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The theoretical description of heavy-ion collisions at intermediate beam energies, 10 MeV ≲, E/A ≲ 150 MeV, is still in an unsatisfactory state. Different factors contribute to that situation. Thus, for one, the nucleon excitation energies are low and in that comparable to nucleon localization energies, indicating a likely importance of the quantal effects. With the change in the beam energy in the discussed range, the dynamics changes from that dominated by the mean field to that dominated by collisions (as evidenced in the appearance of the flow balance-energy). As excitation energies grow, they begin to exceed the average binding energies per nucleon and, within the mentioned range of E/A, a massive production of intermediate-mass fragments (IMF) takes place[1]. The production, in terms of IMF multiplicity or total mass that IMF carry, maximizes at E/A ~ 75 MeV. The description of the intermediate and light fragment production is beyond the capability of common single-particle models of collisions[2]. The single-particle models with fluctuating forces[3,4] can describe fragment production, but miss the shell effects and the discreteness of the mass and charge numbers. The involved limitation is recognized once one realizes that, in the very central Au + Au collisions at 100 MeV/nucleon, the probability for a proton to come out from the reaction as a constituent of an a particle is close[5] to 50%. Within the Boltzmann-Langevin model with the fluctuating forces[3,4], the α particle plays no distinguished role. Statistical models[6,7] account for the shell effects, but miss the reaction dynamics. The importance of the dynamics is seen, in particular, in the fact that the collective outward flow energy in the reactions is comparable to the thermal energy.
130Dynamical effects and intermediate mass fragment production in peripheral and semicentral collisions of Xe+Sn at 50 MeV/nucleon
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39Formation and breakup of extra-large composite system in central Au+Au collisions.
Recent data on multifragmentation in central Au+Au collisions at beam energy 100 MeV/nucleon are analyzed within the statistical and molecular dynamics models. The data are consistent with statistical breakup of a thermalized source with radius of 14 fm, mass number around 250, and excitation energy about 7 meV/nucleon, in good agreement with molecular dynamics calculations. According to this model, fragment formation takes place within a time interval of 50 fm/c after the maximum compression. Intense preequilibrium emission of light particles and strong collective flow of fragments are predicted.
Read more14PACIAE 2.1: An updated issue of the parton and hadron cascade model PACIAE 2.0
We have updated the parton and hadron cascade model PACIAE 2.0 for relativistic nuclear collisions (cf. Comput. Phys. Comm. 183(2012)333) to the new issue of PACIAE 2.1. The PACIAE model is based on PYTHIA. In the PYTHIA model, once the generated particle/parton transverse momentum $p_T$ is randomly selected, its component of $[p_x,p_y]$ was originally put on the circle with radius $p_T$ randomly. It is now replaced by put on the circumference of ellipse with half major and minor axes of $p_T(1+\delta_p)$ and $p_T(1-\delta_p)$, respectively, in order to investigate better the transverse momentum anisotropy in final hadronic state.
Read more25Intermediate mass fragment emission in Fe+Au collisions.
Experimental results are presented on the charge, velocity, and angular distributions of intermediate mass fragments (IMFs) for the reaction Fe+Au at bombarding energies of 50 and 100 MeV/nucleon. Results are compared to the quantum molecular dynamics (QMD) model and a modified QMD which includes a Pauli potential and follows the subsequent statistical decay of excited reaction products. The more complete model gives a good representation of the data and suggests that the major source of IMFs at large angles is due to multifragmentation of the target residue.
Read more20Superasymmetric fission of heavy nuclei induced by intermediate-energy protons
In this work we present the results for the investigation of\nintermediate-mass fragment (IMF) production with the proton-induced reaction at\n660 MeV on 238U and 237Np target. The data were obtained with the LNR Phasotron\nU-400M Cyclotron at Joint Institute for Nuclear Research (JINR), Dubna, Russia.\nA total of 93 isotopes, in the mass range of 30 < A < 200, were unambiguously\nidentified with high precision. The fragment production cross sections were\nobtained by means of the induced-activation method in an off-line analysis.\nMass-yield distributions were derived from the data and compared with the\nresults of the simulation code CRISP for multimodal fission. A discussion of\nthe super-asymmetric fragment production mechanism is also given.\n
Read more75Collective expansion in central Au+Au collisions.
Energy spectra for intermediate mass fragments produced in central Au + Au collisions at EA=100 MeV indicate a collective expansion at breakup. For the first time, values for this collective expansion energy per nucleon are extracted independently for each charge. Typically, these values are one-third to one-half of the incident kinetic energy per nucleon in the c.m. system, but they decrease with Zf, suggesting that all fragments do not participate equally in the collective expansion.Received 29 April 1994DOI:https://doi.org/10.1103/PhysRevLett.73.3367©1994 American Physical Society
Read more1Excitation Energy and Temperature in the Multifragmentation of 1 GeV/Nucleon Au+C
Multifragmentation (MF) is the break-up of colliding nuclei into many species of lighter nuclei, particularly intermediate mass fragments (IMF’s) with 3 ≤ Z IMF ≤ 30 [1]. MF occurs in many different kinds of nuclear reactions when the excitation energy per nucleon is comparable to the nucleon binding energy. At lower excitation energy, a compound nucleus is formed, which decays by evaporation of a few light particles (primarily neutrons, protons, and alphas), leaving a large residual nucleus that contains most of the original mass. At higher energy, the excited system decomposes entirely into light particles. As excitation energy increases, the final mass yields start at low excitation with a double structure peaked at the lowest and highest masses, progress through a power law mass distribution of IMF’s at intermediate excitation, and end at high excitation with an exponential distribution of only light particles. This evolution strongly resembles the progression of a heated fluid from the liquid state through the critical point into the gaseous phase. Since pioneering studies in the early 1980’s [2], intense experimental and theoretical effort has focused on this behavior, attempting to understand the mechanism of MF. Although it is not universally accepted, the idea of a nuclear liquid-gas phase transition has become the leading paradigm used to interpret MF phenomena. Many important questions remain unresolved, including: does MF exhibited by different nuclear reactions have a common underlying physical mechanism? do the excited systems equilibrate sufficiently to apply thermal concepts? is the phase transition first order or continuous, and does this vary for different reactions? what are the thermodynamic properties (e.g. temperature, density, entropy) of these systems? what are the trajectories in the temperature-density plane for various reactions? can the physical properties of nuclear matter, including the equation of state, be extracted from MF data?
Read more18Three paths for intermediate-mass fragment formation at a near-onset excitation energy of 1.3 MeV/nucleon.
Intermediate mass fragments of 4\ensuremath{\le}Z\ensuremath{\le}17 have been studied from the reaction 640 MeV $^{86}\mathrm{o}$/rKr${+}^{63}$Cu. Inclusive energy and angular distribution measurements have been made as well as coincidence measurements with fissionlike fragments of 18\ensuremath{\le}Z\ensuremath{\le}40. The coincidence configuration was such that two-body exit channels were excluded; nevertheless, heavy fragments were recorded both in plane and out of plane with respect to light fragments of 4\ensuremath{\le}Z\ensuremath{\le}10. Three mechanisms have been identified for intermediate mass fragment production. The dominant pathway for all intermediate mass fragments is two-body breakup of fission or evaporationlike character. A second pathway for Z\ensuremath{\le}10 is intermediate mass fragment ejection from the composite nucleus followed by sequential fission. The third path is simultaneous ternary breakup for 4\ensuremath{\le}Z\ensuremath{\le}9 that resembles ternary alpha-accompanied fission at low energies.
Read more2Rapidity correlations in the RHIC Beam Energy Scan Data
A pair-normalized two-particle covariance versus the rapidity of the two particles, called R2, was originally studied in ISR and FNAL data in the 1970's. This variable has recently seen renewed interest for the study of the dynamics of heavy-ion collisions in the longitudinal direction. These rapidity correlations can be decomposed into a basis set of Legendre polynomials with prefactors 〈amn〉, which can be considered the rapidity analog of the decomposition of azimuthal anisotropies into a set of cosine functions with prefactors vn. The 〈amn〉 values have been suggested to be sensitive to the number of particle emitting sources, baryon stopping, viscosities, and critical behavior. The rapidity correlations have been measured by the STAR collaboration as a function of the beam energy for 0–5% central Au+Au collisions with beam energies ranging from 7.7 to 200 GeV. The experimental results and comparisons to the UrQMD model are presented.
Read more4Ω production in p+p, Au+Au and U+U collisions at STAR
Ω production in <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si1.gif" overflow="scroll"><mml:mi>p</mml:mi><mml:mo>+</mml:mo><mml:mi>p</mml:mi></mml:math>, <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si2.gif" overflow="scroll"><mml:mrow><mml:mi mathvariant="normal">Au</mml:mi></mml:mrow><mml:mo>+</mml:mo><mml:mrow><mml:mi mathvariant="normal">Au</mml:mi></mml:mrow></mml:math> and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si3.gif" overflow="scroll"><mml:mi mathvariant="normal">U</mml:mi><mml:mo>+</mml:mo><mml:mi mathvariant="normal">U</mml:mi></mml:math> collisions at STAR
Read more11Vaporization and multifragmentation in the reaction 1.2 GeV [formula omitted] + Cu and Ag
Vaporization and multifragmentation in the reaction 1.2 GeV [formula omitted] + Cu and Ag
4Production of Energetic Light Fragments in Spallation Reactions
Different reaction mechanisms contribute to the production of light fragments (LF) from nuclear reactions. Available models cannot accurately predict emission of LF from arbitrary reactions. However, the emission of LF is important for many applications, such as cosmic-ray-induced single event upsets, radiation protection, and cancer therapy with proton and heavy-ion beams, to name just a few. The cascade-exciton model (CEM) and the Los Alamos version of the quark-gluon string model (LAQGSM), as implemented in the CEM03.03 and LAQGSM03.03 event generators used in the Los Alamos transport code MCNP6, describe quite well the spectra of fragments with sizes up to 4He across a broad range of target masses and incident energies. However, they do not predict high-energy tails for LF heavier than 4He. The standard versions of CEM and LAQGSM do not account for preequilibrium emission of LF larger than 4He. The aim of our work is to extend the preequilibrium model to include such processes. We do this by including the emission of fragments heavier than 4He at the preequilibrium stage, and using an improved version of the Fermi Break-up model, providing improved agreement with various experimental data.
Read more70Midrapidity source of intermediate-mass fragments in highly central collisions of Au + Au at 150A MeV.
Charged particles have been observed in collisions of Au on Au at an incident energy of 150{ital A} MeV using a high-granularity detector system covering approximately the forward hemisphere in the center-of-mass system. Highly central collisions have been studied using a double selection criterion which combines large charged-particle multiplicities with small transverse-momentum directivities. In this class of events about one quarter of the total nuclear charge emerges as intermediate-mass fragments with nuclear charges {ital Z}{gt}2. These fragments are centered at midrapidity and are produced with large transverse velocities.
Read moreAsymmetry effects in fragment production
The production of different fragments has been studied by taking into account the mass asymmetry of the reaction and employing the momentum dependent interactions. Two different set of asymmetric reactions have been analyzed while keeping Atotal fixed using soft momentum dependent equation of state. Our results indicate that the impact of momentum dependent interactions is different in lighter projectile systems as compared to heavier ones. The comparative analysis of IQMD simulations with the experimental data in case of heavier projectile and lighter target system for the reaction of 197Au+27Al (η = 0.7) at E = 600 MeV/nucleon shows that with the inclusion of MDI we are able, upto some extent, to reproduce the experimental universality of rise and fall of intermediate mass fragments (IMFs).
Read more292Identified Baryon and Meson Distributions at Large Transverse Momenta fromAu+AuCollisions atsNN=200 GeV
Transverse momentum spectra of pi+/-, p, and p up to 12 GeV/c at midrapidity in centrality selected Au + Au collisions at square root sNN=200 GeV are presented. In central Au + Au collisions, both pi +/- and p(p) show significant suppression with respect to binary scaling at pT approximately >4 GeV/c. Protons and antiprotons are less suppressed than pi+/-, in the range 1.5 approximately < pT approximately < 6 GeV/c. The pi-/pi+ and p/p ratios show at most a weak pT dependence and no significant centrality dependence. The p/pi ratios in central Au + Au collisions approach the values in p + p and d + Au collisions at pT approximately >5 GeV/c. The results at high pT indicate that the partonic sources of pi+/-, p, and p have similar energy loss when traversing the nuclear medium.
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