S. J. Gaff

Direct observation of the inversion of flow

Abstract The sign of the mean transverse momentum of non-equilibrium light charged particles was determined from the circular polarization of coincident γ-rays emitted from residual nuclei for 14 N-induced reactions on 154 Sm at incident energies, E/A=35, 100 and 155 MeV. Results of the emitted α-particles at mid-central collisions show the predicted transition from mean-field dominated dynamics at low energies to nucleon-nucleon collision dominated dynamics at high energies.

Isospin independence of the H-He double isotope ratio "thermometer"

Abstract Light fragments ( d , t , 3 He, 4 He) were measured for central collisions of 112 Sn+ 112 Sn and 124 Sn+ 124 Sn at E / A =40 MeV. While individual isotope ratios depend strongly on the neutron-to-proton ratio of the entrance channel, the double ratio R H–He =[ Y ( d ) Y ( 4 He)]/[ Y ( t ) Y ( 3 He)] shows no such sensitivity. Within the assumptions of the Albergo thermometric technique, this independence is consistent with the attainment of full chemical equilibrium for each reaction at the same common temperature. However, the same independence is also predicted by calculations based on emission from an expanding emitting source, where the yield for the final double ratio is not obtained at a single well defined source temperature.

Nuclear temperature measurements with helium isotopes

Abstract Temperatures were determined from double isotope ratios and excited state populations and corrected for secondary decay for central Au + Au collisions at 35 AMeV. Consistent emission temperatures were obtained both from double isotope ratios and excited state populations. Center of mass energy spectra have been studied for Helium and Hydrogen isotopes emitted from highly excited residues produced in central Xe + Cu reaction at 30 AMeV. Differences between the spectra for 3He and 4He are observed that can be attributed to the time dependent evaporative cooling of hot nuclei.

Present status of the caloric curve of nuclei

Abstract Spectator decay was studied for the system Au + Au at an energy of 1000 A·MeV and the decay of the interaction region at energies between 50 and 200 A·MeV. In both cases temperatures were derived from several double-ratios of neighboring isotopes and from the population of excited states in 5 Li and 4 He. Agreement was found among the different isotope temperatures and also among the two excited state temperatures. The comparison of isotope and excited state temperatures, however, reveals large differences, which cannot be explained by feeding corrections. At incident energies between 600 and 1000 A·MeV the energy spectra of fragments and also neutrons of the decaying projectile spectator were measured. Whereas the slope parameters of the energy spectra and mean energies for fragments with Z ≥ 2 are independent of the incident energy, a strong dependence is found for the lightest particles, so that preequilibrium contributions to the spectator decay should be taken into account.