Vies moyennes de niveaux de 53Mn excités par la réaction 50Cr (α, pγ)53Mn

Les propriétés électromagnétiques du noyau 53Mn sont étudiées à l'aide de la réaction 50Cr(α, pγ)53Mn à Eα = 14,5 MeV. Les rayonnements γ sont détectés en coïncidence avec les groupes de protons observés à l'aide d'un détecteur annulaire placé à 180° dans l'axe du faisceau. Des rapports d'embranchement de transitions γ sont donnés. La méthode d'atténuation de l'effet Doppler a été utilisée pour déduire les vies moyennes : niveau de 3,42 MeV, τ = 1,0 +0,6-0,4 ps ; niveau de 3,44 MeV, τ = 0,13 +0,05-0,04ps ; niveau de 4,15 MeV, τ = 0,07 ± 0,02 ps. Pour les niveaux à Ex < 3 MeV nos valeurs de τ sont en accord avec des résultats obtenus ailleurs. Pour le niveau de 3,44 MeV nous déduisons Jπ = 15/2- sans ambiguïté. Certaines probabilités de transitions B(M1) et B(E2) sont déterminées et comparées aux prédictions des modèles récents

Abundance of Delta Resonances in 58Ni+58Ni Collisions between 1 and 2 AGeV

Charged pion spectra measured in 58Ni-58Ni collisions at 1.06, 1.45 and 1.93 AGeV are interpreted in terms of a thermal model including the decay of Delta resonances. The transverse momentum spectra of pions are well reproduced by adding the pions originating from the Delta-resonance decay to the component of thermal pions, deduced from the high transverse momentum part of the pion spectra. About 10 and 18% of the nucleons are excited to Delta states at freeze-out for beam energies of 1 and 2 AGeV, respectively.Comment: 14 pages, LaTeX with 3 included figures; submitted to Physics Letters

Stopping and Radial Flow in Central 58Ni + 58Ni Collisions between 1 and 2 AGeV

The production of charged pions, protons and deuterons has been studied in central collisions of 58Ni on 58Ni at incident beam energies of 1.06, 1.45 and 1.93 AGeV. The dependence of transverse-momentum and rapidity spectra on the beam energy and on the centrality of the collison is presented. It is shown that the scaling of the mean rapidity shift of protons established for AGS and SPS energies is valid down to 1 AGeV. The degree of nuclear stopping is discussed; the IQMD transport model reproduces the measured proton rapidity spectra for the most central events reasonably well, but does not show any sensitivity between the soft and the hard equation of state (EoS). A radial flow analysis, using the midrapidity transverse-momentum spectra, delivers freeze-out temperatures T and radial flow velocities beta_r which increase with beam energy up to 2 AGeV; in comparison to existing data of Au on Au over a large range of energies only beta_r shows a system size dependence

Prominent transverse flow of clusters in stopped Au(150 A MeV)+Au reactions

Stopped Au(150 A MeV) + Au collisions have been measured with the FOPI-Detector at GSI by imposing an upper limit on the ratio of the global longitudinal momentum to the collected charge within an event

Charged pions from Ni on Ni collisions between 1 and 2 AGeV

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

Central Collisions of Au on Au at 150, 250 and 400 A MeV

Collisions of Au on Au at incident energies of 150, 250 and 400 A MeV were studied with the FOPI-facility at GSI Darmstadt. Nuclear charge (Z < 16) and velocity of the products were detected with full azimuthal acceptance at laboratory angles of 1-30 degrees. Isotope separated light charged particles were measured with movable multiple telescopes in an angular range of 6-90 degrees. Central collisions representing about 1 % of the reaction cross section were selected by requiring high total transverse energy, but vanishing sideflow. The velocity space distributions and yields of the emitted fragments are reported. The data are analysed in terms of a thermal model including radial flow. A comparison with predictions of the Quantum Molecular Model is presented.Comment: LateX text 62 pages, plus six Postscript files with a total of 34 figures, accepted by Nucl.Phys.

Entropy production in the Au + Au reaction between 150 and 800 A MeV

The entropy per nucleon (S/A) has been extracted for the Au [(150—800)A MeV] + Au reaction by using the phase I setup of the 4 pi facility at GSI, Darmstadt. The entropy has been obtained from the comparison of various observables characterizing the dM/dZ fragment multiplicity distributions, extending up to Z~15, with those calculated with the quantum statistical model. It is the first time that S/A values are determined by considering the full ensemble of charged products detected in the reaction. Consistent values of S/A are found from different methods. These entropy values are shown to be fairly independent of the volume of the ``participant'' region considered. They are somewhat lower than those extracted in earlier works but are in good agreement with hydrodynamic calculations and suggest a low viscosity for the hot and dense nuclear matter

Vie moyenne de l'état 8+ à 6,38 MeV dans 54Fe

The lifetime of the 6.38 MeV 8+ state in 54Fe populated in the 50Cr(6Li, pn) reaction has been measured by using the Doppler shift attenuation method ; γ-rays were observed in coincidence with neutrons. The mean lifetime is τ = 164+40-30 fs. E2-transition strengths in 54Fe are discussed and compared to the predictions of shell-model calculations.La vie moyenne τ de l'état 8+ à 6,38 MeV de 54Fe peuplé par la réaction 50Cr(6Li, pn), a été mesurée en utilisant la méthode d'atténuation de l'effet Doppler ; les rayonnements γ ont été observés en coïncidence avec les neutrons. La valeur obtenue est τ = 164+40-30 fs. Les probabilités de transitions E2 dans 54Fe sont discutées et comparées aux prédictions de calcul de modèles des couches

RECHERCHE D'ÉLÉMENTS TRACES PAR SPECTROMÉTRIE X

Un générateur de rayons X, ainsi qu'un détecteur Si(Li) sont utilisés pour la recherche d'éléments traces par spectrométrie X. La méthode est illustrée dans le cas d'analyses d'échantillons biologiques.A X-ray generator and a Si(Li) detector were used to perform X-ray measurements for trace element analysis. The method is illustrated for the case of biological samples analysis