Zero-to-first sound transition for isovector modes in hot nuclei

The isovector density wave propagation in symmetric nuclear matter (NM) at finite temperature is studied within the Landau-Vlasov theory. In cold NM a zero sound is found, which is changing to a first sound with increasing te mperature. This transition, at variance with the case of a one-component Fe rmi liquid, does not result in the appearance of a maximum in the attenuati on of the collective motion as a function of temperature. The damping width of a volume isovector mode is always monotonically increasing with tempera ture due to the presence of the collisional friction between neutron and pr oton liquids. However, a clear effect of the transition is expected on the structure of the dipole response function. The observable consequences of the transition in finite hot nuclei are disc ussed on the basis of the calculated photoabsorption cross section. The gam ma-spectra from a thermally excited nucleus Pb-208 are evaluated within the statistical model using theoretical photoabsorption cross sections and com pared to experimental data. A transition temperature T-tr similar or equal to 4.5 MeV is deduced. (C) 1999 Published by Elsevier Science B.V.