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Abstract:: The ratio of K- to K+-meson yields has been measured in the systems 96Ru + 96Ru at 1.69 A GeV, 96Ru+96Zr at 1.69 A GeV, and 58Ni+58Ni at 1.93 A GeV incident beam kinetic energy. The yield ratio is observed to vary across the measured phase space. Relativistic transport-model calculations indicate that the data are best understood if in-medium modifications of the kaons are taken into account.

Abstract.: The mass distributions of baryon resonances populated in near-central collisions of Au on Au and Ni on Ni are deduced by defolding the pt spectra of charged pions by a method which does not depend on a specific resonance shape. In addition the mass distributions of resonances are obtained from the invariant masses of (p, π±) pairs. With both methods the deduced mass distributions are shifted by an average value of −60 MeV/c2 relative to the mass distribution of the free Δ(1232) resonance, the distributions descent almost exponentially towards mass values of 2000 MeV/c2. The observed differences between (p, π−) and (p, π+) pairs indicate a contribution of isospin I = 1/2 resonances. The attempt to consistently describe the deduced mass distributions and the reconstructed kinetic energy spectra of the resonances leads to new insights about the freeze out conditions, i.e. to rather low temperatures and large expansion velocities.

Structures of non-statistical character, recently observed in 58Ni+46Tielastic and 58Ni+62Nielastic and inelastic excitation functions, produce damped oscillations in the cross section energy autocorrelation functions. The analysis of these damped oscillations in terms of S-matrix spin and parity decoherence indicates, as a possible interpretation, damping of the coherent rotational motion of the intermediate dinuclear system formed in the reaction.

Charged pions are measured with the 4π detector FOPIat GSIusing the Au on Au reaction at 1.06 AGeV bombarding energy. The pion multiplicities ngpincrease 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 Apartand 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 (Tl,π) and high (Th,π) temperature which change with the cm emission angle Θ of the pions. In the angular range 45dg < Θ< 135° the low temperature < Tl,π−> is larger than < Tl,π+>, the high temperatures < Th,π−>, < Th,π+> are, within experimental uncertainties, the same. The inclusive polar angular distributions of pions are anisotropic, dσ/dΩincreases 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/GEANTcalculations using momentum dependent NNinteractions 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.

The exciton model in its closed form and the Weisskopf-Ewing model have been consistently applied to neutron-induced reactions. The analysis has been performed for 12 nuclei in the rangeA=45–209. The primary particle-emission is considered to be governed by an admixture of precompound and compound-nucleus mechanisms, while all consecutive particles are considered to undergo pure compound-nucleus emission. The calculated primary-neutron spectra and excitation functions for (n, p), (n, 2n) and (n, 3n) reactions atEn=4–24 MeV have been compared with experimental data. The average best-fit values of the exciton-model free parameterK have been found to be ¯K=700 MeV3. Departures form this value appear to cancel out structure effects introduced into transition rates through the structure-sensitive single-particle level density.

The light charged particles emitted in the40Ar+68Zn reaction performed at 14.6, 19.6 and 35 MeV/nucleon have been studied inclusively. The energy spectra have been analysed in terms of preequilibrium emission, moving source and coalescence models. A complete coherence between the present data and those obtained separately from heavy fragment studies is achieved. These data are consistent with the onset of projectile fragmentation between 20 and 35 MeV/nucleon correlated with the formation of a highly excited region of the nuclear system inducing light particle emission. The other sources of light particles can be interpreted as statistical and sequential decay of the quasi projectile.

The systematic decrease in (n,p) cross sections with increasing neutron number for isotopes in reactions induced by 14 MeV neutrons on heavy targets is explained in terms of the preequilibrium model. The relevant physical quantities responsible for this behavior are proton and neutron separation energies, similarly to (n, p) reactions on lighter nuclei, in which proton emission is governed by the compound-nucleus model. The principal transition rate?+ in the preequilibrium exciton-model calculations should be kept constant (for a given number of excitons and constant energy) in order to obtain a good description of data. The calculations are compared with experimental (n,p) cross sections for series of Te, Xe, W, and Os isotopes.

The preequilibrium emission of nucleons from reactions induced by medium-energy heavy ions is analyzed by using (i) the first term of the Griffin-exciton-model expansion and (ii) the modified Harp-Miller-Berne equilibration model. It is shown that both models account well for the high-energy component of angle-integrated emission spectra. The values of the relevant parametern0 (initial number of degrees of freedom which share the available energy), obtained from the analysis with the two models, are practically equal for each reaction studied. The best-fit values ofn0 depend on the incident energy. This behaviour is a novel feature of preequilibrium emission mechanisms in heavy-ion reactions, hitherto unobserved in light-ion reactions.

The (n, 2n) cross sections for about 80 nuclei ranging fromA=45 to 209 were calculated using the evaporation model. The calculation was performed for incident energiesEn which satisfy the conditionUR=En+Qn,2n=6±1 MeV. For 11 nuclei in the above mass region, (n, 2n) excitation functions were calculated from threshold to 20 MeV. In all calculations a single set of input parameters was consistently used. The influence of the level-density parameter and neutron cross sections on the evaporation model was tested. All calculated results were compared with experimental data. The systematic discrepancy between experimental data and evaporation calculations leads to the necessity of introducing the preequilibrium emission mechanism.

Excitation functions and angular distributions of58Ni+58Ni and58Ni+62Ni scattering at energies just above the Coulomb barrier have been measured aroundθcm=90° in energy stepsΔEcm=0.25 MeV fromEcm⋍ 110 MeV toEcm⋍ 120 MeV for58Ni+58Ni and fromEcm⋍ 110 MeV toEcm⋍ 118 MeV for58Ni+62Ni. Evidence for structure of non-statistical character has been found in the angle-summed excitation functions; this evidence is corroborated by the analysis of the angular distributions. This is the first time that non-statistical structure in elastic and inelastic scattering is reported with high confidence level for this mass and excitation energy ranges. Attempts are presented to understand the nature of this structure, including the presence of intermediate dinuclear states and virtual states in a potential well.

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.

Excitation function and angular distributions of the62Ni(58Ni,58Ni)62Ni elastic scattering have been measured at incident58Ni energies from 220.0 to 230.0 MeV in steps of 0.5 MeV. Evidence of two structures was found in the excitation function; a statistical analysis suggests a possible nuclear cluster quasi-molecular nature for these structures.

Particles with strange quark content produced in the system 1.93A·GeV58Ni on58Ni have been investigated at GSI Darmstadt with the FOPI detector system. The correlation of these produced particles was analyzed with respect to the reaction plane.? baryons exhibit a very pronounced sideward flow pattern which is qualitatively similar to the proton flow. However, the kaon (K+,Ks0) flow patterns are significantly different from that of the protons, and their form may be useful to restrict theoretical models on the form of the kaon potential in the nuclear medium.

High-lying 3-states in Ni isotopes have been investigated and their interpretation in terms of core excitation is discussed.

Experimental inclusive proton spectra of58Ni+60Ni at 15.1 MeV/u have been decomposed using three moving sources associated with, respectively, equilibrium, deep-inelastic, and preequilibrium processes. For the last, a new expression, taking into account the anisotropy of preequilibrium particles in the source system, has been applied. The high-energy part of the obtained angle-integrated proton spectrum has been compared with predictions of the diabatic dissipation model.