Search Results (44 total)

The characteristics of intermediate mass fragments (IMFs: 3≤  Z≤  20) produced in midperipheral and central collisions are compared. We compare IMFs detected at midvelocity with those evaporated from the excited projectilelike fragment (PLF^*). On average, the IMFs produced at midvelocity are larger in atomic number, exhibit broader transverse velocity distributions, and are more neutron rich as compared to IMFs evaporated from the PLF^*. These characteristics of midvelocity fragments are consistent with the low-density formation of the fragments. We present in the different kinematical regions studied, the 〈E_⊥〉 for isotopically identified IMFs. For a given Z,〈E_⊥〉 is either constant or decreases slightly with increasing A, in contradiction with a mass-dependent collective expansion in which all IMFs are emitted on average at the same time. Neutron-deficient isotopes of even Z elements manifest higher kinetic energies than heavier isotopes of the same element for both PLF^* and midvelocity emission. This result may be because of the charged-particle decay of long-lived excited states.

By combining data from a charged particle ⁵⁸Ni+⁵⁸Ni experiment at 52 MeV/nucleon with an ³⁶Ar+⁵⁸Ni experiment at 50 MeV/nucleon for which free neutrons have been detected, an increase in the neutron to proton ratio of the whole nuclear material at midrapidity has been experimentally observed in the reaction ⁵⁸Ni+⁵⁸Ni at 52 MeV/nucleon. The neutron-to-proton ratio of the quasi-projectile emission is analyzed for the same reactions and is seen to decrease below the ratio of the initial system. Those observations suggest that an asymmetric exchange of neutrons and protons between the quasiprojectile and the midrapidity region exists.

Alpha particles emitted from an excited projectilelike fragment (PLF^*) formed in a peripheral collision of two intermediate-energy heavy ions exhibit a strong preference for emission towards the targetlike fragment. The interplay of the initial deformation of the PLF^* caused by the reaction, Coulomb proximity, and the rotation of the PLF^* results in the observed anisotropic angular distribution. Changes in the shape of the angular distribution with excitation energy are interpreted as being the result of forming more elongated initial geometries in the more peripheral collisions.

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.

Using symmetric ¹¹²Sn+¹¹²Sn, ¹²⁴Sn+¹²⁴Sn collisions as references, we probe isospin diffusion in peripheral asymmetric ¹¹²Sn+¹²⁴Sn, ¹²⁴Sn+¹¹²Sn systems at an incident energy of E/A=50  MeV. Isoscaling analyses imply that the quasiprojectile and quasitarget in these collisions do not achieve isospin equilibrium, permitting an assessment of isospin transport rates. We find that comparisons between isospin sensitive experimental and theoretical observables, using suitably chosen scaled ratios, permit investigation of the density dependence of the asymmetry term of the nuclear equation of state.

Isotopic yields for light particles and intermediate mass fragments have been measured for central ¹¹²Sn+¹¹²Sn, ¹¹²Sn+¹²⁴Sn, ¹²⁴Sn+¹¹²Sn, and ¹²⁴Sn+¹²⁴Sn collisions at E∕A=50  MeV and compared with predictions of stochastic mean field calculations. These calculations predict a sensitivity of the isotopic distributions to the density dependence of the asymmetry term of the nuclear equation of state. However, the secondary decay of the excited fragments modifies significantly the primary isotopic distributions and these modifications are rather sensitive to theoretical uncertainties in the excitation energies of the hot fragments. The predicted final isotope distributions are narrower than the experimental data and the sensitivity of the predicted yields to the density dependence of the asymmetry term is reduced.

Projectilelike fragments (PLF, 15<~Z<~46) formed in peripheral and midperipheral collisions of ¹¹⁴Cd projectiles with ⁹²Mo nuclei at E/A=50 MeV have been detected at very forward angles, 2.1°<~θ_lab<~4.2°. Calorimetric analysis of the charged particles observed in coincidence with the PLF reveals that the excitation of the primary PLF is strongly related to its velocity damping. Furthermore, for a given V_PLF*, its excitation is not related to its size, Z_PLF*. For the largest velocity damping, the excitation energy attained is large, approximately commensurate with a system at the limiting temperature.

Fusionlike events in ⁵⁸Ni+¹²C at 34.5 MeV/nucleon are isolated by use of the statistical discriminant analysis method applied to a set of 20 global variables in complete events with at least 90% of the total charge of the system. Two-fragment reduced-velocity correlation functions are measured for the emission of intermediate mass fragments (IMF’s) and compared to many-body trajectory calculations. Alpha particle emission sequence has been deduced as a function of excitation energy. At the highest excitations, α particles are emitted simultaneously with IMF’s on a time scale of about 200–300 fm/c. In less excited events, they are emitted on a longer time scale. This hybrid deexcitation mechanism, sequential and prompt, is corroborated by comparing the charge of the heaviest and the second heaviest fragments to predictions of GEMINI and SMM simulations.

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.

In efforts to determine phase transitions in the disintegration of highly excited heavy nuclei, a popular practice is to parametrize the yields of isotopes as a function of temperature in the form Y(z)=z^-τf(z^σ(T-T₀)), where Y(z)’s are the measured yields and τ, σ, and T₀ are fitted to the yields. Here T₀ would be interpreted as the phase transition temperature. For finite systems such as those obtained in nuclear collisions, this parametrization is only approximate and hence allows for extraction of T₀ in more than one way. In this work we look in detail at how values of T₀ differ, depending on methods of extraction. It should be mentioned that for finite systems, this approximate parametrization works not only at the critical point, but also for first-order phase transitions (at least in some models). Thus the approximate fit is no guarantee that one is seeing a critical phenomenon. A different but more conventional search for the nuclear phase transition would look for a maximum in the specific heat as a function of temperature T₂. In this case T₂ is interpreted as the phase transition temperature. Ideally T₀ and T₂ would coincide. We invesigate this possibility, both in theory and from the ISiS data, performing both canonical (T) and microcanonical (e=E^*/A) calculations. Although more than one value of T₀ can be extracted from the approximate parametrization, the work here points to the best value from among the choices. Several interesting results, seen in theoretical calculations, are borne out in experiment.

The defining characteristics of fragment emission resulting from the noncentral collision of ¹¹⁴Cd ions with ⁹²Mo target nuclei at E/A=50 MeV are presented. Charge correlations and average relative velocities for midvelocity fragment emission exhibit significant differences when compared to standard statistical decay. These differences associated with similar velocity dissipation are indicative of the influence of the entrance channel dynamics on the fragment production process.

Investigation of intermediate-velocity particle production is performed on entrance channel mass asymmetric collisions of ⁵⁸Ni+C and ⁵⁸Ni+Au at 34.5 MeV/nucleon. Distinctions between prompt preequilibrium ejections, multiple neck ruptures, and an alternative phenomenon of delayed aligned asymmetric breakup are achieved using source reconstructed correlation observables and time-based cluster recognition in molecular dynamics simulations.

The formation and deexcitation of fusionlike events selected in events with a total charge equal or greater than 16 in ²⁴Mg+¹²C system has been investigated at 25, 35, and 45 MeV/nucleon with a large multidetector array. Central single-source events are selected by use of the statistical discriminant analysis method applied to a set of 26 global variables. The fusion cross section has been extracted for the three bombarding energies and compared to other experimental data and to theoretical predictions. The total multiplicity is found to first increase to a maximum value and then decrease with increasing beam energy. It is shown that this behavior is connected to the opening of multifragmentation channels at 45 MeV/nucleon and the disappearance of channels with only light charged particles.

Isotope ratios of fragments produced at midrapidity in peripheral and central collisions of ¹¹⁴Cd ions with ⁹⁸Mo target nuclei at E/A=50 MeV are compared. The influence of the size (A), density, N/Z, E^*/A, and E_flow/A of the emitting source on the measured isotope ratios was explored by comparison with a statistical model (SMM). The midrapidity region associated with peripheral collisions does not appear to be neutron-enriched relative to central collisions.

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 particle emission at intermediate velocities in mass asymmetric reactions is studied within the framework of classical molecular dynamics. Two reactions in the Fermi energy domain were modeled, ⁵⁸Ni+C and ⁵⁸Ni+Au at 34.5 MeV/nucleon. The availability of microscopic correlations at all times allowed a detailed study of the fragment formation process. Special attention was paid to the physical origin of fragments and emission timescales, which allowed us to disentangle the different processes involved in the midrapidity particle production. Consequently, a clear distinction between a prompt preequilibrium emission and a delayed aligned asymmetric breakup of the heavier partner of the reaction was achieved.

The thermal component of the 8 GeV/c π+ Au data of the ISiS Collaboration is shown to follow the scaling predicted by Fisher’s model when Coulomb energy is taken into account. Critical exponents τ and σ, the critical point (p_c,ρ_c,T_c), surface energy coefficient c₀, enthalpy of evaporation ΔH, and critical compressibility factor C_c^F are determined. For the first time, the experimental phase diagrams, (p,T) and (T,ρ), describing the liquid vapor coexistence of finite neutral nuclear matter have been constructed.

A percolation model of nuclear fragmentation is used to interpret 10.2 GeV/c p+¹⁹⁷Au multifragmentation data. Emphasis is put on finding signatures of a continuous nuclear matter phase transition in finite nuclear systems. Based on model calculations, corrections accounting for physical constraints of the fragment detection and sequential decay processes are derived. Strong circumstantial evidence for a continuous phase transition is found, and the values of two critical exponents, σ  =  0.5±0.1 and τ  =  2.35±0.05, are extracted from the data. A critical temperature of T_c  =  8.3±0.2 MeV is found.

Two-fragment reduced-velocity correlation functions of intermediate-mass fragments emitted from midrapidity component and quasiprojectile (QP) sources formed in ⁵⁸Ni+¹²C and ⁵⁸Ni+¹⁹⁷Au reactions at 34.5 MeV/nucleon have been studied. For the midrapidity component, they show a stronger Coulomb suppression at low relative velocities than for the QP source, suggesting a shorter emission time for it than for the QP source. Comparing the experimental correlation functions with the prediction of many-body Coulomb trajectory code, the emission times of a QP source formed in both reactions were extracted as a function of the excitation energy. The variation of emission time of the QP source with the excitation energy is independent of the reaction system. It decreases monotonically with the excitation energy in the range of (2–6)A MeV from several hundred fm/c to about 100 fm/c. Above an excitation energy of 6A MeV, it becomes very short and saturates, suggesting that the QP source undergoes a multifragmentationlike breakup. The influence of the quasitarget fragment Coulomb interaction on QP emission time is also considered.

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.

We have measured two-fragment correlation functions of the intermediate mass fragments emitted from quasiprojectile sources and midrapidity component formed in ⁵⁸Ni+¹⁹⁷Au at 34.5 MeV/nucleon. The two-fragment correlation functions of midrapidity component show a stronger Coulomb suppression than the quasiprojectile source. This Coulomb suppression for midrapidity component changes very little with the excitation energy of the quasiprojectile source deduced event by event by calorimetry method. By comparing the experimental correlation functions with an N-body Coulomb trajectory code calculation, the emission time of quasiprojectile sources has been extracted as a function of the excitation energy. The emission time decreases monotonically with the excitation energy in the range of 2–6 A MeV from 550 fm/c to about 150 fm/c. Above excitation energy of 6A MeV, the emission time becomes shorter and constant, suggesting that prompt multifragmentation occurs in these quasiprojectile sources.

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.

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.