Search

A comprehensive study of the evolution of the GDR properties from E* = 150 MeV to E* = 430 MeV has been undertaken in nuclei of mass A = 120 = 132. The experimental investigation was performed using MEDEA detector. An onset of a quenching of the GDR gamma yield was found at E* = 270 MeV comparing the experimental gamma-ray spectrum with statistical model calculation. The quenching effect increases at E* = 330 MeV and is even more pronounced when the analysis is extended to higher excitation energies using data from previously performed experiments. The comparison with phenomenological models describing the quenching phenomenon gives a qualitative explanation for the effect but is not able to reproduce its detailed features as a function of excitation energy. A smooth cut-off approach describes reasonably well the progressive disappearance of the dipole strength, which occurs around 220-230 MeV excitation energy, and shows that the GDR quenching is a rather sharp effect. [ABSTRACT FROM AUTHOR]

The dynamical dipole mode was investigated in the mass region of the 192Pb compound nucleus, by using the 40Ca + 152Sm and 48Ca + 144Sm reactions at Elab = 11 and 10.1 MeV/nucleon, respectively. Both fusionevaporation and fission events were studied simultaneously for the first time. Our results show that the dynamical dipole mode survives in reactions involving heavier nuclei than those studied previously, however, its yield is lower than that expected within BNV calculations. [ABSTRACT FROM AUTHOR]

The dynamical dipole mode was investigated in the mass region of the 192Pb compound nucleus, by using the 40Ca + 152Sm and 48Ca + 144Sm reactions at Elab = 11 and 10.1 MeV/nucleon, respectively. Both fusion-evaporation and fission events were studied simultaneously for the first time. Preliminary results obtained with a part of the collected statistics show that the dynamical dipole mode survives in reactions involving heavier mass reaction partners than those investigated in our previous works. As it represents a fast cooling mechanism on the fusion path, it could be used to favor the synthesis of super heavy elements through "hot" fusion reactions. [ABSTRACT FROM AUTHOR]

Various aspects observed in the reactions 40Ca+40,48 Ca,46Ti at E=25 MeV/nucleon, studied by means of the CHIMERA detector, will be discussed. An isospin effect on the competition between heavy residue production and binary-like channels has been observed in incomplete fusion events. From the thermodynamical point of view, kinetic energy spectra of protons emitted in fusion-evaporation events were fitted with two maxwellian functions. Despite of the different neutron richness of the populated sources, the average temperatures obtained for the quasi-fused systems are similar (T≃5.2MeV). We also performed a preliminary reconstruction of the excitation energy reached by quasi-fusion sources populated in the studied reactions. [ABSTRACT FROM AUTHOR]

HADES is a secondary generation experiment operated at GSI Darmstadt with the main goal to study dielectron production in proton, pion and heavy ion induced reactions. The first part of the HADES mission is to reinvestigate the puzzling pair excess measured by the DLS collaboration in C + C and Ca + Ca collisions at 1 A GeV. For this purpose dedicated measurements with the C + C system at 1 and 2 A GeV were performed. The pair excess above a cocktail of free hadronic decays has been extracted and compared to the one measured by DLS. Furthermore, the excess is confronted with predictions of various model calculations. [ABSTRACT FROM AUTHOR]

We present the results of a study of charged-pion production in 12C + 12C collisions at incident beam energies of 1A GeV and 2A GeV using the HADES spectrometer at GSI. The main emphasis of the HADES program is on the dielectron signal from the early phase of the collision. Here, however, we discuss the data with respect to the emission of charged hadrons, specifically the production of $ \pi^{{\pm}}_{}$ mesons, which are related to neutral pions representing a dominant contribution to the dielectron yield. We have performed the first large-angular-range measurement of the distribution of $ \pi^{{\pm}}_{}$ mesons for the 12C + 12C collision system covering a fairly large rapidity interval. The pion yields, transverse-mass and angular distributions are compared with calculations done within a transport model, as well as with existing data from other experiments. The anisotropy of pion production is systematically analyzed. [ABSTRACT FROM AUTHOR]

The evolution with beam energy of the prompt dipole radiation, related with entrance channel charge asymmetry effects, was studied in the fusion reactions: 36Ar+96Zr and 40Ar+92Zr at Elab = 16 and 15.1 MeV/u, respectively. Both reactions populate, through entrance channels having different charge asymmetries, the same compound nucleus at the same average excitation energy and with identical spin distribution. By studying the γ-ray energy spectra of the considered reactions, and by comparing the present result with previous ones obtained at lower energies, we deduce that the prompt dipole γ-ray emission presents a maximum value at 9 MeV/u and decreases toward lower and higher energies. Moreover, the centroid and the width of the preequilibrium dipole component were found to remain constant, within the errors, by increasing the beam energy. [ABSTRACT FROM AUTHOR]

We review some results on energetic particle production in heavy-ion collisions below roughly 100A·MeV, both theoretically and experimentally. We discuss the possible mechanisms of particle production, as well as the possibility to gather information on the nuclear equation of state (EOS) from data. Results on subthreshold pions, energetic photons, nucleons and light charged particles ( Z ⩽ 2) are discussed and contrasted to microscopic models. Important information about the first stages of the reaction are obtained by such probes. At present, we can conclude that we have at least a qualitative understanding of the processes involved when such particles are produced. However, a quantitative determination of relevant EOS parameters is still missing. The production mechanism close to the kinematical threshold (incoherent, cooperative or statistical) is not completely elucidated either. This calls for new data using more modern detector systems and comparison to more refined microscopic models. [ABSTRACT FROM AUTHOR]

Using the 4π CHIMERA detector system we have observed a dynamical, non-equilibrated fission of projectile-like fragments as well as an equilibrated fission component in collisions 124Sn + 64Ni and 112Sn + 58Ni at 35A MeV. The velocity and angular distributions of projectile-like fission fragments were analyzed. The contribution of the "dynamical fission" component is larger in the neutron rich 124Sn + 64Ni system than in the neutron poor 112Sn + 58Ni one. [ABSTRACT FROM AUTHOR]

The identification of the light mesons in the HADES spectrometer is based on an invariant mass reconstruction of their decay into e+e- pairs. In the dilepton (e+e-) signal reconstruction, particularly important is the reduction of a huge combinatorial background arising from wrong combinations of e+ and e- to unlike sign pairs. Methods of the dilepton signal and combinatorial background reconstruction in the HADES spectrometer will be presented. [ABSTRACT FROM AUTHOR]

The HADES spectrometer and a time-of-flight detector based on long-sized (50–250 cm) scintillation counters are described. The detector is designed for identification of electrons, pions, and protons with momenta of 0.1–1.5 GeV/ c at a flight distance of 220–250 cm. The operating conditions are analyzed, and the requirements to the detector and counter design are formulated. The design of the counters and their arrangement in the spectrometer are considered. The results of experimental studies of such characteristics of long counters as the light attenuation length in a scintillator and temporal and spatial resolutions, dependeing on the location of the point of the particle’s hit on the counter, are presented. Analysis of the data has shown that the main factor determining the spatial resolution of long counters is the quality of the polishing of the side faces of the scintillators, which determines the effective (averaged over the scintillator side surfaces) coefficient of total internal reflection R. It is shown that, for R>-0.99, it is quite possible to achieve temporal and spatial resolutions of 100 ns and 2–3 cm, respectively, for scintillators with a length of up to 250 cm and a cross section of 1–20 cm2. Foreign and Russian photomultipliers were used in the counters. [ABSTRACT FROM AUTHOR]

The emission of hard photons and pre-equilibrium protons has been investigated in exclusive experiments performed with MEDEA detector. The observation of the γ-proton anticorrelation indicates that the dominant production mechanism is the first-chance neutron-proton (np) collisions. The anisotropic angular distributions point out the presence of screening effects and are reminiscent of the free np scattering thus giving information of the space-time origin of the energetic protons. Protons, with energy more than twice the kinematical limit for nucleon-nucleon (NN) collisions, have been observed in several reactions around the Fermi energy. [ABSTRACT FROM AUTHOR]

The event-by-event determination of the impact parameter from particle multiplicity emitted in heavy-ion collisions is considered. The associate problem of the determination of the particle multiplicity on both multi-event and event-by-event basis is formulated and discussed. It is shown that a multi-event data analysis procedure allows an accurate determination of the multipolicity distribution of the emitted particles even with not necessarily large total efficiency and granularity detection systems. In the event-by-event data analysis procedure a fold-to-multiplicity conversion based on the Bayes strategy is adopted. It is shown that a reliable event-by-event multiplicity determination and multiplicity resolution definition cannot prescind from a multi-event analysis of the full data set. It is found that the resolution of the event-by-event fold-to-multiplicity conversion procedure depends on the characteristics of the detection system, but not exclusively. The dependence on the particular multiplicity distribution investigated is discussed. It is concluded that event-by-event high-resolution impact parameter measurements can only be performed with very efficient and granular detection systems, but a gross event classification into central and peripheral collisions can be performed even with detection efficiency as low as Ω=0.3. [ABSTRACT FROM AUTHOR]

Presents information on the development of the time of flight (TOF) wall subdetector for the High Acceptance DiElectron Spectrometer (HADES) collaboration. Details on the detector system; Information on the TOF wall; Performance of the detector; Evaluation of the charge particle multiplicity of the planes of the TOF wall.

Photons emitted in the Si+ Ni and S+ Ni reactions at 143 MeV and 156 MeV incident energy respectively, have been detected in coincidence with the ejected charged fragments. An array of 48 BaF γ-rays detector and 6 solid state silicon detector telescopes have been used. Photon energy spectra measured in the energy range from 2 to 20 MeV in coincidence with ejectiles coming from deep inelastic reactions, are consistent with statistical emission from the reaction products. [ABSTRACT FROM AUTHOR]