Search

Complex fragments with 4 ⩽ Z ≲ 50 have been detected in the reactions of 139 La + 12 C and 27 Al at E / A = 14.7 and 18.0 MeV. From the measured angular distributions, the cross sections for the isotropic, target-like and projectile-like components were extracted. The roles of deep-inelastic, fast-fission, and incomplete fusion processes, and the statistical, compound nucleus emission of complex fragments are discussed.

As online trigger for events containing Λ hyperons in p+p collisions at 3.1 GeV a silicon-based device has been designed and built. This system has been integrated close to the target region within the FOPI spectrometer at GSI and was also employed as a tracking device to improve the vertex reconstruction of secondary decays. The design of the detector components, read-out, the trigger capability as well as the tracking performance are presented. An enrichment factor of about 14 was achieved for events containing a Λ-hyperon candidate.

The design of a general-purpose PreAmplifier-DIscriminator ASIC chip, PADI, is presented in this article. PADI is intended to be used as Front-End-Electronics (FEE) for reading out the timing Resistive-Plate Chambers (RPCs) in the time-of-flight (ToF) wall of the CBM detector for the future FAIR facility in Darmstadt-Germany, which will comprise about 100,000 channels in a 150 m2 area. The evolution of this 0.18 μm CMOS technology design will be presented, from the first prototype PADI-1 to the last one, PADI-8, as well as its features and test results.

The production of K$^+$, K$^-$ and $\varphi$(1020) mesons is studied in Al+Al collisions at a beam energy of 1.9A GeV which is close or below the production threshold in NN reactions. Inverse slopes, anisotropy parameters, and total emission yields of K$^{\pm}$ mesons are obtained. A comparison of the ratio of kinetic energy distributions of K$^-$ and K$^+$ mesons to the HSD transport model calculations suggests that the inclusion of the in-medium modifications of kaon properties is necessary to reproduce the ratio. The inverse slope and total yield of $\phi$ mesons are deduced. The contribution to K$^-$ production from $\phi$ meson decays is found to be [17 $\pm$ 3 (stat) $^{+2}_{-7}$ (syst)] %. The results are in line with previous K$^{\pm}$ and $\phi$ data obtained for different colliding systems at similar incident beam energies., Comment: 16 pages, 11 figures

Using the large acceptance apparatus FOPI, we study central and semi-central collisions in the reactions (energies in A GeV are given in parentheses): 40Ca + 40Ca (0.4, 0.6, 0.8, 1.0, 1.5, 1.93), 58Ni + 58Ni (0.15, 0.25, 0.4), 96Ru + 96Ru (0.4, 1.0, 1.5), 96Zr + 96Zr (0.4, 1.0, 1.5), 129Xe + CsI (0.15, 0.25, 0.4), 197Au + 197Au (0.09, 0.12, 0.15, 0.25, 0.4, 0.6, 0.8, 1.0, 1.2, 1.5). The observables include directed and elliptic flow. The data are compared to earlier data where possible and to transport model simulations. A stiff nuclear equation of state is found to be incompatible with the data. Evidence for extra-repulsion of neutrons in compressed asymmetric matter is found.

Using the large acceptance apparatus FOPI, we study central collisions in the reactions (energies in A GeV are given in parentheses): Ca-40 + Ca-40 (0.4, 0.6, 0.8, 1.0, 1.5, 1.93), Ni-58 + Ni-58 (0.15, 0.25, 0.4), Ru-96+Ru-96 (0.4, 1.0. 1.5), (96)zr+(96)zr 1.0, 1.5), Xe-129+CsI (0.15, 0.25, 0.4), Au-197 + Au-197 (0.09, 0.12, 0.15, 0.25, 0.4, 0.6, 0.8, 1.0, 1.2, 1.5). The observables include cluster multiplicities, longitudinal and transverse rapidity distributions and stopping, and radial flow. The data are compared to earlier data where possible and to transport model simulations. (C) 2010 Elsevier B.V. All rights reserved.

During the development of the Multi-Strip Multi-Gap Resistive Plate Chambers (MMRPCs) for the time-of-flight (ToF) upgrade of the FOPI detector system, we have designed different versions of the front-end electronics (FEE). The signals from a MMRPC are read out on each side of the anode strips by an amplifier followed by a leading-edge discriminator. They are digitized in a time-to-amplitude converter (TAC) followed by a charge-to-digital converter (QDC). These counters are designed to provide a ToF resolution below sigmat les 60 ps requiring an amplifier-diecriminator stage with an intrinsic electronic resolution below sigmat(FEE) les 20 ps. We describe the design steps of the FEE-card from a 4-channel version (FEE1) for the R&D phase to the final 16-channel card (FEE5) for the readout of 4800 channels of the total detector array. Through these design steps we have kept the following key characteristics: low noise sigman les 25 muV RMS (to the amplifier input), high gain (200) and a high bandwidth (1 GHz).

Using the large acceptance apparatus FOPI, we study pion emission in the reactions (energies in A GeV are given in parentheses): 40Ca + 40Ca (0.4, 0.6, 0.8, 1.0, 1.5, 1.93), 96Ru + 96Ru (0.4, 1.0, 1.5), 96Zr + 96Zr (0.4, 1.0, 1.5), 197Au + 197Au (0.4, 0.6, 0.8, 1.0, 1.2, 1.5). The observables include longitudinal and transverse rapidity distributions and stopping, polar anisotropies, pion multiplicities, transverse momentum spectra, ratios ( π + / π − ) of average transverse momenta and of yields, directed flow, elliptic flow. The data are compared to earlier data where possible and to transport model simulations.

$\ensuremath{\phi}$ and ${\mathrm{K}}^{\ensuremath{-}}$ mesons from Ni+Ni collisions at the beam energy of $1.91A$ GeV have been measured by the FOPI spectrometer, with a trigger selecting central and semicentral events amounting to 52% of the total cross section. The phase-space distributions, and the total yield of ${\mathrm{K}}^{\ensuremath{-}}$, as well as the kinetic energy distribution and the total yield of $\ensuremath{\phi}$ mesons are presented. The $\ensuremath{\phi}/$ ${\mathrm{K}}^{\ensuremath{-}}$ ratio is found to be $0.44\ifmmode\pm\else\textpm\fi{}0.07{(\text{stat})}_{\ensuremath{-}0.10}^{+0.16}(\text{syst})$, meaning that about 22% of ${\mathrm{K}}^{\ensuremath{-}}$ mesons originate from the decays of $\ensuremath{\phi}$ mesons, occurring mostly in vacuum. The inverse slopes of direct kaons are up to about 15 MeV larger than the ones extracted within the one-source model, signaling that a considerable share of gap between the slopes of ${\mathrm{K}}^{+}$ and ${\mathrm{K}}^{\ensuremath{-}}$ could be explained by the contribution of $\ensuremath{\phi}$ mesons to negative kaons.

We report on the status of the Time-of-Flight upgrade with M ultistrip- M ultigap- R esistive- P late- C hambers (MMRPCs) for the FOPI detector at GSI [J.L. Ritman and the FOPI collaboration: Nucl Phys. B – Proc. Supp. 44 (1995) 708–715 ]. Final detector parameters for the mass production of the counters and the electronic readout system are presented. These final parameters are based on measurements which took place over the last years where the operating range of the MMRPCs has been mapped out with respect to time resolution, efficiency and cluster size. For this purpose we show results at different beam intensities, electric fields and at different preamplifier gains (FEE-gain).

New results on nuclear collective flow are presented for central and semi-central Ru + Ru collisions at 400 A MeV measured with the FOPI detector at GSI-Darmstadt. The source shape parameters, flow angle and aspect ratios, are extracted from Gaussian fits to in-plane and out-of-plane momentum distributions. The orientation and the shape of the source exhibit different trends according to the investigated phase space region. The shape parameters of the participant source are studied as a function of the particle mass and collision centrality. The flow angle is found to be independent of the particle mass. Both the flow angle and the aspect ratios depend sensitively on the impact parameter. Detailed comparisons with the predictions of the isospin quantum molecular dynamics model are performed. It is shown in particular that the source shape parameters permit to extract information on the in-medium nucleon–nucleon cross section.

The characteristics of a symmetric multigap resistive plate chamber with multistrip readout electrode, recently developed by us, continued to be investigated. Studies of the time resolution, efficiency, average charge and dark rate as a function of applied voltage and the influence of the angle of incidence of the detected particle on these observables have been performed. Different type of discriminators have been tested.

A completely new configuration of a glass resistive-plate chamber (GRPC) was built and tested. It consists of a double two-gap structure of electrodes with an active area of about 400 cm 2 and is read out via a central multistrip printed circuit board. In measurements with a 60 Co source and p, d particles of 1.5 A GeV time resolutions better than 80 ps , position resolution along the strips of 5– 6 mm and efficiencies larger than 95% were obtained using available fast standard electronics. These results open the possibility of constructing compact TOF detectors of high resolution and high granularity.

A spark counter of the type developed by Pestov has been tested with the aim of searching for an optimum time response function, changing voltage, content of noble and quencher gases, pressure and energy-loss. Replacing the usual argon by neon has brought an improvement of the resolution and a significant reduction of tails in the time response function. It has been proven that a counter as long as 90 cm can deliver, using neon gas mixture, a time resolution σ ps with about 1% absolute tail and an efficiency of about 90%.

Azimuthal emission patterns of $K^\pm$ mesons have been measured in Ni + Ni collisions with the FOPI spectrometer at a beam kinetic energy of 1.91 A GeV. The transverse momentum $p_{T}$ integrated directed and elliptic flow of $K^{+}$ and $K^{-}$ mesons as well as the centrality dependence of $p_{T}$ - differential directed flow of $K^{+}$ mesons are compared to the predictions of HSD and IQMD transport models. The data exhibits different propagation patterns of $K^{+}$ and $K^{-}$ mesons in the compressed and heated nuclear medium and favor the existence of a kaon-nucleon in-medium potential, repulsive for $K^{+}$ mesons and attractive for $K^{-}$ mesons.

Small-angle correlations of pairs of nonidentical light charged particles produced in central collisions of heavy ions in the A= 100 mass region at a beam energy of 400 A· MeV are investigated with the FOPI detector system at GSI Darmstadt. The difference of longitudinal correlation functions with the relative velocity parallel and anti-parallel to the center-of-mass velocity of the pair in the central source frame is studied. This method allows extracting the apparent space-time differences of the emission of the charged particles. Comparing the correlations with results of a final-state interaction model delivers quantitative estimates of these asymmetries. Time delays as short as 1~fm/c or – alternatively – source radius differences of a few tenth fm are resolved. The strong collective expansion of the participant zone introduces not only an apparent reduction of the source radius but also a modification of the emission times. After correcting for both effects a complete sequence of the space-time emission of p, d, t, 3He, α particles is presented for the first time.

We present new experimental results concerning the azimuthal distributions of proton-likes, light and intermediate mass fragments at midrapidity for Au(100-800 AMeV)+Au collisions measured with the FOPI phase-I detector at GSI in Darmstadt. The azimuthal distributions are investigated as a function of the collision centrality, the incident energy, the fragment charge and transverse momentum. The azimuthal anisotropy is maximum for impact parameters around 7 fm. Intermediate mass fragments present a stronger out-of-plane emission signal than light fragments and a saturation is reached for Z greater than or equal to 4. The azimuthal anisotropy increases with the fragment transverse momentum and decreases as the incident energy increases. The azimuthal anisotropy of Z = 2 particles investigated as a function of the scaled fragment transverse momentum follows an universal curve for bombarding energies between 250-800 AMeV. A signature for a transition from in-plane to out-of-plane emission is evidenced at the lowest beam energies.

Charged pions are measured with the 4π detector FOPI at GSI using the Au on Au reaction at 1.06 AGeV bombarding energy. The pion multiplicities ngp increase with the number of participants Apart. The average pion multiplicities per participant are #### and #### These values are only half as large as extrapolated from the low-mass systems studied by Harris et al. The ratio #### increases with Apart and decreases with the pion kinetic energies. The pion kinetic energy spectra have concave shapes, their parametrization in terms of thermal Boltzmann distributions yields a low (T l,π) and high (Th,π) temperature which change with the cm emission angle Θ of the pions. In the angular range 45dg is larger than , the high temperatures , are, within experimental uncertainties, the same. The inclusive polar angular distributions of pions are anisotropic, dσ/dΩ increases for forward and backward angles. The forward — backward enhancements are independent of the pion kinetic energies or the number of participants. In addition to the preferred forward — backward emission, also the enhanced emission into the transverse direction Θ = 90° is observed for pions with high energies or for pions from near-central collisions. These observations and the shape of the rapidity spectra suggest that pions, emitted from the central rapidity region, are partly rescattered by spectator matter. The strength of the rescattering process depends only weakly on the number of participants. The experimental data are compared to the results of IQMD/GEANT calculations using momentum dependent NN interactions and a hard equation of state. The calculated pion multiplicities are approximately 50% larger than experimentally determined; the existence of secondary pion sources is reproduced by the calculation, but their predicted strengths are larger than experimentally observed.

Using the FOPI facility at GSI, charged particles (1 ≤Z≤6) produced in the Au(150 A MeV)+Au reaction have been measured at laboratory angles 1.20 < Θlab < 300. Highly central collisions have been selected with two criteria, both dealing with the longitudinal and transverse degrees of freedom of the reaction. The relevance of this selection method is supported by QMD calculations which indicate that such criteria are able to select mean impact parameters less than 2 fm. Bias effects introduced by the criteria have been evaluated. The centre-of-mass polar angle distributions of low energy clusters emitted in these central collisions, have been extracted: the intensity ratio deduced for a transverse to longitudinal emission is found to be R= 1.4 −0.4 +0.2 . Model comparisons using QMD are presented. The value of R appears to depend sensitively on the nucleon-nucleon cross section, σnn. Within this model, a value of σ=25+ - 5 mb is derived.

Events with 2, 3 and 4 heavy fragments (A≥20) detected in the reactions100Mo +100Mo at 18.7, 23.7 A·MeV and120Sn +120Sn at 18.4 A·MeV were analyzed by means of an improved version of the kinematic coincidence method. The phase-space distributions prove that 3- (and possibly 4-) body events predominantly originate from a two-step mechanism and are compatible with the hypothesis of a binary deep-inelastic interaction followed by the further fissionlike decay of one (or both) of the primary fragments. The characteristics of the fission step — mass asymmetry, relative velocity, in-plane and out-of-plane angles — have been reconstructed for the 3-body events and indications are found that nonequilibrium effects at the end of the deep-inelastic phase may influence the fissionlike decay.

Light-particle emission from Au+Au collisions has been studied in the bombarding-energy range 100–250 A ·MeV, using ΔE − E R telescopes in coincidence with the FOPI detector in its phase I configuration. Center-of-mass energy spectra have been measured for Z = 1,2 isotopes emitted in central collisions at CM polar angles between 60° and 90°. Evidence for a collective expansion is reported, on the basis of the mean kinetic energies of hydrogen isotopes. Comparison is presented with statistical calculations (WIX code). For CM kinetic energy spectra, fair agreement is found between data and a recently developed transport model.

The key element providing hadron identification in the future Compressed Baryonic Matter spectrometer at FAIR is a time-of-flight wall placed at 10 m distance from the target. The most promising technological option for such a task consists on a 150 m 2 carpet based on Resistive Plate Chambers. Due to the fixed-target geometry, the conceptual design foresees two extreme regions: an outermost region (low rate/low multiplicity) covered by float glass RPCs in multi-strip fashion, and a central region (high rate/high multiplicity) consisting of densely packed read-out cells made with low resistive electrodes. The status of the ongoing R&D efforts in both regions is presented. © 2010 Elsevier B.V. All rights reserved.

The production of neutral pions has been studied in the reactions 40 Ar + nat Ca , 86 Kr + nat Zr and 197 Au + 197 Au at 1 A GeV. For high energy pions emitted from the heavier systems a steeper than linear rise of the pion multiplicity with the centrality of the reaction is observed, indicating a pion production process other than binary nucleon-nucleon collisions. At low transverse momenta an enhancement of the π 0 -multiplicity increasing with the mass of the collision system is found. Systematic discrepancies between the experimental results and recent BUU, QMD and Cascade calculations are discussed.

The FOPI detector [1] at the heavy-ion synchrotron SIS-18 at GSI in Darmstadt has upgraded part of its time-of-flight (ToF) system by adding a new sub-detector shell (called the MMRPC Barrel) that is made out of Multi-strip Multi-gap Resistive-Plate Counters (MMRPCs). The MMRPC Barrel has an active area of 5 m2 covered by 2400 individual anode strips [2] , [3] which are read out at both ends by custom-designed electronics [4] , [5] . With these multi-strip anodes we have obtained an enhanced detector granularity with 95% efficiency and a ToF resolution σ ToF better than 90 ps which increases the identification limit for charged kaons up to laboratory momenta of at least 1 GeV/c. In this paper we report on the design, construction and operational characteristics of the MMRPC Barrel and describe its performance during the first experiments.

The multiplicities of p and \ensuremath{\alpha} particles detected in coincidence with fragments emitted in fully relaxed collisions in the reactions of 18.5A MeV $^{136}\mathrm{Xe}$${+}^{48}$Ti have been measured for different exit channel mass asymmetries. A kinematic source analysis of the spectra and angular distributions of the light particles has been used to separate the total multiplicities into prescission and postscission contributions. From these results, the excitation energies at scission are determined using an empirical technique based upon previous measurements of light charged particle multiplicities observed in coincidence with evaporation residues. These excitation energies are found to decrease from \ensuremath{\sim}400 MeV to 110 MeV as the fragment mass asymmetry, ${\mathit{A}}_{\mathit{H}}$/${\mathit{A}}_{\mathit{L}}$, varies from 4.8 to 1.0. A corresponding increase of the mean lifetime of the scissioning nucleus from \ensuremath{\sim}5\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}22}$ s to \ensuremath{\sim}1\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}20}$ s is derived using calculated statistical model decay widths. The extent to which this variation of lifetime with mass asymmetry may be attributed to completely damped deep inelastic collisions or to dynamic delays in the decay of a compound nucleus is discussed as is the need for inclusion of dynamics in the deexcitation calculations for hot nuclei. Observed three fragment events are also discussed.

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 [ital R][similar to]14 fm.

A new measurement of subthreshold ${K}^{*}(892){}^{0}$ and ${K}^{0}$ production is presented. The experimental data complete the measurement of strange particles produced in $\mathrm{Al}+\mathrm{Al}$ collisions at 1.9$A$ GeV measured with the FOPI detector at SIS at GSI (Darmstadt). The ${K}^{*}(892){}^{0}/{K}^{0}$ yield ratio is found to be $0.0315\ifmmode\pm\else\textpm\fi{}0.006(\mathrm{stat}.)\ifmmode\pm\else\textpm\fi{}0.012(\mathrm{syst}.)$ and is in good agreement with the transport model prediction. These measurements provide information on the in-medium cross section of ${K}^{+}$-${\ensuremath{\pi}}^{\ensuremath{-}}$ fusion, which is the dominant process in subthreshold ${K}^{*}(892){}^{0}$ production.

During the last years FOPI has developed a new ToF system as an upgrade of the existing detector based on Multi-strip Multi-gap Resistive Plate Chambers (MMRPCs). The intention is to increase the charged Kaon identification up to a laboratory momentum of 1 GeV / c and to enhance the azimuthal detector granularity. The new ToF barrel has an active area of 5 m 2 with 2400 individual strips ( 900 × 1.6 mm 2 ) [A. Schuttauf, et al., Nucl. Phys. B 158 (2006) 52] which are read out on both sides by a custom designed electronics [M. Ciobanu, et al., IEEE Trans. Nucl. Sci. NS-54 (4) (2007) 1201; K. Koch, et al., IEEE Trans. Nucl. Sci. NS-52(3) (2005) 745]. To reach the envisaged goal a time resolution of 100 ps is needed, at a flight path of 1–1.3 m. Due to the rare production of the K − at SIS energies the efficiency of the MMRPCs has to be above 95%. We report on measurements with the detectors and electronics from the mass production line. For this purpose we used a proton beam at 2.0 and 1.25 GeV, at rates between 0.1 and 5 kHz / cm 2 to determine the timing, efficiency and rate capability of the MMRPCs.

Inclusive cross sections of intermediate mass fragments from the reaction {sup 40}Ar+{sup 197}Au at {ital E}/{ital A}=30 and 220 MeV were measured with a low threshold and over a large range of angles. The integrated cross sections are about 2 b at both energies but the emission characteristics change considerably. The increased isotropy of the angular distributions and the strongly reduced Coulomb repulsion at the higher bombarding energy suggest that the emission process evolves towards fragmentation of excited spectator matter in central ion-ion collisions.

Binary reaction products from reactions of a {sup 238}U beam at energies 5.4, 5.9, 6.7, and 7.5 MeV/nucleon with targets of {sup 16}O, {sup 26}Mg, {sup 27}Al, {sup 32}S, {sup 35}Cl, {sup 40}Ca, {sup 48}Ca, and {sup nat}Zn have been measured with a kinematic coincidence method. The {gamma}-ray multiplicities are measured at the same time with NaI detectors and presented as a function of fragment mass. The results indicate that all aligned and statistically excited spin modes of the fragments are fully populated in compound nucleus fission reactions. In quasifission reactions the relaxation processes seem to terminate before full statistical spin equilibrium is reached. The results are analyzed and discussed in terms of various dinuclear spin modes with different characteristic relaxation times. The influence of various terminal freeze-out shapes is also discussed.

First measurement of subthreshold {sigma}(1385) production is presented. Experimental data are presented for Al+Al reactions at 1.9A GeV measured with the FOPI detector at SIS/GSI. The {sigma}(1385)/{lambda} ratio is found to be in good agreement with the transport and statistical model predictions. The results allow for a better understanding of subthreshold strangeness production and strangeness exchange reaction which is the dominant process for K{sup -} production below and close-to threshold.

Results on $K^+$ and $K^0$ meson production in $^{96}_{44}$Ru + $^{96}_{44}$Ru and $^{96}_{40}$Zr + $^{96}_{40}$Zr collisions at a beam kinetic energy of 1.528$A$ GeV, measured with the FOPI detector at GSI-Darmstadt, are investigated as a possible probe of isospin effects in high density nuclear matter. The measured double ratio ($K^+/K^0$)$_{Ru}$/($K^+/K^0$)$_{Zr}$ is compared to the predictions of a thermal model and a Relativistic Mean Field transport model using two different collision scenarios and under different assumptions on the stiffness of the symmetry energy. We find a good agreement with the thermal model prediction and the assumption of a soft symmetry energy for infinite nuclear matter while more realistic transport simulations of the collisions show a similar agreement with the data but also exhibit a reduced sensitivity to the symmetry term.

New results concerning the production of neutral strange particles, $K^0$ and $\Lambda$ in Ni + Ni collisions at 1.93$A$ GeV, measured with the FOPI detector at GSI-Darmstadt, are presented. Rapidity density distributions and Boltzmann slope parameter distributions are measured in nearly the full phase space of the reaction. The observables are compared to existing $K^+$ and proton data. While the $K^0$ data agree with previously reported $K^+$ measurements, the $\Lambda$ distributions show a different behavior relative to that of protons. The strangeness balance and the production yield per participating nucleon as a function of the centrality of the reaction are discussed, for the first time at SIS energies., Comment: 10 pages, 8 figures

Four-gap resistive plate chamber (RPC) prototypes with silicate glass electrodes (bulk resistivity ∼10 8 –10 9 Ω cm) were studied for suitability in time-of-flight (TOF) applications at high rates. These studies were performed using a continuous electron beam of 34 MeV at the FZ Dresden–Rossendorf electron linac ELBE. Time resolutions of about 100 ps and efficiencies larger than 95% were obtained for flux densities up to 20 kHz/cm 2 .

We report on the first analysis of directed and elliptic flow with the new method of Lee--Yang zeroes. Experimental data are presented for Ru+Ru reactions at 1.69 AGeV measured with the FOPI detector at SIS/GSI. The results obtained with several methods, based on the event-plane reconstruction, on Lee--Yang zeroes, and on multi-particle cumulants (up to 5th order) applied for the first time at SIS energies, are compared. They show conclusive evidence that azimuthal correlations between nucleons and composite particles at this energy are largely dominated by anisotropic flow., Comment: 5 pages, 3 figures, submitted to Phys. Rev. C Rapid Com

Small-angle correlations of pairs of protons emitted in central collisions of Ca + Ca, Ru + Ru and Au + Au at beam energies from 400 to 1500 MeV per nucleon are investigated with the FOPI detector system at SIS/GSI Darmstadt. Dependences on system size and beam energy are presented which extend the experimental data basis of pp correlations in the SIS energy range substantially. The size of the proton-emitting source is estimated by comparing the experimental data with the output of a final-state interaction model which utilizes either static Gaussian sources or the one-body phase-space distribution of protons provided by the BUU transport approach. The trends in the experimental data, i.e. system-size and beam energy dependences, are well reproduced by this hybrid model. However, the pp correlation function is found rather insensitive to the stiffness of the equation of state entering the transport model calculations., 9 pages, 8 figures, accepted at Eur. Phys. Journ. A

We present measurements of the excitation function of elliptic flow at midrapidity in Au+Au collisions at beam energies from 0.09 to 1.49 GeV per nucleon. For the integral flow, we discuss the interplay between collective expansion and spectator shadowing for three centrality classes. A complete excitation function of transverse momentum dependence of elliptic flow is presented for the first time in this energy range, revealing a rapid change with incident energy below 0.4 AGeV, followed by an almost perfect scaling at the higher energies. The equation of state of compressed nuclear matter is addressed through comparisons to microscopic transport model calculations., Comment: 10 pages, 4 eps figures, submitted for publication. Data files will be available at http://www.gsi.de/~fopiwww/pub/

We present new experimental data on directed flow in collisions of Au+Au, Xe+CsI and Ni+Ni at incident energies from 90 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 allow 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 case of Ni+Ni the model underpredicts flow for any EoS. At 90A MeV incident beam energy, none of the IQMD parametrizations studied here is able to consistently explain the experimental data., RevTeX, 20 pages, 30 eps figures, accepted for publication in Phys. Rev. C. Data files available at http://www.gsi.de/~fopiwww/pub/

We present experimental data on directed flow in semi-central 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 a function of incident energy and for different regions in rapidity., 5 pages, Latex, 5 eps figures, accepted for publication in Phys. Rev. C (Rapid Communications). Data files available at http://www-linux.gsi.de/~andronic/fopi/v1.html

Charged pions from Ni + Ni reactions at 1.05, 1.45 and 1.93 AGeV are measured with the FOPI detector. The mean $\pi^{\pm}$ multiplicities per mean number of participants increase with beam energy, in accordance with earlier studies of the Ar + KCl and La + La systems. The pion kinetic energy spectra have concave shape and are fitted by the superposition of two Boltzmann distributions with different temperatures. These apparent temperatures depend only weakly on bombarding energy. The pion angular distributions show a forward/backward enhancement at all energies, but not the $\Theta = 90^0$ enhancement which was observed in case of the Au + Au system. These features also determine the rapidity distributions which are therefore in disagreement with the hypothesis of one thermal source. The importance of the Coulomb interaction and of the pion rescattering by spectator matter in producing these phenomena is discussed., Comment: 22 pages, Latex using documentstyle[12pt,a4,epsfig], to appear in Z. Phys.A

Semi-inclusive triple differential multiplicity distributions of positively charged kaons have been measured over a wide range in rapidity and transverse mass for central collisions of $^{58}$Ni with $^{58}$Ni nuclei. The transverse mass ($m_t$) spectra have been studied as a function of rapidity at a beam energy 1.93 AGeV. The $m_t$ distributions of K^+ mesons are well described by a single Boltzmann-type function. The spectral slopes are similar to that of the protons indicating that rescattering plays a significant role in the propagation of the kaon. Multiplicity densities have been obtained as a function of rapidity by extrapolating the Boltzmann-type fits to the measured distributions over the remaining phase space. The total K^+ meson yield has been determined at beam energies of 1.06, 1.45, and 1.93 AGeV, and is presented in comparison to existing data. The low total yield indicates that the K^+ meson can not be explained within a hadro-chemical equilibrium scenario, therefore indicating that the yield does remain sensitive to effects related to its production processes such as the equation of state of nuclear matter and/or modifications to the K^+ dispersion relation., Comment: 24 pages Latex (elsart) 7 PS figures to be submitted to Nucl. Phys A

Small-angle correlations of proton pairs produced in central Ni+Ni collisions at a beam energy of 1.93 A.GeV are investigated with the FOPI detector system at GSI Darmstadt. Simultaneous comparison of longitudinal and transverse correlation functions with the predictions of the Koonin model allows to unravel the space-time ambiguity of the source extension. The determination of the source rest frame is found essential for the identification of this effect. For the present system, the selection of central events comprising about 8 % of the total cross section allows for the isolation of a compact source in momentum space which is centered around the c.m. velocity of the colliding nuclei. Taking into account the strong collective expansion of the participant zone, which introduces a reduction of the extracted source radius of more than 30 %, r.m.s. radius and emission time parameters of R-rms = (4.2 +/- 1.2) fm and t(rms) = (11(-5)(+7)) fm/c are extracted, respectively. In contrast, the analysis of the angle-integrated correlation function mixes the spatial and temporal size of the source and gives an upper limit R-rms = (7.0 +/- 0.8) fm of the source radius.

Velocity correlations of intermediate mass fragments (IMF), produced in collisions of Au+Au at 100, 150, 250, and 400{ital A} MeV beam energy, are extracted from measurements with the 4{pi} detector system (FOPI) in construction stage I at Schwerionen-Synchrotron (SIS) at the Gesellschaft fuer 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 {ital R}{sub {ital s}}{congruent}13 fm for 100{ital A} 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 relativemore » IMF velocities, found for the given beam energy range, is attributed to rather constant averaged next-neighbor distances {l_angle}{ital d}{sub IMF}{r_angle}=8.6{plus_minus}0.2 fm of the IMF charge centers within the source at breakup time.« less

We present new experimental data obtained with the FOPI detector at SIS, for the Au + Au heavy-ion collisions at 400 A MeV incident energy. The sideward flow, determined from a method without reaction-plane reconstruction, and the nuclear stopping are studied as a function of the centrality of the collisions. In order to study the nuclear in-medium effects, which act on the NN cross sections and potential and hence on experimental observables like the nuclear-matter flow and stopping, these results are compared with the predictions of two different QMD versions. The first one offers a fully microscopic calculation of the cross sections and potential in the G-matrix formalism and naturally includes the in-medium effects (this version is for the first time confronted with experiment). The second one uses a standard Skyrme potential plus a momentum-dependent term in order to mimic the in-medium effects.

Events with 2, 3 and 4 heavy-fragments (A≧20) have been detected in the reactions100Mo+100Mo atE/A =18.7, 23.7 MeV and120Sn+120Sn atE/A=18.4 MeV. The experiments were performed with an array of 12 detectors which together covered a large fraction of the forward hemisphere and allowed a high detection efficiency for these events. Masses and energies of all fragments have been reconstructed by means of an improved version of the kinematic coincidence method. The probabilitiesP 3 andP 4 of producing 3- and 4-body events were found to depend mainly on the dissipated energy rather than on the bombarding energy, thus indicating that their origin lies more in the decay properties of the excited fragments than in the dynamics of the interaction. Emission of light particles from the composite system is shown to become more relevant with increasing bombarding energy and may explain the drop of theP 3 andP 4 curves at high energy losses. Small deviations of theP 3 andP 4 curves at 23.7A · MeV from those at lower bombarding energies were used to estimate the amount of a possible pre-equilibrium light particle emission as a function of impact parameter.