Tracing the Evolution of Temperature in Near Fermi Energy Heavy Ion Collisions

The kinetic energy variation of emitted light clusters has been employed as a clock to explore the time evolution of the temperature for thermalizing composite systems produced in the reactions of 26A, 35A and 47A MeV $^{64}$Zn with $^{58}$Ni, $^{92}$Mo and $^{197}$Au. For each system investigated, the double isotope ratio temperature curve exhibits a high maximum apparent temperature, in the range of 10-25 MeV, at high ejectile velocity. These maximum values increase with increasing projectile energy and decrease with increasing target mass. The time at which the maximum in the temperature curve is reached ranges from 80 to 130 fm/c after contact. For each different target, the subsequent cooling curves for all three projectile energies are quite similar. Temperatures comparable to those of limiting temperature systematics are reached 30 to 40 fm/c after the times corresponding to the maxima, at a time when AMD-V transport model calculations predict entry into the final evaporative or fragmentation stage of de-excitation of the hot composite systems. Evidence for the establishment of thermal and chemical equilibrium is discussed.Comment: 9 pages, 5 figure

Reaction dynamics and multifragmentation in Fermi energy heavy ion reactions

The reaction systems, $^{64}$Zn+$^{58}$Ni, $^{64}$Zn+$^{92}$Mo, $^{64}$Zn+${197}$Au, at 26, 35, and 47 A MeV, have been studied both in experiments with a 4$\pi$ detector array, NIMROD, and with antisymmetrized molecular dynamics model calculations employing effective interactions corresponding to soft and stiff equation of state (EOS). Direct experimental observables, such as multiplicity distributions, charge distributions, energy spectra and velocity spectra, have been compared in detail with those of the calculations and a reasonable agreement is obtained for both EOS's. No conclusive preference for either EOS has been observed. Neither of the above direct observables nor the strength of the elliptic flow are also sensitive to changes in the in-medium nucleon-nucleon cross sections. A detailed analysis of the central collision events revealed that multifragmentation with cold fragment emission is a common feature predicted for all reactions studied here. A possible multifragmentation scenario is presented; after the preequilibrium emission ceases in the composite system, cold light fragments are formed in a hotter gas of nucleons and stay cold until the composite system underdoes multifragmentation. For reaction with $^{197}$Au at 47A MeV a significant radial expansion takes place. For reactions with $^{58}$Ni and $^{92}$Mo at 47A MeV semitransparency becomes prominent. The differing reaction dynamics drastically change the kinematic characteristics of emitted fragments. This scenario gives consistent explanations for many existing experimental results in the Fermi energy domain

Energy deposition in 209Bi(alpha,alpha') reactions at 240 MeV

The energy deposition associated with inelastic ct particle scattering on (209)Bi at 240 MeV has been determined using the TAMU neutron ball. A comparison of the reconstructed average excitation energies with the beam energy losses demonstrates that only part of the missing beam energy is usually deposited as thermal excitation in the target nucleus. Requiring an additional coincidence with a light charged particle or fission fragment leads to selection of a significant higher average excitation energy

Energy deposition and GDR emission in inelastic alpha particle scattering

Neutron fold distributions measured for the reaction 209Bi(alpha,alpha') at 240 MeV have been analyzed with the help of Statistical Model calculations to determine the distribution of excitation energy in the primary target fragments as a function of the projectile energy loss, EL. Results show that the distributions in excitation energy feature a plateau which extends from the kinematical limit Ex=EL to very small excitations, suggesting a variety of interactions of the beam particles with the target nucleus. Requiring an additional coincidence with a light charged particle leads to selection of a significant higher average excitation energy. This effect is extrapolated to explore results of previous GDR decay measurements in the case of a 208pb target. Corrections of derived GDR parameters due to the partial transfer of excitation energy are suggested

Reaction dynamics and multifragmentation in Fermi energy heavy ion reactions

The reaction systems, Zn-64+Ni-58, Zn-64+Mo-92, Zn-64+Au-197, at 26, 35, and 47 A MeV, have been studied both in experiments with a 4pi detector array, NIMROD, and with antisymmetrized molecular dynamics model calculations employing effective interactions corresponding to soft and stiff equation of state (EOS). Direct experimental observables, such as multiplicity distributions, charge distributions, energy spectra and velocity spectra, have been compared in detail with those of the calculations and a reasonable agreement is obtained for both EOS's. No conclusive preference for either EOS has been observed. Neither of the above direct observables nor the strength of the elliptic flow are also sensitive to changes in the in-medium nucleon-nucleon cross sections. A detailed analysis of the central collision events revealed that multifragmentation with cold fragment emission is a common feature predicted for all reactions studied here. A possible multifragmentation scenario is presented; after the preequilibrium emission ceases in the composite system, cold light fragments are formed in a hotter gas of nucleons and stay cold until the composite system underdoes multifragmentation. For reaction with Au-197 at 47A MeV a significant radial expansion takes place. For reactions with Ni-58 and Mo-92 at 47A MeV semitransparency becomes prominent. The differing reaction dynamics drastically change the kinematic characteristics of emitted fragments. This scenario gives consistent explanations for many existing experimental results in the Fermi energy domain