M. Bini

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

Abstract 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 ⩽ 15) and velocity of the products were detected with full azimuthal acceptance at laboratory angles 1° ⩽ θ lab ⩽ 30°. Isotope separated light charged particles were measured with movable multiple telescopes in an angular range of 6–90°. Central collisions representing about 1% of the reaction cross section were selected by requiring high total transverse energy, but vanishing side flow. 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.

Evidence for collective expansion in light-particle emission following Au+Au collisions at 100, 150 and 250 A·MeV

Abstract 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.

Application of digital sampling techniques to particle identification in scintillation detectors

In this paper, the use of a fast digitizing system for identification of fast charged particles with scintillation detectors is discussed. The three-layer phoswich detectors developed in the framework of the FIASCO experiment for the detection of light charged particles (LCP) and intermediate mass fragments (IMF) emitted in heavy-ion collisions at Fermi energies are briefly discussed. The standard analog electronics treatment of the signals for particle identification is illustrated. After a description of the digitizer designed to perform a fast digital sampling of the phoswich signals, the feasibility of particle identification on the sampled data is demonstrated. The results obtained with two different pulse shape discrimination analyses based on the digitally sampled data are compared with the standard analog signal treatment. The obtained results suggest, for the present application, the replacement of the analog methods with the digital sampling technique.

Sideward flow in Au + Au collisions at 400 A.MeV

Abstract 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.

Intermediate mass fragment emission pattern in peripheral heavy-ion collisions at Fermi energies.

The emission pattern in the V_perp - V_par plane of Intermediate Mass Fragments with Z=3-7 (IMF) has been studied in the collision 116Sn + 93Nb at 29.5 AMeV as a function of the Total Kinetic Energy Loss of the reaction. This pattern shows that for peripheral reactions most of IMFs are emitted at mid-velocity. Coulomb trajectory calculations demonstrate that these IMFs are produced in the early stages of the reaction and shed light on geometrical details of these emissions, suggesting that the IMFs originate both from the neck and the surface of the interacting nuclei.

Isospin transport in 84 Kr + 112,124 Sn collisions at Fermi energies

Isospin transport phenomena in dissipative heavy ion collisions have been in- vestigated at Fermi energies with a beam of 84 Kr at 35AMeV. A comparison of theh Ni /Z of light and medium products forward-emitted in the centre of mass frame when the beam impinges on two different targets, the n-poor 112 Sn and the n-rich 124 Sn, is presented. Data were collected by means of a three-layer telescope with very good performances in terms of mass identification (full isotopic resolution up to Z�20 for ions punching through the first detector layer) built by the FAZIA Collaboration and located just beyond the grazing angle for both reactions. Theh Ni /Z of the products detected when the n-rich target is used is always higher than that associated to the n-poor one; since the detector was able to measure only fragments coming from the QuasiProjectile decay and/or neck emission, the observed behaviour can be ascribed to the isospin diffusion process, driven by the isospin gradient between QuasiProjectile and QuasiTarget. Moreover, for light fragments theh Ni /Z as a function of the lab velocity of the fragment is observed to increase when we move from the QuasiProjectile velocity to the centre of mass (neck zone). This effect can be interpreted as an evidence of isospin drift driven by the density gradient between the QuasiProjectile zone (at normal density) and the more diluted neck zone.

GARFIELD + RCo digital upgrade: A modern set-up for mass and charge identification of heavy-ion reaction products

An upgraded GARFIELD + Ring Counter (RCo) apparatus is presented with improved performances as far as electronics and detectors are concerned. On the one hand fast sampling digital read out has been extended to all detectors, allowing for an important simplification of the signal processing chain together with an enriched extracted information. On the other hand a relevant improvement has been made in the forward part of the set-up (RCo): an increased granularity of the CsI(Tl) crystals and a higher homogeneity in the silicon detector resistivity. The renewed performances of the GARFIELD + RCo array make it suitable for nuclear reaction measurements both with stable and with Radioactive Ion Beams (RIB), like the ones planned for the SPES facility, where the physics of isospin can be studied.

Non-statistical decay and α-correlations in the 12C+ 12C fusion-evaporation reaction at 95 MeV

Multiple alpha coincidences and correlations are studied in the reaction 12C+12C at 95 MeV for fusion–evaporation events completely detected in charge. Two specific channels with carbon and oxygen residues in coincidence with α −particles are addressed, which are associated with anomalously high branching ratios with respect to the predictions of Hauser–Feshbach calculations. Triple alpha emission appears kinematically compatible with a sequential emission from a highly excited Mg. The phase space distribution of α − α coincidences suggests a correlated emission from a Mg compound, leaving an oxygen residue excited above the threshold for neutron decay. These observations indicate a preferential α emission of 24Mg at excitation energies well above the threshold for 6 − α decay.

A semiconductor-scintillator telescope using a single silicon diode

A novel type of detector telescope suited for charged particle identification is proposed. It consists of a large area silicon PIN diode optically coupled to a CsI(Tl) crystal having about the same cross section as the silicon detector: the peculiarity of the proposed telescope is that the silicon detector acts as a ΔE element and at the same time as a photosensitive device for the fluorescence light produced in the crystal by the penetrating particle. A proper filtering of the signal has been used in order to reconstruct the complementary information (ΔE and Eres) from the composite output waveform of the charge preamplifier coupled to the silicon detector. Prototypes of the telescope have been built and used to detect light particles produced in heavy ion collisions at low (8 MeV/n) and high (300 MeV/n) bombarding energies; results are presented and discussed.

Thermal properties of light nuclei from 12C + 12C fusion–evaporation reactions

The 12C + 12C reaction at 95 MeV has been studied through the complete charge identification of its products by means of the GARFIELD+RCo experimental set-up at INFN Laboratori Nazionali di Legnaro (LNL). In this paper, the first of a series of two, a comparison to a dedicated Hauser–Feshbach calculation allows selecting a set of dissipative events which corresponds, to a large extent, to the statistical evaporation of highly excited 24Mg. Information on the isotopic distribution of the evaporation residues in coincidence with their complete evaporation chain is also extracted. The set of data puts strong constraints on the behaviour of the level density (LD) of light nuclei above the threshold for particle emission. In particular, a fast increase of the LD parameter with excitation energy is supported by the data. Residual deviations from a statistical behaviour are seen in two specific channels, and tentatively associated with a contamination from direct reactions and/or α-clustering effects. These channels are studied in further details in the second paper of the series.