[en] This article reviews the current understanding of collective dipole vibrations in hot atomic nuclei. Such nuclei are produced in energetic nuclear collisions using heavy ions. The experimental data establish the universality of GDR in excited nuclei. The systematics of the measured strengths, resonance energies, and widths show that resonances are stable features, that gamma-ray decay is statistical, and that the resonances couple strongly to the shape degrees of freedom of excited nuclei. This discussion focuses on experimental methods, GDR observables and methods of analysis, the shapes and fluctuations of hot nuclei, limits of collectivity in hot nuclear matter, and pre-fission gamma rays from giant resonances. 148 refs., 19 figs

[en] Gamma rays of high energies provide information on the initial stages of the decay of excited nuclear systems produced in violent nuclear collisions. In particular, gamma ray emission in the energy range 10-30 MeV, is mainly related to the deexcitation of giant resonance modes (highly collective vibrations of the nucleons) in the hot conglomerate system. The details of the gamma ray spectrum, angular distribution patterns, etc., are sensitive to the coupling of the resonances to other degrees of freedom of the excited nucleus, and as such constitute a new tool to study the properties of hot nuclear matter. In the talk the present situation of the field of excited state giant resonances will be reviewed with particular emphasis on the coupling of the GDR to the shape degrees of freedom of the compound nucleus, new insights into the general mechanism underlying the relaxation of collective excitations in hot nuclei

[en] High-energy γ-ray spectra have been measured in coincidence with products from the reaction /sup 40/Ar+ /sup 70/Ge at 15 and 24 MeV/nucleon. The evolution of the giant dipole resonance (GDR) is studied as a function of excitation energy (E/sup */). Up to E/sup */≅300 MeV the measured GDR strength is consistent with full collectivity. At higher E/sup */ a strong inhibition of the GDR γ decay is observed. At the highest transition energies, the spectra are consistent with bremsstrahlung emission associated with nucleon-nucleon collisions

[en] The gamma decay of the giant dipole resonance is a sensitive tool for investigating how nuclear shape changes with spin and excitation energy, but the information is coded in a subtle way, inasmuch as the shape and orientation of nuclei at finite temperature display large fluctuations. At the time of the conference, the three systems ^109-110Sn, ^161-162Yb and ^165-167Er had recently been studied on the HECTOR spectrometer. The Sn nuclei are spherical in their ground states, and are expected to become oblate under the stress of rotation. The Yb and Er nuclei are prolate, and are expected to become first spherical, then oblate. While the patterns of the measured angular anisotropies are consistent with this general picture, many questions still remain open. 3 refs., 1 tab., 3 figs

[en] The enhancement of the π^-/π⁺ ratio as a function of the properties of the participant fireball are explored and the sensitivity to dynamical features such as the total participant charge, the transverse expansion velocity and the temporal and spatial extent of the source, using a simple dynamical Monte Carlo model are investigated

[en] The lowest (π, α) = (+, +), (+, -) and (-, +) rotational sequences in ¹⁶⁵,¹⁶⁷Yb as well as the lowest (-, -) sequence in ¹⁶⁵Yb have been established to high spins. The single-quasineutron portions of these sequences are used to construct two- and three-quasineutron states which are compared to the corresponding configurations in ¹⁶⁵,¹⁶⁶,¹⁶⁷Yb. Such a comparison indicates that the residual interactions between two quasineutrons are nearly independent of hω/2π and configuration (for those studied), are attractive and are equal to about 300 keV. For three-quasineutron configurations and hω/2π dependence is observed in the residual interaction, which can be interpreted as a reduction in alignment for the three-quasineutron configurations, a result of reduced pairing. Difficulties in reproducing the signature-dependent energy-splitting and band-crossing frequencies in the positive-parity decay sequence are not completely understood. The signature dependence of the interband-intraband branching ratios as well as of the interband M1/E2 mixing ratios provide an experimental justification of the signature dependence of B(M1) transition rates recently suggested by Hamamoto. (orig.)

[en] Gamma-ray transition-energy spectra up to E/sub γ/approx. =33 MeV have been measured for the decay of the compound nucleus ¹¹¹Sn( populated at excitation energies up to E(approx. =100 MeV. Above E/sub γ/approx. =20 MeV an excess of gamma rays over the contribution from the isovector giant dipole resonance is observed in the energy range where the isovector giant quadrupole resonance built on excited states is expected. The width of the giant dipole resonance in /sup 108,111/Sn( shows a large increase with increasing nuclear temperature and angular momentum

[en] Experiments on ¹⁶⁰Yb indicate three negative-parity bands up to I > or = 21h. All three experience a band crossing around a frequency hω=0.36 MeV, a value intermediate to those of the first and second yrast backbends. The i/sub 13/2/ band in ¹⁶¹Yb is observed up to ⁴⁹⁺₂; it also has a strong upbend at this intermediate frequency. The fingerprint of a band crossing based upon the crossing frequency and gain in aligned angular momentum is demonstrated and the results compared to cranking-model calculations to suggest quasiparticle assignments for the bands

[en] The giant dipole resonance built on excited states was studied in /sup 110,112/Sn nuclei at excitation energies of 62, 80, 110, and 130 MeV, by measuring γ rays following heavy ion fusion reactions /sup 16/O+ /sup 94/Mo and /sup 19/F+ /sup 93/Nb. The energy of the dipole resonance was found to be 14.8 +- 0.4 MeV independent of excitation energy. This value is approximately 1 MeV below the extrapolated ground state value. The width increases almost quadratically from 6.7 MeV at 62 MeV to 10.8 MeV at 130 MeV. This agrees qualitatively with a theoretical prediction that the increase in width for these nuclei is mainly due to an averaging of strength functions over various oblate shapes induced at high spin and temperature. The sensitivity to the isovector quadrupole vibration was found to be small even at the highest excitation energy. No convincing conclusion could be made for its existence in these Sn nuclei

[en] A study of the spectral and angular distributions of the high energy gamma-rays emitted by the giant dipole resonance (GDR) in hot compound nuclei for the two mass regions A=110 and A=160-170 at a variety of excitation energies (E^*=50-90 MeV) is presented. A comparison of the data with model predictions at the equilibrium deformation and with thermal fluctuations in the adiabatic limit is discussed. In general, the adiabatic calculations give a fair account of the data exception made for Sn at very high spins. In addition the combined analysis of the strength function and of the angular distribution suggests that large deformations caused by angular momentum effects are responsible of the width increase while collisional damping is constant at least up to T=2 MeV. (author). 16 refs., 9 figs., 1 tab