Search Results (4 total)

The 4π multidetector AMPHORA has been used to measure yield distributions and energy spectra for products of the collisions in the reactions of ⁴⁰Ca with ⁴⁰Ca at 35 MeV/nucleon. Events of high multiplicity (≥10) for which ≥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/b_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 ¹⁹⁷Au with Cu indicates that a similar multifragmenting system is produced in the two very different reaction systems.

Excitation energy depositions in the reactions of 40A MeV ⁴⁰Ar with ²³²Th have been determined from measurements of neutron multiplicities in coincidence with mass identified heavy reaction products. For the most central collisions the derived excitation energies of 880 MeV are 200–300 MeV above previous estimates and in excellent agreement with momentum transfer systematics. Heavy evaporation residues are observed for these collisions indicating apparent dynamic delays in the fission channel of (1–5)×10^-20 s. While a massive-transfer simulation incorporating preequilibrium emission is in generally good agreement with the experimental results evidence is also found for strongly damped collisions.

The multiplicities of p and α particles detected in coincidence with fragments emitted in fully relaxed collisions in the reactions of 18.5A MeV ¹³⁶Xe+⁴⁸Ti have been measured for different exit channel mass asymmetries. A kinematic source analysis of the spectra and angular distributions of the light particles has been used to separate the total multiplicities into prescission and postscission contributions. From these results, the excitation energies at scission are determined using an empirical technique based upon previous measurements of light charged particle multiplicities observed in coincidence with evaporation residues. These excitation energies are found to decrease from ∼400 MeV to 110 MeV as the fragment mass asymmetry, A_H/A_L, varies from 4.8 to 1.0. A corresponding increase of the mean lifetime of the scissioning nucleus from ∼5×10^-22 s to ∼1×10^-20 s is derived using calculated statistical model decay widths. The extent to which this variation of lifetime with mass asymmetry may be attributed to completely damped deep inelastic collisions or to dynamic delays in the decay of a compound nucleus is discussed as is the need for inclusion of dynamics in the deexcitation calculations for hot nuclei. Observed three fragment events are also discussed.

The multifragment emission of ‘‘completely characterized’’ events in the ⁴⁰Ca+⁴⁰Ca system at 35 MeV/nucleon has been compared to the predictions of several models. The observed multifragment emission is not in agreement with models based on conventional statistical binary decay, but is in agreement with both a simultaneous multifragmentation model and a sequential emission model in which expansion is treated.