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Spectator decay was studied for the system Au + Au at an energy of 1000 A·MeV and the decay of the interaction region at energies between 50 and 200 A·MeV. In both cases temperatures were derived from several double-ratios of neighboring isotopes and from the population of excited states in 5 Li and 4 He. Agreement was found among the different isotope temperatures and also among the two excited state temperatures. The comparison of isotope and excited state temperatures, however, reveals large differences, which cannot be explained by feeding corrections. At incident energies between 600 and 1000 A·MeV the energy spectra of fragments and also neutrons of the decaying projectile spectator were measured. Whereas the slope parameters of the energy spectra and mean energies for fragments with Z ≥ 2 are independent of the incident energy, a strong dependence is found for the lightest particles, so that preequilibrium contributions to the spectator decay should be taken into account.

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.

Cross sections for the charge pickup of {sup 238}U projectiles were measured at {ital E}/{ital A}=600 and 1000 MeV for seven different targets (Be, C, Al, Cu, In, Au, and U). Events with two fission fragments with a sum charge of 93 in the exit channel were selected. Due to the significant excitation energy, the majority of the produced Np nuclei fission instead of decaying by evaporation to residues. The observed cross sections can be well reproduced by intranuclear-cascade-plus-evaporation calculations and, therefore, confirm recent results that no exotic processes are needed to explain charge-pickup processes. {copyright} {ital 1996 The American Physical Society.}

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.

Multifragment decays of heavy nuclei have been studied at the ALADIN spectrometer system at beam energies between 100 and 1000 MeV per nucleon. The observed fragment distributions signal a universality of spectator decays at bombarding energies E A ≥ 400 MeV . The role of the radial flow for the fragmentation process is explored by comparing fragment distributions measured for Au+Au collisions at E A = 100 MeV in central collisions and at E A = 1000 MeV in more peripheral reactions. At both energies the maximum multiplicity of intermediate mass fragments (IMFs) normalized to the size of the decaying system is about one IMF per 30 nucleons but the element distributions show significant differences. Within a coalescence picture the suppression of heavy fragments in central collisions at E A = 100 MeV may be related to a reduction of the density in momentum space which is caused by a large collective expansion velocity component.

Fragment distributions have been measured for Au+Au collisions at [ital E]/[ital A]=100 and 1000 MeV. A high detection efficiency for fragments was obtained by combining the ALADIN spectrometer and the MSU-Miniball/WU-Miniwall array. At both energies the maximum multiplicity of intermediate mass fragments (IMF) normalized to the size of the decaying system is about one IMF per 30 nucleons but the element distributions show significant differences. Within a coalescence picture the suppression of heavy fragments in central collisions at [ital E]/[ital A]=100 MeV may be related to a reduction of the density in momentum space which is caused by the collective expansion.

We investigate the beam energy dependence of neutron and proton squeeze-out in collisions of197Au+197Au atE/A=400—800 MeV. The azimuthal anisotropy that describes the enhanced emission of mid-rapidity neutrons perpendicular to the reaction plane rises strongly with the transverse momentum of the neutrons. This dependence of the azimuthal anisotropy follows a universal curve — independent of beam energy — if the neutron momenta are measured in fractions of the projectile momentum per mass unit. Analogously, the kinetic energy spectra of mid-rapidity neutrons exhibit a universal behaviour as a function of the kinetic energy of the projectile.

Multifragmentation has been measured for [sup 197]Au+[sup 197]Au collisions at [ital E]/[ital A]=100, 250, and 400 MeV. The mean fragment multiplicity increases monotonically with the charged particle multiplicity at [ital E]/[ital A]=100 MeV, but decreases for central collisions with incident energy, consistent with the onset of nuclear vaporization. Molecular dynamics calculations follow some trends but underpredict the observed fragment multiplicities. Including the statistical decay of excited residues improves the agreement for peripheral collisions but worsens it for central collisions.

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.

We present data on collective flow of neutrons in collisons of [sup 197]Au + [sup 197]Au at 400 MeV/nucleon. The azimuthal distribution about the beam axis is investigated with respect to the reaction plane as determined from light charged particles. The squeezeout'' of neutrons, perpendicular to the reaction plane, is observed for the first time. Quantitative agreement to a high level of accuracy is found between the behavior of neutrons and hydrogen ions.

The nuclear response of a medium-mass nucleus ([sup 136]Xe) to electromagnetic excitation in a near-relativistic heavy-ion collision was investigated in the reaction[sup 136]Xe(0.7[ital A] GeV)+Pb. From an exclusive measurement of the neutron decay of the excited [sup 136]Xe projectiles, strong excitations of giant resonances and, in particular, of the double isovector giant dipole resonance were identified. A resonance energy of 28.3[plus minus]0.7 MeV, a width of 6.3[plus minus]1.6 MeV, and a total cross section of 215[plus minus]50 mb were found for the double giant dipole resonance.

At the SIS/ESR accelerator facility at GSI in Darmstadt the 4π-detector system FOPI is under construction at present. It is designed for the investigation of central collisions of heavy ions in the energy range up to 2 A GeV. As phase I of this detector a forward wall has been built and used in various experiments. It comprizes a total number of 764 scintillators with an additional shell of 188 thin ΔE -detectors in front of it and covers the full azimuth of the polar angles from 1° to 30°. The velocity and the nuclear charge of the fragments are determined by a combined time-of-flight and ΔE measurement.

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.

Multi-fragment decays of 129Xe, 197Au, and 238U projectiles in collisions with Be, C, Al, Cu, In, Au, and U targets at energies between E/A = 400 MeV and 1000 MeV have been studied with the ALADIN forward-spectrometer at SIS. By adding an array of 84 Si-CsI(Tl) telescopes the solid-angle coverage of the setup was extended to \theta_lab = 16 degree. This permitted the complete detection of fragments from the projectile-spectator source. The dominant feature of the systematic set of data is the Z_bound universality that is obeyed by the fragment multiplicities and correlations. These observables are invariant with respect to the entrance channel if plotted as a function of Z_bound, where Z_bound is the sum of the atomic numbers Z_i of all projectile fragments with Z_i \geq 2. No significant dependence on the bombarding energy nor on the target mass is observed. The dependence of the fragment multiplicity on the projectile mass follows a linear scaling law. The reasons for and the limits of the observed universality of spectator fragmentation are explored within the realm of the available data and with model studies. It is found that the universal properties should persist up to much higher bombarding energies than explored in this work and that they are consistent with universal features exhibited by the intranuclear cascade and statistical multifragmentation models. PACS numbers: 25.70.Mn, 25.70.Pq, 25.75.-q, Comment: Plain Tex, 49 pages including 20 eps figures. Also available from http://www-kp3.gsi.de/www/kp3/aladin_publications.html

Neutron and He-6 momentum distributions from He-8 break-up reactions in a C target have been measured at 240 MeV/u. The two-neutron removal cross section was found to be sigma(-2n) = 0.27 +/- 0.03 b. The nature of the momentum distributions is interpreted in some simple reaction scenarios.

Fragment distributions resulting from $\mathrm{Au}+\mathrm{Au}$ collisions at an incident energy of $E/A\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}600\phantom{\rule{0ex}{0ex}}\mathrm{MeV}$ are studied. From the measured fragment and neutron distributions the mass and the excitation energy of the decaying prefragments were determined. A temperature scale was derived from observed yield ratios of He and Li isotopes. The relation between this isotope temperature and the excitation energy of the system exhibits a behavior which is expected for a phase transition. The nuclear vapor regime takes over at an excitation energy of 10 MeV per nucleon, a temperature of 5 MeV, and may be characterized by a density of 0.15--0.3 normal nuclear density.

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

Charge, velocity, and angular correlations between intermediate mass fragments (IMF) are presented for 50 and 100 MeV/nucleon Fe bombardments of Ta, Au, and Th targets. Correlation functions generated as a function of the relative velocity and the opening angle between two IMF's are qualitatively independent of the projectile energy and target mass and show a suppression at small relative velocities and opening angles due to the Coulomb repulsion between the fragments. The correlations are consistent with IMF's emitted primarily from a highly excited target residue following a rapid preequilibrium cascade. The correlation data are compared to model calculations using the event generator meneka and the quantum molecular dynamics (QMD) code with a statistical deexcitation of residual fragments utilizing the multifragmentation code smm. All data are consistent with a simultaneous multifragmentation at a freeze-out density of 0.1--0.3 times normal nuclear matter density or a more sequential emission with time constant \ensuremath{\tau}\ensuremath{\le}500 fm/c.

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.

Electromagnetic fission of238U projectiles at E/A =600 and 1000 MeV was studied with the ALADIN spectrometer at the heavy-ion synchrotron SIS. Seven different targets (Be, C, Al, Cu, In, Au and U) were used. By considering only those fission events where the two charges added up to 92, most of the nuclear interactions were excluded. The nuclear contributions to the measured fission cross sections were determined by extrapolating from beryllium to the heavier targets with the concept of factorization. The obtained cross sections for electromagnetic fission are well reproduced by extended Weizsacker-Williams calculations which include E1 and E2 excitations. The asymmetry of the fission fragments' charge distribution gives evidence for the excitation of the double giant-dipole resonance in uranium.

Neutron angular distributions from violent break-up reactions of Li-11 and Be-11 have been measured at 28 MeV/u and 280 MeV/u and at 41 MeV/u and 460 MeV/u, respectively. The derived neutron momentum distributions show a narrow component in transverse momentum that is within uncertainties independent of beam energy and target charge. This component is suggested to be simply related to the momentum distribution of the loosely bound halo neutron(s) in the projectiles.

Collective flow of nuclear matter is one important aspect of the research performed at heavy ion accelerator laboratories. The phenomenon was predicted on the basis of hydrodynamical calculations [1], and experimental evidence was first presented for the systems 93 Nb + 93 Nb and 197 Au + 197 Au in the projectile energy range between 150 and 1050 MeV/u [2]. The comparison to microscopic calculations shows that nuclear matter is compressed to about two to three times the ground state density and that a substantial fraction of the kinetic energy in the entrance channel is converted into compressional energy [3]. In these calculations, the relation between density and compressional energy depends on the parameterization of the nucleon-nucleon inter-action. The values of its parameters will hopefully be restricted by a comparison of the calculations to experimental data, and thus one can obtain information on the nucleon-nucleon interaction within the nuclear medium which is not accessible from other sources.

A detailed deexcitation calculation is presented for target residues resulting from intermediate-energy heavy ion reactions. The model involves an intranuclear cascade, subsequent fast nucleon emission, and final decay by statistical evaporation including fission. Results are compared to data from bombardments with Fe and Nb projectiles on targets of Ta, Au, and Th at 100 MeV/nucleon. The majority of observable features are reproduced with this simple approach, making obvious the need for involving new physical phenomena associated with multifragmentation or other collective dissipation mechanisms.

Cross sections, parallel and perpendicular momentum transfers, charge loss, and velocity systematics are presented for fission following reactions of Fe and Nb projectiles at 50\char21{}100 MeV/nucleon on targets of Ta, Au, and Th. Data are compared to simple models for peripheral heavy ion collisions.

Radiochemical techniques were applied to determine the actinide yields for nuclides with mass numbers below and above that of the target from the reaction 48 Ca + 248 Cm at energies near the interaction barrier. This work was initiated in the hope that the very neutron-rich projectile 48 Ca might give access to new neutron-rich isotopes of target-like actinide elements by nucleon transfer reactions. However, for trans-target elements, the use of 48 Ca did not result in increased production rates for such isotopes compared with those obtained in previous investigations. In contrast, rather neutronrich isotopes of below-target elements were observed at a remarkably high cross-section level. This observation is interpreted as a massive mass drift towards symmetry for this system. These investigations have been extended to the system 40 Ca + 248 Cm to explore the effect of the very different neutron-to-proton N Z ratios of 40 Ca and 48 Ca on the production of heavy target-like products.

Excitation functions have been measured for production of isotopes of Bk through Fm in bombardments of /sup 248/Cm with 234- to 294-MeV /sup 40/Ca ions and with 239- to 318-MeV /sup 48/Ca ions. The maxima of the isotopic distributions for these elements occur at only 2 to 3 mass numbers larger for /sup 48/Ca than for /sup 40/Ca reactions. The shapes of the distributions and the half-widths of about 2.5 mass numbers are quite similar to those observed previously for reactions of /sup 16/O, /sup 18/O, /sup 20/Ne, and /sup 22/Ne with /sup 248/Cm. In general, the excitation functions for /sup 40/Ca show maxima near the Coulomb barrier while those for /sup 48/Ca are about 20 MeV above the barrier. The cross sections decrease rather slowly with increasing projectile energy over the energy range studied, indicating that the additional projectile energy is not manifested as excitation energy of these actinide products.