Search Results (21 total)

We present a complete systematics (excitation functions and system-size dependences) of global stopping and side flow for heavy ion reactions in the energy range between 0.09A and 1.93A  GeV. For the heaviest system, Au+Au, we observe a plateau of maximal stopping extending from about 0.2A to 0.8A  GeV with a fast drop on both sides. The degree of stopping, which is shown to remain significantly below the expectations of a full stopping scenario, is found to be highly correlated to the amount of side flow.

Detailed studies of the azimuthal dependence of the mean fragment and flow energies in the Au+Au and Xe+CsI systems are reported as a function of incident energy and centrality. Comparisons between data and model calculations show that the flow energy values along different azimuthal directions could be viewed as snapshots of the fireball expansion with different exposure times. For the same number of participating nucleons more transversally elongated participant shapes from the heavier system produce less collective transverse energy. Good agreement with Boltzmann-Uehling-Uhlenbeck calculations is obtained for a soft nuclear equation of state.

We present new experimental data on directed flow in collisions of Au+Au, Xe+CsI, and Ni+Ni at incident energies from 90A to 400A MeV. We study the centrality and system dependence of integral and differential directed flow for particles selected according to charge. All the features of the experimental data are compared with isospin quantum molecular dynamics (IQMD) model calculations in an attempt to extract information about the nuclear matter equation of state (EoS). We show that the combination of rapidity and transverse momentum analysis of directed flow allows to disentangle various parametrizations in the model. At 400A MeV, a soft EoS with momentum dependent interactions is best suited to explain the experimental data in Au+Au and Xe+CsI, but in the case of Ni+Ni the model underpredicts flow for any EoS. At 90A MeV incident beam energy, none of the IQMD parametrizations studied here are able to consistently explain the experimental data.

We present the centrality dependence of proton and deuteron rapidity distributions in Ru+Ru collisions at 400A MeV. Data are compared with isospin quantum molecular dynamics (IQMD) calculations under various assumptions on the nucleon-nucleon cross section in the medium. The rapidity spectra of both particles can be reproduced by IQMD with a free nucleon-nucleon cross section for the most central collisions. The ratio of baryon rapidity distributions in isospin asymmetric collision systems shows incomplete mixing and partial transparency of the projectile and target nuclei at this beam energy.

We present experimental data on directed flow in semicentral Au+Au collisions at incident energies from 90 to 400 A MeV. For the first time for this energy domain, the data are presented in a transverse momentum differential way. We study the first order Fourier coefficient v₁ for different particle species and establish a gradual change of its patterns as function of incident energy and for different regions in rapidity.

Four different combinations of ₄₄⁹⁶Ru and ₄₀⁹⁶Zr nuclei, both as projectile and target, were investigated at the same bombarding energy of 400A MeV using a 4π detector. The degree of isospin mixing between projectile and target nucleons is mapped across a large portion of the phase space using two different isospin-tracer observables, the number of measured protons and the tritium to ³He yield ratio. The experimental results show that the global equilibrium is not reached even in the most central collisions. Quantitative measures of stopping and mixing are extracted from the data. They are found to exhibit a quite strong sensitivity to the in-medium ( n,n) cross section used in microscopic transport calculations.

The production of charged pions, protons, and deuterons has been studied in central collisions of ⁵⁸Ni on ⁵⁸Ni at incident beam energies of 1.06A, 1.45A, and 1.93A GeV. The dependence of transverse-momentum and rapidity spectra on the beam energy and on the centrality of the collison is presented. It is shown that the scaling of the mean rapidity shift of protons established for between 10A and 200A GeV at the Brookhaven AGS and the CERN SPS accelerators energies is valid down to 1A GeV. The degree of nuclear stopping is discussed; quantum molecular dynamics calculations reproduce the measured proton rapidity spectra for the most central events reasonably well, but do not show any sensitivity between the soft and the hard equation of state. A radial flow analysis, using the midrapidity transverse-momentum spectra, delivers freeze-out temperatures T and radial flow velocities β_r which increase with beam energy up to 2A GeV; in comparison to the existing data of Au on Au over a large range of energies, only β_r shows a system size dependence.

The ratio of the total charge bound in fragments with Z between 2 and 15 to the hydrogen yield, (tsum₂¹⁵M_iZi)/M_H, has been measured, and the neutron-to-proton ratio n/p has been estimated from the data of central Au + Au reactions between 100 and 400A MeV, measured with the phase I setup of the detector system FOPI and GSI, Darmstadt, in the polar-angle range between 7° and 30°. These two quantities were used to determine the entropy per nucleon S/A by comparing them with the predictions of the freesco code. The analysis allows the simulataneous extraction of the values of the baryonic entropy, temperature, and collective flow. The extracted values are in good agreement with the values obtained in earlier FOPI studies, and, for the baryonic entropy, with recent hydrodynamic calculations.

Complete distributions of the light and intermediate mass fragments ( Z  =  1–6) produced within the polar angular range 1^∘≤Θ_lab≤30^∘ in highly central collisions of 250 A MeV Au + Au are presented. The results of this measurement and a model analysis are used to study the expansion and clustering of the hot and compressed transient state formed in central collisions of such a heavy system. The influence of the initial conditions on the final observables is discussed.

Velocity correlations of intermediate mass fragments (IMF), produced in collisions of Au+Au at 100, 150, 250, and 400A MeV beam energy, are extracted from measurements with the 4π detector system (FOPI) in construction stage I at Schwerionen-Synchrotron (SIS) at the Gesellschaft für Schwerionenforschung (GSI) in Darmstadt. The IMF correlation functions of peripheral and semicentral events are found to be strongly affected by the collective sideward motion of nuclear matter. The sideflow causes an enhancement of correlations at small relative velocities. This enhancement results from the mixing of differently azimuthally oriented events; it vanishes if the events are rotated into a unique reaction plane. Selecting violent central collisions, the comparison of the data with a Coulomb dominated final-state interaction model points to a radius of the expanding and multifragmenting source of R_s≃13 fm for 100A MeV which appears shrinking by 20% when increasing the projectile energy to 400 MeV per nucleon. The deduced source radii are found to depend on the radial explosion energy used in the model. The inclusion of such a collective expansion is necessary for a reasonable description of the experimental single-particle spectra of the IMF. The unique Coulomb suppression of small relative IMF velocities, found for the given beam energy range, is attributed to rather constant averaged next-neighbor distances 〈d_IMF〉=8.6±0.2 fm of the IMF charge centers within the source at breakup time.

Using the FOPI facility at GSI Darmstadt complete data of Au on Au collisions at 150A MeV were collected for charged products (Z=1–15) at laboratory angles 1°≤Θ_lab≤30°. Central collisions were selected by applying various criteria. The kinetic energy spectra of fragments from an isolated midrapidity source are investigated in detail for center-of-mass angles 25°≤Θ_c.m.≤45°. The heavy products (Z≥3) are used to determine the collective energy which is found to be at least 10A MeV.

Velocity correlations of intermediate mass fragments (IMFs), produced in central collisions of Au+Au at 150 MeV beam energy, are extracted from measurements with the FOPI (phase I) detector system at SIS in GSI Darmstadt. The IMF correlation function for semicentral events is found to be affected by the directed sideward flow. When rotating the events into a unique reaction plane an enhancement of correlations, resulting from event mixing effects, vanishes. Selecting violent collisions with a high degree of azimuthal symmetry the correlation function appears nearly independent of additional event or single particle gate conditions. The comparison of the data with a Coulomb dominated final-state interaction model points to an expanding and multifragmenting soure with radius R∼14 fm.

The entropy per nucleon (S/A) has been extracted for the Au [(150–800)A MeV] + Au reaction by using the phase I setup of the 4π facility at GSI, Darmstadt. The entropy has been obtained from the comparison of various observables characterizing the dM/dZ fragment multiplicity distributions, extending up to Z∼15, with those calculated with the quantum statistical model. It is the first time that S/A values are determined by considering the full ensemble of charged products detected in the reaction. Consistent values of S/A are found from different methods. These entropy values are shown to be fairly independent of the volume of the ‘‘participant’’ region considered. They are somewhat lower than those extracted in earlier works but are in good agreement with hydrodynamic calculations and suggest a low viscosity for the hot and dense nuclear matter.

The results of intranuclear cascade calculations (ideal gas with two-body collisions and no mean field), complemented by a simple percolation procedure, are compared with experimental data on protons and light nuclear fragments (d, t, ³He, and ⁴He) measured in 400 and 800 MeV/nucleon Ne+Nb collisions using a large solid angle detector. The model reproduces quite well global experimental observables like nuclear fragment multiplicity distributions or production cross sections, and nuclear fragment to proton ratios. For rapidity distributions the best agreement occurs for peripheral reactions. Transverse momentum analysis confirms once again that the cascade, although being a microscopic approach, gives too small a collective flow, the best agreement being reached for Z=2 nuclear fragments. Nevertheless these comparisons are encouraging for further improvements of the model. Moreover, such an approach is easy to extend to any other models that could calculate the nucleon phase space distribution after the compression stage of the reaction, when light nuclear fragments emitted at large angles are constructed from percolation.

Charged particles have been observed in collisions of Au on Au at an incident energy of 150A 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 Z>2. These fragments are centered at midrapidity and are produced with large transverse velocities.

Deuteron spectra at laboratory angles from 30° to 90° were measured in α+(Pb, Cu, and C) collisions at 800, 600, and 200 MeV/nucleon, and α+(Pb and C) collisions at 400 MeV/nucleon. The coalescence relation between protons and deuterons was examined for the inclusive part of the spectra. The size of the interacting region was evaluated from the observed coalescence coefficients. The rms radius is typically 4–5 fm, depending of the target mass. The proton and deuteron energy spectra corresponding to central collisions were fitted assuming emission from a single source moving with a velocity intermediate between that of the projectile and the target. The extracted ‘‘temperatures’’ are independent of the nature of the emitted particle, indicating that the fragments have a common source. The best fits were achieved for 200- and 400-MeV/nucleon reactions. Spectra of deuteron-like pairs, including real deuterons and neutron-proton pairs that may be contained in a larger nuclear cluster, are compared to the prediction of an intranuclear cascade model incorporating a clustering algorithm based on a classical coalescence prescription. Best agreements between experimental and predicted deuteron-like spectra occur for 800- and 600-MeV/nucleon collisions.

Charged pions and light nuclei (p, d, t, ³He, and ⁴He) have been measured in the interaction of proton beams with C, Nb, and Pb targets at 0.8 and 1.6 GeV incident energies, using a large solid angle detector. From slices on the multiplicity of protonlike particles (free protons and protons bound in light fragments), the events have been sorted out into two classes corresponding to more peripheral and more central collisions. For each class of events, the mean value and the dispersion of the π⁺ and π^- multiplicity distributions have been studied as a function of target mass and incident energy. Comparisons to the Liege intranuclear cascade predictions exhibit some discrepancies which are discussed.

Triple differential cross sections of charged pions were measured for collisions of Ne projectiles at E/A=800 MeV with NaF, Nb, and Pb targets. The reaction plane was estimated event by event from the light-baryon momentum distribution. For heavy targets, preferential emission of charged pions away from the interaction zone towards the projectile side was observed in the transverse direction. Such a preferential emission, which is not predicted by cascade calculations, may be attributed to a stronger pion absorption by the heavier spectator remnant.

A p-γ coincidence experiment has been performed with 400-MeV protons. The angular and momentum analysis of the scattered proton demonstrates a primary quasifree process. The residual nuclei which have been identified from the γ-ray spectra characterize the final states and indicate an evaporation process or a sequential ejection at low energy of some correlated nucleons. In addition, preliminary results with triple coincidences (2p-γ) indicate a promising way to clarify the mechanism of quasifree reactions.

Various light- and medium-mass nuclei have been bombarded with 150-, 230-, 600-, and 960-MeV protons. Prompt γ rays were measured with a Ge(Li) detector. A large number of lines in the γ-energy spectra correspond to the de-excitation of the first levels of 4n nuclei (N=Z even). The dependence of this effect on the incident energy and the mass of the target is studied. The data are compared with recent pion-bombardment results.