Search Results (17 total)

Effects of in-medium cross sections and of optical potential on preequilibrium emission and on formation of a thermal source are investigated by comparing the results of transport simulations with experimental results from the p+¹⁹⁷Au reaction at 6.2–14.6  GeV∕c. The employed transport model includes light-composite-particle production and allows for inclusion of in-medium particle-particle cross-section reduction and of momentum dependence in the particle optical potentials. Compared to the past, the model incorporates improved parametrizations of elementary high-energy processes. The simulations indicate that the majority of energy deposition occurs during the first 25  fm∕c of a reaction. This is followed by a preequilibrium emission and readjustment of system density and momentum distribution toward an equilibrated system. Within different variants of calculations, the best agreement with data, on the d∕p and t∕p yield ratios and on the residue mass and charge numbers, is obtained at the time of about 65  fm∕c from the start of a reaction, for simulations employing reduced in-medium cross sections and momentum-dependent optical potentials. By that time, the preequilibrium nucleon and cluster emission, as well as mean field readjustments, drive the system to a state of depleted average density, ρ∕ρ₀∼1∕4–1∕3 for central collisions, and low-to-moderate excitation, i.e., the region of nuclear liquid-gas phase transition.

Multifragment breakup of ^natAg and ¹⁹⁷Au nuclei bombarded by 1.8–4.8  GeV ³He ions has been studied with the Indiana Silicon Sphere 4π detector array. To investigate the properties of the emitting source as a function of excitation energy, a two-component moving-source analysis has been performed on the intermediate-mass-fragment spectra, gating on excitation energy. The results provide evidence for nuclear expansion∕dilution to a value of ρ∕ρ₀≲1∕3 prior to breakup. For the most violent events, relatively low source velocities of v∕c~0.01 and slope temperatures of T∼15  MeV are obtained for the dominant thermal-like source. The dependence of isotope ratios on deposition energy and ejectile kinetic energy is examined for H and He isotopes, and the caloric curves for the 4.8  GeV data are presented.

The relation between excitation energy and reaction observables has been examined for (6.0–14.6)-GeV/c protons, (5.0–9.2)-GeV π^-, and 8.0-GeV/c antiprotons incident on a ¹⁹⁷Au target. Relative to proton and π^- beams, 8.0-GeV/c antiprotons are found to be the most effective projectile for depositing high excitation energies in the targetlike residue. For protons and π^- the excitation-energy distributions are nearly identical and appear to be independent of beam momentum above 6–8 GeV/c. It is found that total measured charge, total thermal energy, and total charged-particle multiplicity scale most directly with excitation energy, whereas IMF multiplicity and total transverse energy exhibit large fluctuations. Correlations of the observed fragment multiplicity, charge, and kinetic-energy distributions with excitation energy indicate a transition in the reaction observables near E^*/A≈4–6 MeV. These experimental signals are consistent with a multifragmentation mechanism that becomes the dominant deexcitation mode above in the range E^*/A∼4–6 MeV.

Excitation-energy-gated two-fragment correlation functions have been studied between E^*/A  =  (2–9)A MeV for equilibriumlike sources formed in 8–10 GeV/c π^- and p+¹⁹⁷Au reactions. Comparison with an N-body Coulomb-trajectory code shows an order of magnitude decrease in the fragment emission time in the interval E^*/A  =  (2–5)A MeV, followed by a nearly constant breakup time at higher excitation energy. The decrease in emission time is strongly correlated with the onset of multifragmentation and thermally induced radial expansion, consistent with a transition from surface-dominated to bulk emission expected for spinodal decomposition.

Exclusive studies of angular distributions for intermediate-mass fragments (IMFs) produced in GeV hadron-induced reactions have been performed with the Indiana Silicon Sphere (ISiS) 4π detector array. Special emphasis has been given to understanding the origin of sideways peaking, which becomes prominent in the angular distributions for beam momenta above about 10 GeV/c. Both the magnitude of the effect and the peak angle increase as a function of fragment multiplicity and charge. When gated on IMF kinetic energy, the angular distributions evolve from forward-peaked to near isotropy as the fragment kinetic energy decreases. Fragment-fragment angular-correlation analyses show no obvious evidence for a dynamic mechanism that might signal shock wave effects or the breakup of exotic geometric shapes such as bubbles or toroids. Moving-source and intranuclear cascade simulations suggest that the observed sideways peaking is of kinematic origin, arising from significant transverse momentum imparted to the heavy recoil nucleus during the fast cascade stage of the collision. A two-step cascade and statistical multifragmentation calculation is consistent with this assumption.

To investigate the source size and time dependence of multifragmentation reactions, small- and large-angle relative velocity correlations between coincident complex fragments have been measured for the 1.8–4.8 GeV ³He+^natAg, ¹⁹⁷Au systems. The results support an evolutionary scenario for the fragment emission process in which lighter IMFs (Z≲6) are emitted from a hot, more dense source prior to breakup of an expanded residue. For the most highly excited residues, for which there is a significant yield of fragments with very soft energy spectra (E/A<~3 MeV), comparisons with an N-body simulation suggest a breakup time of τ∼50 fm/c for the expanded residue. Comparison of these data with both the evolutionary expanding emitting source model and the Copenhagen statistical multifragmentation model shows good agreement for heavier IMF’s formed in the final breakup stage, but only the evolutionary model is successful in accounting for the lighter IMFs.

The charge-moment technique has been used to study the fragment charge distribution for the ³He(4.8 GeV)+¹⁹⁷Au reaction. A large variety of fragment charges characterized by a relative variance ∼2.3, is observed for excitation energies around 5.5 MeV/nucleon. Similar signals related to a phase transition are predicted by the percolation model and the statistical multifragmentation model. Effects of detector acceptance and contribution from fission are discussed.

Exclusive studies of sideways-peaked angular distributions for intermediate-mass fragments (IMFs) produced in hadron-induced reactions have been performed with the Indiana silicon sphere (ISiS) detector array. The effect becomes prominent for beam momenta above about 10 GeV/c. Both the magnitude of the effect and the peak angle increase as a function of fragment multiplicity and charge. When gated on IMF kinetic energy, the angular distributions evolve from forward peaked to nearly isotropic as the fragment energy decreases. Fragment-fragment correlation studies show no evidence for a preferred angle that might signal a fast dynamic breakup mechanism. Moving-source and intranuclear cascade simulations suggest a possible kinematic origin arising from significant transverse momentum imparted to the recoil nucleus during the fast cascade. A two-step cascade and statistical multifragmentation calculation is consistent with the data.

Small-angle relative velocity correlations have been measured for the emission of soft intermediate mass fragments (0.7<~E/A<~3.0 MeV) in high-deposition-energy events produced in the 4.8 GeV ³He+¹⁹⁷Au reaction. The experimental correlations are compared with an N-body Coulomb-trajectory simulation that accounts for the fragment spectra, multiplicity, and charge distributions. In order to reproduce the data, a small residue (Z∼10–20) that is located randomly in the breakup volume is required. This analysis indicates breakup time scales of order τ=20–50 fm/c.

4π studies of multiple charged-particle emission in GeV π^-- and proton-induced reactions on a Au target have been performed with the ISiS detector array. Multiplicity, charge, and angular distributions yield nearly identical results for both p and π^- beams, suggesting an independence of hadron type in initiating the fast cascade and subsequent energy deposition in the struck nucleus. The excitation functions show little sensitivity to beam momentum, consistent with a saturation in deposition energy and the concept of limiting fragmentation. However, the intermediate mass fragment multiplicities and fragment charge distributions depend strongly on collision violence.

Multifragmentation of ^natAg and ¹⁹⁷Au nuclei induced by 1.8–4.8 GeV ³He ions has been studied with the Indiana Silicon Sphere 4π detector array. Rapidity, moving source, and sphericity-coplanarity analyses are consistent with near-simultaneous emission from a source in approximate kinetic equilibrium. For the most dissipative collisions, the spectral peaks are broadened and shifted to very low energies, indicative of emission from an extended nuclear system with ρ/ρ₀∼1/3. Predictions of an intranuclear cascade/expanding, emitting source model compare well with experimental multiplicity distributions and the evolution of fragment spectral shapes. © 1996 The American Physical Society.

The Indiana Silicon Sphere 4π detector has been used to measure light-charged particles and intermediate-mass fragments (IMFs) emitted in the 18–4.8 GeV ³He+^natAg, ¹⁹⁷Au reactions. Ejectile multiplicity and total event kinetic energy distributions scale systematically with projectile energy and target mass, except for the ^natAg target at 3.6 and 4.8 GeV. For this system, a saturation in deposition energy is indicated by the data, suggesting the upper projectile energy for stopping has been reached. Maximum deposition energies of ∼950 MeV for the ^natAg target and ∼1600 MeV for the ¹⁹⁷Au target are inferred from the data. The results also demonstrate the importance of accounting for fast cascade processes in defining the excitation energy of the targetlike residue. Correlations between various observables and the average IMF multiplicity indicate that the total thermal energy and total observed charge provide useful gauges of the excitation energy of the fragmenting system. Comparison of the experimental distributions with intranuclear cascade predictions shows qualitative agreement. © 1996 The American Physical Society.

Multifragmentation of ^natAg and ¹⁹⁷Au nuclei induced by a 4.8-GeV ³He ions has been studied with the Indiana Silicon Sphere 4π detector array. Rapidity and moving source analyses are consistent with thermal emission from a source in approximate kinetic equilibrium. For the most dissipative collisions, the spectral Coulomb peaks are broadened to very low energies, indicative of emission from an extended nuclear system. Predictions of a model with an intranuclear cascade and an expanding, emitting source compare well with experimental multiplicity distributions and the evolution of fragment spectral shapes.

Projectilelike fragments were detected and characterized in terms of A, Z, and energy for the reactions ³⁷Cl on ⁴⁰Ca and ²⁰⁹Bi at E=7.3 MeV/nucleon, and ³⁵Cl on ²⁰⁹Bi at E=15 MeV/nucleon, at angles close to the grazing angle. Mass and charge distributions were generated in the N-Z plane as a function of energy loss, and parametrized in terms of their centroids, variances, and coefficients of correlation. The results are compared to the predictions of two current models based on a stochastic nucleon exchange mechanism. The drifts of the charge and mass centroids for the system ³⁷Cl on ⁴⁰Ca are consistent with a process of mass and charge equilibration mediated by nucleon exchange between the two partners, followed by evaporation. The asymmetric systems show a strong drift toward larger asymmetry, with the production of neutron-rich nuclei. It is concluded that this is the result of a net transfer of protons from the light to the heavy partner, and a net flow of neutrons in the opposite direction. Model predictions fail to reproduce in detail the evolution of the centroids for asymmetric systems. The variances for all systems increase with energy loss, as would be expected from a nucleon exchange mechanism. However, the variances for the reaction ³⁷Cl on ⁴⁰Ca are higher than those expected from that mechanism, and the variances for the reaction ³⁵Cl on ²⁰⁹Bi start decreasing after about 100 MeV of energy loss. The coefficients of correlation indicate that the transfer of nucleons between projectile and target is correlated, as expected from Q-value constraints to the valley of β stability.

Inclusive yields of isotopically resolved intermediate-mass fragments (IMF) with Z=3–6 have been measured for the reaction of E/A=50 MeV ⁴He with ¹¹⁶Sn and ¹²⁴Sn targets. The isotopic yield patterns reflect the N/Z ratio of the target for both equilibriumlike and nonequilibrium components of the spectra. In addition, the IMF elemental cross sections for the ¹²⁴Sn target decrease much more rapidly with Z than for ¹¹⁶Sn. This effect is presumably due to lower barriers for IMF emission from residues formed with the neutron-deficient ¹¹⁶Sn target and increased competition from pre-IMF neutron emission for the neutron-excess ¹²⁴Sn target. The experimental cross sections are compared with statistical and accreting-source calculations.

An unusual low-energy component has been observed in the spectrum of equilibriumlike intermediate-mass fragments (IMFs) emitted from systems formed in high-linear-momentum-transfer reactions of 270-MeV ³He with ²³²Th. The properties of this component include strong focusing of IMFs at an angle perpendicular to the fission fragment separation axis, kinetic energies about one-half those for normal IMFs, and a preference for neutron-excess fragments. This behavior is consistent with emission of IMFs from the neck region of a fissioning nucleus.

Excitation functions have been measured for complex fragments with atomic number Z=3–15 emitted in collisions of E/A=20–100 MeV ¹⁴N ions with targets of ^natAg and ¹⁹⁷Au. The results are analyzed in terms of a three-source model which includes: projectilelike fragments from peripheral processes, nonequilibrium emission, and statistical decay of fully equilibrated compound nuclei. Nonequilibrium emission of complex fragments is found to increase rapidly relative to equilibrium emission up to E/A≊60 MeV and then remain approximately constant thereafter. Evidence for enhanced production of heavier intermediate-mass fragments is found in the data for reactions with the ^natAg target at energies above E/A≊60 MeV. Fit parameters for the equilibrium and nonequilibrium sources are examined. In particular, it is observed that the temperature parameter for nonequilibrium emission of complex fragments is independent of bombarding energy up to E/A=100 MeV.