The y-ray spectrum in the giant dipole resonance (GDR) region associated with the reaction 40 Ar+ 70 Ge at 10 MeV/nucleon has been measured in coincidence with residues of the heavy composite systems whose excitation energy was E* -230 MeV. From the statistical-model analysis, it is deduced that the GDR strength is consistent with 100% of the energy-weighted sum rule; the energy is 16 ± 1 MeV while the width is 13±1 MeV. This value is not very different from the one measured at E* =130 MeV, thus pointing to saturation effects in the damping of the GDR.PACS numbers: 24.30.Cz, 25.70.Gh, 27.60.+J Information on the properties of nuclei at high temperature can be obtained by measuring the high-energy y rays which are emitted when they decay, in particular in the energy region of the giant dipole resonance (GDR) decay. In fact, studies of the energy, width, structure, and strength of the GDR as a function of excitation energy and spin provide direct information on the coupling of the GDR to fluctuations of the nuclear surface and on the size and strength of the average potential at finite temperature. Studies of this type have been carried out for a number of nuclei up to moderate excitation energies. The width of the GDR built on excited states in the Sn isotopes 1,2 has been found to increase nearly quadratically with the excitation energy of the compound nucleus up to E* « 130 MeV. Thermal fluctuations exploring the ensemble of nuclear shapes can account for only part of the observed increase. Indeed, the angular momentum transferred to the compound nucleus increases with bombarding energy and leads to a broadening of the GDR strength function due to deformation effects. This is supported by calculations 3 of the potential-energy surfaces of Sn nuclei as a function of nuclear temperature T and spin /, which predict that the Sn isotopes evolve from spherical shapes at low / to well deformed, predominantly oblate shapes at / > 40. Assuming that the dipole vibration couples adiabatically to the nuclear surface vibrations, the width of the GDR increases.In the present paper we report on a study of the structure of the GDR up to excitation energies is*«230 MeV in 1,0 Sn nuclei. We find that the width of the GDR at this E* does not deviate appreciably from the one measured at 130 MeV. This saturation opens up for new insights into the damping mechanism of the GDR at finite temperature.A 1-mg/cm 2 70 Ge target was bombarded by a 400-MeV 40 Ar beam from the coupled cyclotron SARA of the Institut des Sciences Nucleaires, Grenoble. The reaction products were detected in two position-sensitive parallel-plate avalance counters (PPAC's) with a sensitive area of 15x20 cm 2 . The PPAC's were located symmetrically on both sides of the beam in the forward direction and subtended an angle of ± (3°-20°) in the laboratory system. The PPAC provided information on the time of flight from the target and the energy loss of projectilelike fragments and residues from fusion and incomplete fusion. The high-energy y rays were measured in an a...

No abstract

No abstract