 208 Pb and 91 Zr in equilibrium states have been calculated with a new fugacity approximation namely a thermal wavelength approximation. In this approximation, the fugacity is directly proportional to the nucleon density. In contrast with the constant fugacity, the thermal wavelength approximation gives a simpler way to calculate the nuclei constant radius and the density profile. The calculated 208 Pb nuclear density is about 0.17 (fm -3 ) in which the discrepancy is 0.1% higher than the experimental one. The level density parameters are 14% higher than the experimental results due to neglecting of the shell correction.

The thermal evolution of the energies and free energies of a set of spherical and near-spherical nuclei spanning the whole periodic table are calculated in the subtracted finite-temperature Thomas- Fermi framework with the zero-range Skyrme-type KDE0 and the finite-range modified Seyler-Blanchard interaction. The calculated energies are subjected to a global fit in the spirit of the liquid-drop model. The extracted parameters in this model reflect the temperature dependence of the volume symmetry and surface symmetry coefficients of finite nuclei, in addition to that of the volume and surface energy coefficients. The temperature dependence of the surface symmetry energy is found to be very substantial whereas that of the volume symmetry energy turns out to be comparatively mild., Comment: 6 page including 5 figures, Phys. Lett. B (in press)

Peripheral collisions give access to a set of events where hot quasi-projectile (QP) sources are produced exploring a large range of excitation energy. In this range evaporation and multifragmentation are both observed for a similar charge/size of the sources. In this work, this experimental fact will be described in terms of coexistence of two classes of events in the first order phase transition formalism. For this, a detailed study of the experimental correlation between the size/charge of the biggest cluster/fragment (Z1) and the excitation energy (E*) is made. Making the parallel with the same correlation derived from the canonical ensemble description, a first value of the latent heat and boundaries of spinodal and coexistence zone are extracted., Comment: 7 pages

It is matter of a critical methodological study of the calorimetry and the thermometry of hot nuclei produced by collisions Xe+ Sn between 25 and 100 MeV/u. The apparatus of hot nucleus physical characterization is the four pi detector array INDRA. This study was made by using the event generators GEMINI, SIMON and HIPSE and a data-processing filter simulating the complete operation of the multidetector. An alternative method of calorimetry was proposed and partially validated with the help of th event generator HIPSE., Il s'agit d'une étude méthodologique critique de la calorimétrie et de la thermométrie de noyaux chauds formés lors de collisions Xe+ Sn entre 25 et 100 MeV/u. L'instrument de caractérisation physique des noyaux chauds est le multidétecteur 4 pi INDRA. Cette étude a été faite en utilisant les générateurs d'événements GEMINI, SIMON et HIPSE et un filtre informatique simulant le fonctionnement complet du multidétecteur.Une méthode alternative de calorimétrie a été proposée et partiellement validée à l'aide du générateur d'événements HIPSE.

Measured hard photon multiplicities from second-chance nucleon-nucleon collisions are used in combination with a kinetic thermal model, to estimate the break-up times of excited nuclear systems produced in nucleus-nucleus reactions at intermediate energies. The obtained nuclear break-up time for the $^{129}${Xe} + $^{nat}${Sn} reaction at 50{\it A} MeV is $\Delta$$\tau$ $\approx$ 100 -- 300 fm/$c$ for all reaction centralities. The lifetime of the radiating sources produced in seven other different heavy-ion reactions studied by the TAPS experiment are consistent with $\Delta$$\tau$ $\approx$ 100 fm/$c$, such relatively long thermal photon emission times do not support the interpretation of nuclear breakup as due to a fast spinodal process for the heavy nuclear systems studied., Comment: 11 pages, 9 figures, submitted to EPJA

The S-32 + Mo-100 and S-36 + Mo-96 fusionlike reactions were studied at incident energy of E-lab = 298 MeV and 320 MeV, respectively, with the aim of probing the influence of the entrance channel charge asymmetry on the dipole gamma-ray emission. The excitation energy and spin distribution of the compound nucleus created in these reactions were identical, the only difference being associated with the unequal charge asymmetry of the two entrance channels. High-energy gamma-rays were detected in an array of 9 seven-pack BaF2 clusters. Coincidence with fusionlike residues detected in four PPAC ensured the selection of central reaction events. By studying the differential gamma-ray multiplicity associated with the two reactions it was shown that the dipole strength excited in the compound nucleus increases with the entrance channel charge asymmetry. From the linearized spectra, the increase of the GDR gamma-ray intensity was found to be similar to25% for the more charge asymmetric system. The results are discussed and compared with those of previous data obtained at different incident energies.

The fragment yields from the multifragmentation of gold, lanthanum and krypton nuclei obtained by the EOS Collaboration are examined in terms of Fisher's droplet formalism modifed to account for Coulomb energy. The critical exponents sigma and tau and the surface energy coefficient c0 are obtained. Estimates are made of the pressure-temperature and temperature-density coexistence curves of finite neutral nuclear matter as well as the location of the critical point., Comment: 14 pages, 13 figures, 6 tables, submitted to Phys. Rev. C

This work is devoted to the description of both quantal and statistical aspects in a coherent theoretical framework that goes beyond a mean-field approximation. It is shown that among the possible methods, the Extended mean-field theory (ETDHF) is able to describe relaxation by including correlation effects. This theory is first applied to cold and hot giant resonances. After having developed a extended version of the linear response theory the effect of collisions on the response of 40Ca is studied. It is shown that collisions contribute to the spreading width of giant resonances and become a dominant decay channel at high temperature. The generalization of these methods through the modelization of the Hamiltonian as a series of couplings in cascade has enabled to go beyond ETDHF. Thus, phenomena as multi-scale Ericson fluctuations and interferences in the nuclear response are predicted. The study of larger amplitudes requires the introduction of transport theories for the nuclear dynamics. Thus the deexcitation of hot and compressed nuclei has been studied with mean-field theories. This analysis has clarified the link between the equation of state and the dynamical expansion in finite Systems. A comparison with semi-classical has further shown the importance of quantum effects and has demonstrated the necessity to take into account nucleon collisions in a quantum framework. The possibility to apply ETDHF to nuclear dynamics in the general case is then discussed. A rapid and reliable method, tested on a schematic model is proposed to solve the extended mean-field. The application of ETDHF to the monopolar expansion of 16O has finally demonstrated the interesting perspectives of this theory for describing dissipative aspects in quantum dynamics. Finally, the introduction of quantum stochastic approaches is discussed., Ce mémoire traite de la possibilité de reunir a la fois les aspects quantiques et statistiques dans un même cadre théorique en allant au de la du champ moyen. Il est montre que parmi les méthodes possibles, la théorie de champ moyen étendu (ETDHF) en incluant les effets des corrélations a deux corps, permet une description des phénomènes de relaxation. Cette théorie a tout d'abord été appliquée a la décroissance des résonances géantes chaudes et froides. Après avoir développe une version étendu de la théorie de réponse linéaire (RPA etendue), l'effet des collisions sur la réponse collective du 40Ca a été étudie. En particulier, il a été montré que ces collisions contribuent a la largeur d’étalement des résonances géantes, en devenant une voie de décroissance dominante a haute température. La généralisation de ces méthodes à travers une modélisation du hamiltonien nucléaire comme une série de couplages en cascade a permis d'aller au delà de ETDHF. Ainsi, des phénomènes tels que les fluctuations d'Ericson a plusieurs échelles ou des interférences dans la réponse sont prédits. L’étude des mouvements de plus grandes amplitudes requiert l'introduction des théories de transport pour la dynamique des noyaux. Ainsi, le champ moyen a permis l’étude de la désexcitation des noyaux chauds et comprimes. Cette étude a clarifié le lien entre l’équation d’état et la dynamique d'expansion dans les systèmes finis. Une comparaison avec les approches semi-classiques a de plus démontré l'importance des effets quantiques. Il est donc nécessaire de prendre en compte les collisions entre nucléons dans un cadre quantique. La possibilité d'appliquer ETDHF a la dynamique nucléaire dans un cadre général a finalement été discutée. Une méthode fiable et rapide, testée avec succès sur un modèle schématique, a été proposée pour résoudre le champ moyen étendu dans la dynamique des noyaux. L'utilisation de ETDHF pour l’étude de l'expansion monopolaire d'un noyau d'16O a finalement montré les perspectives intéressantes d'une telle théorie en décrivant les aspects dissipatifs dans une dynamique quantique. L'introduction d'une dynamique stochastique quantique allant au delà de ETDHF a finalement été discutée.

Central Au+Au collisions at 35 A MeV are analyzed to find the characteristics of a thermal source, in the framework of the statistical multifragmentation model SMM. A recently introduced backtracing protocol provides an effective comparison of theory and experiment. For the first time the distributions of the central source parameters (density, mass number, excitation energy) are found. The collective energy of primary fragments is also deduced. It is shown that the backtracing procedure allows an estimation of the pre-equilibrium emission. (C) 1998 Published by Elsevier Science B.V.

Hot nuclei are formed in heavy ion collisions covering the Fermi energy domain. According to the excitation energy deposited into these nuclei, several de-excitation processes can be observed, in particular the emission of complex fragments (Z ≥ 3) which remains poorly understood. The GANIL facility offers the possibility to cover the excitation function for the Ar on Ni reaction over a broad energy range from 32 to 95 MeV/u where the hot nuclei evolve from classical “evaporation” to complete “vaporization” into light particles (neutrons,isotopes of H, He). The study of reaction mechanisms shows that from peripheral to central collisions the total cross section is dominated by binary dissipative collisions. Both partners coming from well-characterized events with the INDRA detector are reconstructedusing the “Minimum Spanning Tree” method. Thus excitation energy up to 20 MeV/A are reached in the most violent collisions at the highest bombarding energy. The deposited energy is not shared in the mass ratio between the quasi-projectile and the quasi-target,the quasi-projectile being hoter. For total excitation energies ranging roughly from 2 to 8 MeV/A, the proportion of “multifragmentation” events increases to reach a plateau at about 10 MeV/A due to the rising probability to have complete “vaporization” of the system above 8 MeV/A. The steady increase of the temperature extracted from the double isotopic He-Liratios with excitation energy for the quasi-projectile suggests a progressive evolution of the de-excitation processes as predicted by statistical models. No signal of first order liquid-gas phase transition is seen in our data., Les collisions d'ions lourds aux énergies de Fermi conduisent à la formation de noyaux chauds. Selon l'énergie d'excitation mise en jeu, plusieurs modes de décroissance sont observés, en particulier une émission de fragments complexes (Z>2), dont l'origine est encore mal comprise. Le GANIL permet d'explorer sur une large gamme en énergie, entre 32 et 95 MeV/u, la fonction d'excitation du système Ar+Ni depuis l'« évaporation » jusqu'à la «vaporisation » complète des noyaux chauds en particules légères (neutrons, isotopes de H, He). L'analyse des mécanismes de réaction a montré, pour des petits paramètres d'impact, la domination des collisions binaires très inélastiques. Ces dernières ont motivé une analyse en deux sources, des événements bien mesurés par le détecteur INDRA, à l'aide d'une méthode de reconstruction fondée sur l'«arbre minimum». Des énergies d'excitation voisines de 20 MeV/A sont atteintes, à 95 MeV/u, dans les collisions centrales. Pour les collisions violentes, le partage de l'énergie n'est plus équilibrée entre les deux partenaires, le quasiprojectile ayant une énergie d'excitation par nucléon plus élevée que la quasi-cible. Entre 2 et 8 MeV/A d'énergie d'excitation totale du système, correspond une phase de production importante de fragments de masses intermédiaires qui sature autour de 10 MeV/A. Le déclin du régime de «multifragmentation», au-delà de 15 MeV/A, coïncide avec la probabilité croissante d'observer la «vaporisation» des deux noyaux chauds, dont le seuil se situe aux environs de 8 MeV/A. Par ailleurs, l'augmentation régulière de la température extraite des rapports isotopiques He-Li en fonction de l'énergie d'excitation du quasi-projectile suggère une évolution progressive des modes de décroissance des noyaux chauds, en accord avec les prédictions de modèles statistiques. Aucun signe d'une transition de phase du premier ordre de type liquide-gaz n'est observé pour ces noyaux légers.

Intermediate-mass-fragment emission has been studied in central E/A = 30 MeV Xe-129 + Cu-nat reactions. The measured fragment multiplicities, reduced-velocity correlation functions, and emission velocities have been compared with schematic three-body trajectory calculations and with three statistical models with input based upon a dynamical BNV code. The statistical models which include expansion either explicitly or implicitly are able to generate a sufficient number of fragments. The three-body trajectory calculations indicate a mean emission time of approximate to 200 fm/c, consistent with sequential decay. Dynamical expanding-emitting source calculations predict a similar time scale for fragment emission and give satisfactory agreement with the experimental correlation functions if the experimental angular distributions are incorporated into the model. The Berlin multifragmentation model gives good agreement with the experimental charge distributions, and, depending upon the choice of radius parameter, can provide agreement with either the correlation functions or the fragment emission velocities, but not with both simultaneously. Although an overall good agreement is obtained in the statistical model comparisons, even in the most violent collisions the angular distributions and fragment emission velocities are incompatible with completely equilibrated decay from a single source.

The measured properties of the Giant Dipole Resonance in hot rotating nuclei are successfully described with the model of thermal fluctuations, even though there are still some open problems especially at very low (T < 1.2MeV), very high (T > 2.5MeV) temperatures and missing data in some mass regions. Recent experimental works have addressed more specific problems regarding the nuclear shape and its behaviour in very particular and delimited phase space regions. In this paper will be discussed new exclusive measurements of the GDR decay in heavy 216Rn nuclei (where the shape of nuclei surviving fission have been probed) and some preliminary data on the 132Ce nuclei at very high excitation energy.