[en] Short communication

[en] The present status of our understanding of low mass dilepton production in relativistic heavy ion collisions is discussed. The focus of the discussion will be the sensitivity of dilepton measurements to in medium changes of hadrons and the restoration of chiral symmetry. We will finally discuss how the presence of strong long wavelength pion modes, i.e. disoriented chiral condensates can be seen in the dilepton spectrum. copyright 1997 American Institute of Physics

[en] Preliminary results from the first experiment of the Stopped Beam RISING campaign are presented. The relativistic projectile fragmentation of a 750 MeV/u beam of ¹⁰⁷Ag populated isomeric states in very neutron deficient nuclei at the proton dripline around mass 80-90. Nuclei were unambiguously identified using the FRagment Separator (FRS) and its ancillary detectors located at GSI. The ions produced were slowed down from relativistic energies by means of an Al degrader and implanted in the centre of the high-efficiency Stopped RISING array. This allowed the identification of new excited states in the N=Z=43 nucleus, ⁸⁶Tc, populated following the de-excitation of a microsecond isomer. Preliminary results of this analysis, as well as previously unobserved isomeric states in ^87,88Tc, are reported. (author)

[en] With the addition of krypton and silver projectiles we have extended our previous studies of the fragmentation of heavy relativistic nuclei in targets ranging in mass from hydrogen to lead. These projectiles were studied at a number of discrete energies between 450 and 1500A MeV. The total and partial charge-changing cross sections were determined for each energy, target, and projectile, and the values compared with previous predictions. A new parametrization of the dependence of the total charge-changing cross sections on the target and projectile is introduced, based on nuclear charge radii derived from electron scattering. We have also parametrized the energy dependence of the total cross sections over the range of energies studied. New parameters were found for a previous representation of the partial charge-changing cross sections in hydrogen and a new parametrization has been introduced for the nonhydrogen targets. The evidence that limiting fragmentation has been attained for these relatively light projectile nuclei at Bevalac energies is shown to be inconclusive, and further measurements at higher energies will be needed to address this question

[en] The first preliminary results of pion interferometry in S+Ag and S+Au collisions at 200 GeV/nucleon in a wide rapidity range 0.5< Y<4 are presented. Dependence of longitudinal and transverse source size parameters on rapidity is reported and briefly discussed. Dependence of the longitudinal source size parameter on rapidity may yield very interesting information about the dynamical correlations. Transverse size parameter is found to be larger around and beyond rapidity Y=3, than at lower rapidities

[en] The investigation of deep-inelastic reactions with the Darmstadt-Heidelberg Crystal Ball has been continued. Pronounced structures in the delayed multiplicity for 640 MeV ⁹⁶Mo→⁹⁶Mo near mass symmetry are interpreted as additionnally emitted neutrons caused by nuclear structure effects, in agreement with calculations performed with the CASCADE evaporation code. A new method has been developed employing the delayed multiplicity information for the preparation of the reaction mechanism. As a result it could be shown for the first time that the v-shaped dependence of γ-multiplicity from mass persists up to the highest energy losses. A comparison of theoretical predicitons for the γ-multiplicities vs. energy loss with measured distributions shows that theory overestimates the rise of the observed γ-multiplicity, but is otherwise in qualitative agreement. (orig.)

[en] Short communication

[en] Total and partial charge-changing cross-sections of 600 A MeV Si¹⁴⁺ ions in a thick polyethylene target were measured by CR39 track etch detectors using an optical microscope DM 6000 M system installed with Leica QWin Plus software. The CR39 nuclear track detectors were used to identify the incident charged particles and their fragments using the cone-height distribution method. The charge resolution of CR39 detectors was achieved to be 0.17e in the present condition. The value of the total charge-changing cross-section was calculated to be σ _tot = (766 ±17) mb. To calculate the partial charge-changing cross-sections for ΔZ = -11, -10,.., -1, number of events corresponding to each fragment were determined from multiple Gaussian fitting of cone-height distributions within 95.5% confidence level and the number of incident and survived beam ions were counted within 99.7% confidence level. Possible odd-even pattern in the values of the partial charge-changing cross-section is also presented. The measured charge pick-up cross-section for ΔZ = +1 is (7.2 ±1.7) mb. (orig.)

[en] The HADES (High Acceptance DiElectron Spectrometer) detector, located at GSI, Darmstadt, has been built to study dense baryonic matter and hadronic properties such as form-factors using in particular electromagnetic probes. Currently several upgrades are done in order to improve the HADES performance at SIS18 during FAIR phase 0 experiments. The photon detector of the RICH detector is replaced by H12700 MAPMTs. In this contribution the expected performance of the RICH detector and studies of e⁺e^- pair identification in heavy ion collisions are presented: Simulated UrQMD events of semi-central (44 %) Ag+Ag collisions at 1.65 AGeV beam energy enriched with an additionally embedded PLUTO signal of vector-meson decays have been used for these studies. The high efficient ring finding of the upgraded RICH detector allows for so far unreached background suppression and enables the extraction of an invariant mass spectrum of di-electrons from 50 MeV/c² up to masses beyond the phi meson pole.

[en] The effect of vacuum polarization on the ground-state mass systematics of nuclei is considered. Its contribution to the ground-state energy of heavy nuclei is found to be a few MeV and can be simulated by using an effective nuclear radius in the Coulomb energy term of the mass formula. Its effect on the fission barriers seems to be small. The sub-barrier heavy-ion fusion cross section is found to be reduced at most by an order of magnitude with the inclusion of vacuum polarization in the interaction potential. (orig.)