Apparent Temperatures in Hot Quasi-Projectiles and the Caloric Curve

Abstract

The dependence of nuclear temperature upon excitation energy has been experimentally studied with increasing values of excitation energy over the years. At excitation energies per nucleon, E*/A, lower than 6 MeV the temperatures deduced from the kinetic properties of the emitted particles and clusters follow the Fermi gas law : E* = (A/k).T ². The value of the inverse level density parameter k was found to be in the range 8 to 13^[1]. When excitation energies up to 10 MeV per nucléon were reached, temperatures obtained from the relative populations of excited levels in the emitted light nuclei did not overcome 5–6 MeV^[2], but this limitation could be explained by side-feeding effects. Such hot nuclei were formed in fusion or deep inelastic reactions. At incident energies above 40–50 MeV/u, binary dissipative collisions dominate and the quasi-projectiles reach excitation energies per nuclEon and kinetic temperatures above 10 MeV^{[4, 5]}. The study of projectile “spectators” in reactions at several hundreds of MeV/u made it possible to reach similar excitation energies^[3]. In this Aladin experiment at GSI, the temperature was obtained via the relative abundances of two isotope pairs^[6]. The relation between this temperature Tr ⁰ and E*/A was interpreted as indicating a phase transition, with the nuclear gas regime dominating above E*/A = 10 MeV, as predicted^[7]. However, a monotonic increase of the temperature with excitation energy was observed in similar conditions^[8] and questions were raised about the significance of these caloric curves^{[9, 10, 11, 12]}, about the role played by the mass dependence of the decaying nucleus upon E*/A ^[13], as well as the strong effects of side-feeding, especially at high temperatures ^[14]. This point will be discussed by Xi Hong Fei at this meeting.^[15].