Violent collisions and multifragment final states in theCa40+40Ca reaction at 35 MeV/nucleon

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Abstract

The 4\ensuremath{\pi} multidetector AMPHORA has been used to measure yield distributions and energy spectra for products of the collisions in the reactions of $^{40}\mathrm{Ca}$ with $^{40}\mathrm{Ca}$ at 35 MeV/nucleon. Events of high multiplicity (\ensuremath{\ge}10) for which \ensuremath{\ge}85% of the total entrance channel atomic number is detected have been isolated and found to result from the most violent collisions which lead to excitation energies near 6 MeV/nucleon. A large fraction of these collisions lead to multifragment final states. A detailed comparison of the experimental data with results of various models indicates that statistical models which allow for expansion of the system or treat the multifragmentation process as a simultaneous disassembly are more successful than normal sequential binary models at reproducing the yield data and the event complexity inherent in the multifragment events. Quantum molecular dynamic (QMD) calculations are found to provide generally good agreement with the data but overestimate the proton and neutron emission. The agreement is significantly improved if an appropriate afterburner is used to deexcite the separated primary QMD fragments. The sensitivity of such hybrid calculations to the assumed matching time between the dynamical calculation and the afterburner has been explored. The experimentally filtered QMD calculations which provide good agreement with the experimental observables suggest that the most complex events observed in this work come not from the most central collisions, which decay more by light particle emission, but from a region of impact parameter b/${\mathit{b}}_{\mathrm{max}}$=0.5. This suggests that angular momentum effects play an important role in the multifragment decay modes. A comparison of the present results with those for projectile fragmentation in intermediate impact parameter collisions of 600 MeV/nucleon $^{197}\mathrm{Au}$ with Cu indicates that a similar multifragmenting system is produced in the two very different reaction systems.

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