Search Results (10 total)

Experimental information has been derived from the neutron-proton correlation function in order to deduce the time sequence of neutrons and protons emitted at 45° in the E/A  =  45 MeV ⁵⁸Ni+²⁷Al reaction.

The strength of the neutron-neutron correlation function from the E=45A MeV ⁵⁸Ni+²⁷Al, ^natNi, and ¹⁹⁷Au reactions depends on the neutron parallel velocity. This indicates the presence of multiple sources of neutron emission. We find these sources consistent with a dissipative, binary reaction mechanism as it is described by, e.g., Boltzmann-Uehling-Uhlenbeck calculations.

Small angle neutron-neutron correlations have been measured for the E/A=45 MeV ⁵⁸Ni+²⁷Al, ^natNi, and ¹⁹⁷Au reactions. Two-neutron correlation functions, both integrated and gated on the total momentum of the neutron pair, have been constructed. In order to explain these data, a fraction of fast “dynamical” emission is needed in addition to slower evaporative emission. The overall emission time scale is shorter for the symmetric system, indicating that the dynamical component is stronger in this case.

Multifragment emission following ¹²⁹Xe+¹⁹⁷Au collisions at 30A, 40A, 50A, and 60A MeV has been studied with multidetector systems covering nearly 4π in solid angle. The correlations of both the intermediate mass fragment and light charged particle multiplicities with the transverse energy are explored. A comparison is made with results from a similar system ¹³⁶Xe+²⁰⁹Bi at 28A MeV. The experimental trends are compared to statistical model predictions.

Two-fragment reduced-velocity correlation functions were measured for small-impact-parameter collisions of ⁸⁶Kr +⁹³Nb at E/A=50 MeV and compared to results of many-body Coulomb-trajectory calculations performed for instantaneous and sequential multifragment breakup scenarios. The correlation functions indicate emission on a very short time scale and appear consistent with an instantaneous breakup scenario, even though they exhibit a pronounced dependence on fragment kinetic energy when fragments are emitted at large transverse momenta. For the case of instantaneous breakup, sensitivities to initial-state momentum correlations due to total momentum conservation and to different emission patterns are investigated. For fragments emitted with large transverse momenta, momentum conservation constraints can cause a dependence of reduced-velocity correlation functions on fragment energy and fragment charge similar to those observed experimentally.

Energy spectra have been measured for helium and hydrogen isotopes emitted from highly excited residues produced in central ¹²⁹Xe + ^natCu collisions at E/A=30 MeV. The observed differences between the spectra for ³He and ⁴He nuclei can be attributed to the time dependent evaporative cooling of the residues. This cooling dynamics is also reflected in a characteristic dependence of isotope ratios on the kinetic energy of the emitted particles in reasonable agreement with present evaporative models.

The breakup temperatures for central Au+Au collisions at 35A MeV have been determined from the relative populations of excited states of ⁵Li, ⁴He, and ¹⁰B fragments and nine double ratios involving the yields of elements with 1≤Z≤6. Unlike results reported at significantly higher energies, all thermometers yield temperatures that are consistent within the experimental uncertainties. Extrapolation of the data to zero impact parameter yields T_em  =  4.6±0.4 MeV, somewhat lower than the temperature assumed in statistical multifragmentation model calculations which describe most of the other features of this reaction.

The charge distributions and their dependence on fragment multiplicity have been studied for the multifragmentation of 30 MeV/nucleon Xe+Au, Cu. For both targets, the charge distributions are approximately independent of the fragment multiplicity n. However, a residual systematic dependence on n is detectable at the largest values of the total charge multiplicity N_c. Such n dependence obeys a simple scaling law and suggests the presence of an entropic term possibly related to the mechanism of multifragmentation. Thermal scaling between the different bins of N_c seems to occur.

Two-neutron relative-momentum correlation functions have been measured in the 130 MeV ¹⁸O + ²⁶Mg reaction. Differences in the longitudinal and transverse correlation functions, observed for the first time for neutrons, allow an independent determination of the spatial extent and the time scale for decay of the ⁴⁴Ca compound nucleus. A comparison with theoretical calculations indicates a radius of 4.4±0.3 fm and an average neutron emission time scale of 1100 ± 100 fm/c for ⁴⁴Ca at 100 MeV excitation energy. Correlation functions selected by cuts on the total momentum of the neutron pair give a quantitative characterization of the cooling of a compound nucleus.

Intermediate-mass-fragment emission has been studied in central E/A=30 MeV ¹²⁹Xe+^natCu reactions. The measured fragment multiplicities, reduced-velocity correlation functions, and emission velocities have been compared with schematic three-body trajectory calculations and with three statistical models with input based upon a dynamical BNV code. The statistical models which include expansion either explicitly or implicitly are able to generate a sufficient number of fragments. The three-body trajectory calculations indicate a mean emission time of ≊200 fm/c, consistent with sequential decay. Dynamical expanding-emitting source calculations predict a similar time scale for fragment emission and give satisfactory agreement with the experimental correlation functions if the experimental angular distributions are incorporated into the model. The Berlin multifragmentation model gives good agreement with the experimental charge distributions, and, depending upon the choice of radius parameter, can provide agreement with either the correlation functions or the fragment emission velocities, but not with both simultaneously. Although an overall good agreement is obtained in the statistical model comparisons, even in the most violent collisions the angular distributions and fragment emission velocities are incompatible with completely equilibrated decay from a single source.