Search Results (28 total)

Fragments from the breakup of 32 MeV/nucleon ²⁰Na, ²⁰Ne, and ²⁰F projectile nuclei were measured with isotopic resolution using the FAUST detector array. Complete reconstruction of the fragmenting source was performed on an event-by-event basis. Events analyzed in this study were restricted to those with a summed fragment charge equal to the beam in which all fragments were isotopically identified. The change in neutron content of the projectile could be inferred from the complete isotopic reconstruction of the quasiprojectile. The 〈N/Z〉 present in the fragmenting source is not equal to the N/Z of the initial beam; there has been a shift toward the valley of stability. Additionally, there is now a distribution in the N/Z of the fragmenting system. Multineutron transfer from the neutron-rich target was observed for proton-rich projectiles.

The relationship between nuclear temperature and excitation energy of hot nuclei formed by 8 GeV/c negative pion and antiproton beams incident on ¹⁹⁷Au has been investigated with the ISiS 4π detector array at the BNL AGS accelerator. The double-isotope-ratio technique was used to calculate the temperature of the hot system. The two thermometers used, (p/d-³He/⁴He) and (d/t-³He/⁴He), are in agreement below E^*/A∼8 MeV when corrected for secondary decay. Caloric curves derived from successive segments of the H and He kinetic energy spectra show a systematic decrease in temperature as the kinetic energy bin decreases, consistent with “cooling curve” behavior. When extrapolated to the evaporative-peak region, these results provide good agreement with caloric curves measured for similar systems. The caloric curves from this experiment are also compared with the predictions from the SMM multifragmentation model.

Experimental data from the reaction of an 8.0 GeV/c π^- beam incident on a ¹⁹⁷Au target have been analyzed in order to investigate the breakup time scale for hot residues. Helium nuclei angular distributions and energy spectra supported by a momentum tensor analysis suggest that at large excitation energy, above 3-5 MeV/nucleon, highly excited heavy fragments are separated promptly after the thermalization. A binary fission-like mechanism fits the experimental data at low excitation energies, but seems unable to reproduce the data at excitation energies above 3-5 MeV/nucleon.

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.

The event-by-event reconstruction procedure and related uncertainties involved in the derivation of excitation energy and source-size distributions are investigated for GeV hadron-induced reactions. The analysis is performed for the 5.0–14.6 GeV/c proton-, π^- and antiproton-induced reactions on ¹⁹⁷Au, measured with the Indiana silicon sphere charged-particle detector array at the Brookhaven AGS accelerator. The relative contributions of the three major components of the excitation-energy calorimetry: charged-particle kinetic-energy sums, neutrons, and Q values from reconstructed events, are found to be relatively constant for excitation energies above about 500 MeV. Effects on the results imposed by various assumptions necessary to account for experimental factors are examined and a corresponding deconvolution of the excitation-energy distribution is performed. The major uncertainties in the calorimetry are found to be (1) separation of nonequilibrium and thermal-like charged particles, and (2) the unmeasured neutron component. The self-consistency of the procedure is tested via comparisons with the SMM and SIMON codes for the disintegration of hot nuclei.

A binomial reducibility and thermal scaling analysis is performed on well-chacracterized thermal-like sources formed in 8 GeV/c π^-+¹⁹⁷Au reactions. The fragment probability distributions are shown to be binomial when plotted as a function of the measured excitation energy E^* and the binomial elementary probability p is shown to follow the expected Boltzmann factor: ln(p)∝exp(-B/sqrt[E^*/A]). Binomial reducibility and thermal scaling are explored also using global variables other than E^*, and the effect of source size on the binomial parameter p and m is shown. Finally, the extracted probability p is found to be correlated with the experimentally deduced fragment emission time up to about 6A MeV of excitation energy, hinting at a possible transition in decay mechanism above that excitation energy.

The enhancement in the production of even-Z nuclei observed in nuclear fission has also been observed in fragments produced from heavy ion collsions. Beams of ⁴⁰Ar, ⁴⁰Cl, and ⁴⁰Ca at 25 MeV/nucleon were impinged on ⁵⁸Fe and ⁵⁸Ni targets. The resulting fragments were detected using the MSU 4π detector array, which had additional silicon detectors for better isotopic resolution. Comparison of the ratios of yields for each element showed enhancement of even-Z fragment production. The enhancement was more pronounced for reactions with a greater difference in the N/Z of the compound system. However, this effect was less for systems that were more neutron rich. The average N/Z for fragments also displayed an odd-even effect with a lower average N/Z for the even-Z fragments. This is related to the greater availability of neutron-poor isotopes for even-Z nuclei.

The multifragmentation of quasiprojectiles was studied in the reactions of a ²⁸Si beam with ¹¹²Sn and ¹²⁴Sn targets at projectile energies of 30 and 50 MeV/nucleon. The quasiprojectile observables were reconstructed using isotopically identified charged particles with Z_f<~5 detected at forward angles. The nucleon exchange between projectile and target was investigated using the isospin and the excitation energy of the reconstructed quasiprojectile. For events with total reconstructed charge equal to the charge of the beam (Z_tot=14), the influence of the beam energy and target isospin on the neutron transfer was studied in detail. Simulations were carried out employing a model of deep inelastic transfer, a statistical model of multifragmentation, and a software replica of the FAUST detector array. The concept of deep inelastic transfer provides a good description of the production of highly excited quasiprojectiles. The isospin and excitation energy of the quasiprojectile were described with good overall agreement. The fragment multiplicity, charge and isospin were reproduced satisfactorily. The range of contributing impact parameters was determined using a backtracing procedure.

Isotopic yields of IMF’s produced in the ⁵⁸Ni+¹²C,²⁴Mg reactions at 34.5A MeV are investigated. Analysis of experimental data from the CRL-Laval 4π multidetector array focuses on events where at least 75% (60%) of the charge and momentum were detected for the ⁵⁸Ni+¹²C (⁵⁸Ni+²⁴Mg) system. Averaged isospin ratios (N/Z) for IMF’s with Z=3 and 4 are plotted as a function of emission angle and parallel velocity in the center-of-mass frame. Results from simulations with the statistical codes SMM and GEMINI, assuming an equilibrated source, are compared to the experimental ratios. The ratios seem to indicate the presence of a midrapidity necklike structure that would produce IMF’s richer in neutrons than the two main emitters, even for very central collisions.

We have created quasiprojectiles of varying isospin via peripheral reactions of ²⁸Si+¹¹²Sn and ¹²⁴Sn at 30 and 50 MeV/nucleon. The quasiprojectiles have been reconstructed from completely isotopically identified fragments. The difference in N/Z of the reconstructed quasiprojectiles allows the investigation of the disassembly as a function of the isospin of the fragmenting system. The isobaric yield ratio ³H/³He depends strongly on N/Z ratio of quasiprojectiles. The dependences of mean fragment multiplicity and mean N/Z ratio of the fragments on N/Z ratio of the quasiprojectile are different for light charged particles and intermediate mass fragments. Observation of a different N/Z ratio of light charged particles and intermediate mass fragments is consistent with an inhomogeneous distribution of isospin in the fragmenting system.

Fragment kinetic energy spectra for reactions induced by 8.0 GeV/c π^- beams on a ¹⁹⁷Au target have been analyzed. The average fragment kinetic energies are observed to increase systematically with fragment charge but are nearly independent of excitation energy. Near E*/A=5 MeV, the data are well accounted for by two statistical multifragmentation models, SMM and SIMON-explosion. However, at higher excitation energies, a small amount of extra energy, proportional to the fragment mass, is required in the models in order to match the experimental fragment’s kinetic energies. This extra expansion energy is small relative to the radial expansion observed in heavy-ion-induced reactions, consistent with the interpretation that the latter expansion may be driven primarily by collective dynamical effects that are not present in light-ion-induced collisions.

This paper introduces a new method for the selection of central single-source events, based on classical multivariate techniques. The resulting discriminating variable is shown to be valid for different hypotheses on the nuclear source deexcitation mechanism. It enables the selection of events which are representative of the whole set of single-source events. Application to the Ni+Ni at 32A MeV system measured with the INDRA multidetector has allowed the determination of the fusion probability as a function of the impact parameter and the evaluation of the corresponding cross section.

The isospin dependence of light and heavy fragments emitted from excited nuclear systems and the change in isospin behavior between light and heavy fragments are studied in this report. The 〈N/Z〉 is calculated using data reported in the literature and from the results of the simulation code SMM. A transition in the isospin behavior between light and heavy fragments may support the recently reported two-phase bifurcation of excited nuclear matter into a neutron-rich gas phase and a more symmetric liquid phase.

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.

Comparison of the heating effect produced by 8 GeV/c π^- and antiproton beams incident on ¹⁹⁷Au nuclei has been conducted with the Indiana silicon sphere 4π detector array. Event reconstruction indicates formation of thermal-like heavy residues with excitation energies up to 1.7 GeV. Enhanced energy deposition is observed for antiprotons relative to negative pions. For events with excitation energies that exceed 1000 MeV, there is a 50% increase in cross section for the antiproton beam relative to the π^- beam. The predominant decay mode at these high excitation energies is multifragmentation in which three or more Z≥3 fragments are emitted.

Light charged particles emitted by the projectilelike fragment were measured in the direct and reverse collision of ⁹³Nb and ¹¹⁶Sn at 25A MeV. The experimental multiplicities of hydrogen and helium particles as a function of the primary mass of the emitting fragment show evidence for a correlation with net mass transfer. The ratio of hydrogen and helium multiplicities points to a dependence of the angular momentum sharing on the net mass transfer.

Peripheral reactions of ²⁸Si with ¹¹²Sn and ¹²⁴Sn at 30, 40, and 50 MeV/nucleon were used to elucidate the effect of the neutron content of the target on the process of projectile fragmentation. It is demonstrated that the fragments that result from these projectile fragmentation reactions can be divided into those which are the result of statistical emission of the quasiprojectile and those that are part of a direct component. The statistical part is independent of the target whereas the isotopic composition of fragments from the direct component is dependent on the neutron content of the target.

The characteristics of the midrapidity and target sources (apparent temperatures, velocities, and neutron multiplicities) extracted from the neutron energy spectra, have been measured for various quasiprojectile (QP) excitation energies, reconstructed from charged particles of well defined peripheral events in the ³⁵Cl+^natTa reaction at 43 MeV/nucleon. The reconstructed excitation energy of the QP is always smaller than the excitation energy calculated from its velocity, assuming pure dissipative binary collision. The latter observation combined with the neutron multiplicity at midrapidity and the apparent temperature suggests important preequilibrium and/or dynamical effects in the entrance channel. The midrapidity source moves at a velocity lower than the nucleon-nucleon center of mass velocity showing the importance of the attractive mean-field potential from the target even at 43 MeV/nucleon. The above picture is confirmed by comparison to Boltzman-Nordheim-Vlasov (BNV) simulations.

The peripheral and semiperipheral reactions in ³⁵Cl+¹⁹⁷Au have been studied at 30 and 43 MeV/nucleon. The nonequilibrium α and IMF components have been observed in the experiment. The fraction of nonequilibrium emission decreases with an increase in the atomic number of the projectilelike fragments but, for a given projectilelike fragment, it increases with the charge of the emitted particles. The characteristics of quasiprojectiles reconstructed from their decay products reveal several features reminiscent of damped reactions at lower bombarding energies. The atomic number and deflection angle of projectilelike fragments depend strongly on their kinetic energy or dissipated energy. At 30 MeV/nucleon, the experimental data can be explained by a deep inelastic transfer model. One-body dissipation is still the main mechanism for the energy and angular momentum dissipation. However, at 43 MeV/nucleon, deep inelastic transfer models can predict only the experimental tendency. Two-body dissipation plays a more important role at higher incident energies. The similarity observed in the decay product distributions, as a function of excitation energy, suggests that the excited quasiprojectiles formed in binary collisions might approach thermal equilibrium for both incident energies. The decay products have been analyzed with sequential-binary and simultaneous-disassembly statistical decay models. Both statistical models are able to provide good agreement with the experimental observables except for the mean kinetic energy of the products.

The mass-symmetric reactions ⁵⁸Fe,⁵⁸Ni +⁵⁸Fe,⁵⁸Ni were studied at a beam energy of E_beam=30 MeV/nucleon in order to investigate the isospin dependence of fragment emission. Ratios of inclusive yields of isotopic fragments from hydrogen through nitrogen were extracted as a function of laboratory angle. A moving source analysis of the data indicates that at laboratory angles around 40° the yield of intermediate mass fragments (IMF’s) beyond Z=3 is predominantly from a midrapidity source. The angular dependence of the relative yields of isotopes beyond Z=3 indicates that the IMF’s at more central angles originate from a source which is more neutron deficient than the source responsible for fragments emitted at forward angles. The charge distributions and kinetic energy spectra of the IMF’s at various laboratory angles were well reproduced by calculations employing a quantum molecular-dynamics code followed by a statistical multifragmentation model for generating fragments. The calculations indicate that the measured IMF’s originate mainly from a single source. The isotopic composition of the emitted fragments is, however, not reproduced by the same calculation. The measured isotopic and isobaric ratios indicate an emitting source that is more neutron rich in comparison to the source predicted by model calculations.

Vaporization and high multiplicity events, where the nuclear system is almost completely disassembled in light charged particles (Z<~2), were analyzed for events with total charge detected (ΣZ=23) in the ³⁵Cl+¹²C reaction at 43 MeV/nucleon. Kinematic characteristics have been studied separately for Z=1, Z=2, and Z=3–4 particles. When those are compared to simulations, noticeable differences in privileged emission direction have been found between Z>~2 particles and protons. For violent reactions, these differences could be explained by the apparent “symmetrization” of the reaction partners via the formation of a large necklike structure, and its subsequent breakup into many light ions.

The experimental signature of the formation of a necklike structure, with a velocity between that of the projectilelike emitter and that of the targetlike emitter, is investigated with the same beam and experimental setup for targets lighter and heavier than the projectile. The reactions are ³⁵Cl on ¹²C and on ¹⁹⁷Au at 43 MeV/nucleon. Particle velocity distributions are compared with two-source statistical simulations and the presence of a necklike structure is inferred from the data. In the second part of the paper, dynamical model simulations with the formation of a necklike structure are presented for the ³⁵Cl+¹²C system at 43 MeV/nucleon.

Fragment production has been studied as a function of the source mass and excitation energy in peripheral collisions of ³⁵Cl+¹⁹⁷Au at 43 MeV/nucleon and ⁷⁰Ge+^natTi at 35 MeV/nucleon. The results are compared to the Au+Au data at 600 MeV/nucleon obtained by the ALADIN Collaboration. A mass scaling, by A_source∼35 and 190, strongly correlated to excitation energy per nucleon, is presented, suggesting a thermal fragment production mechanism. Comparisons to a standard sequential decay model and the lattice-gas model are made. Fragment emission from a hot, rotating source is unable to reproduce the experimental source size scaling.

Characteristics of ³⁵Cl+¹²C collisions at 43 MeV/nucleon have been studied for events with total charge detected ( ΣZ  =  23). It is shown that, while single-source events are present in the data, the binary nature of the collision is dominant. For binary events, the emitting sources (projectilelike and targetlike) were reconstructed independently allowing a direct measurement of the total dissipated energy. It is found that up to 75% of the available energy is dissipated and the significant momentum transfer of the selected events leads to the “equal temperature limit.”

The same hot nuclear system (ΣZ=18) has been studied for two different entrance channels with reaction products detected in a forward array of scintillators: central collisions of ²⁴Mg on a ¹²C target at 25A and 35A MeV and peripheral pickup reactions of ³⁵Cl on a ¹⁹⁷Au target at 43A MeV. The detection-efficiency-corrected charge distributions, multiplicity of charged particles and cross sections as a function of excitation energy are compared. The reaction mechanism is investigated, through comparison to simulations with statistical observables. The central reaction ²⁴Mg+¹²C at 35A MeV is well characterized by a dissipative binary collision scenario. Data at 25A MeV show less evidence of such dynamical characteristics. The intermediate-mass fragments (3≤Z≤8) production for each reaction is compared to model calculations for different values of excitation energy. The systems formed in the central collision at 25A MeV and the pickup reaction at 43A MeV show similar source characteristics, both statistically and in momentum space. However, the yields of the various exit channels, from evaporation and/or fission to multifragmentation and vaporization, differ for the two reactions. © 1996 The American Physical Society.