Search Results (28 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.

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.

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.

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.

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.

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 decay structure of the particle-unstable nucleus ¹⁰Li was studied using the method of sequential neutron decay spectroscopy (SNDS) at 0°. The decay energies of ¹⁰Li can be derived from the relative velocity spectrum of the ⁹Li daughter and the neutron measured in coincidence. Evidence for low-lying s-wave strength was observed with a scattering length of <-20 fm, corresponding to a peak energy of <50 keV.

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.

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.

The proton-unbound nucleus ¹¹N has been studied via kinematic reconstruction of the emitted proton in coincidence with the residual ¹⁰C daughter nucleus. Resonances in ¹¹N were populated by using a 40 MeV/nucleon radioactive beam of ¹²N to induce the reaction ⁹Be(¹²N,¹¹N), followed by the proton decay of ¹¹N. The decay energy spectrum was constructed from the energies and separation angle of the ¹⁰C and the proton. In addition to protons from the known 1/2^- state, at 2.24 MeV above the proton decay threshold, another peak is seen near 1.45 MeV. This peak could potentially be due to the predicted 1/2⁺ ground state and/or due to the decay of the 3/2^- state to the first excited state of ¹⁰C

Isotopically resolved intermediate-mass fragments and light charged particles have been detected from the reactions ⁴⁰Ar and ⁴⁰Ca with ⁵⁸Fe and ⁵⁸Ni at E_beam=33 and 45 MeV/nucleon. There is an angular dependence to the isotopic ratios. A moving source analysis shows that fragments emitted at Θ_lab=40° can be attributed primarily to a composite source while the fragments emitted at backward angles are primarily from a targetlike source. The results are compared to predictions of QMD, BUU, and GEMINI. QMD generally reproduces the charge distribution and energy spectra and has partial success with the isobaric ratios when the system is chemically equilibrated. All of the models have difficulty reproducing the isotopic ratios when the system is not chemically equilibrated.

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.

Distributions of parallel and transverse momenta for ⁷Be fragments formed in the breakup of ⁸B have been measured at 41A MeV. The p_∥ distributions are narrow ( 81±4 and 62±3 MeV/c FWHM for Be and Au targets, respectively), comparable to those of neutron halo nuclei. Reaction mechanisms influence the ⁷Be momentum distributions, so they do not directly reflect the valence proton momentum wave function. We present reaction models that reproduce the distributions.

We modify the method of Albergo et al. for determining the temperature of an excited nucleus from double ratios of isotope yields and present a statistical model which accounts for the population and decay of excited states of the emitted fragments. Nuclear temperatures are extracted using experimental ratios of isotopic yields of fragments from helium through carbon for the reactions ⁴⁰Ca + ⁵⁸Ni, ⁴⁰Ar + ⁵⁸Ni, ⁴⁰Ca + ⁵⁸Fe, and ⁴⁰Ar + ⁵⁸Fe at 33 MeV/nucleon projectile energy. Using the model we obtain consistent values for the temperature from various isotope combinations within the experimental error when accounting for the population and decay of the excited fragments.

High energy γ rays from the decay of the giant resonance in hot ¹²⁰Sn nuclei were measured in the excitation energy range of 30–130 MeV. The excited nuclei were populated by inelastic scattering of α particles at 40 and 50 MeV/nucleon. The resonance width was observed to increase monotonically with increasing excitation energy, from 5 MeV at the ground state to ∼12 MeV at the largest excitation energy. Inelastic scattering predominantly populates low angular momentum states, and the observed width increase is thus attributed to fluctuations in the nuclear shape induced by temperature.