No abstract

Abstract. The goal of the FAZIA Collaboration is the design of a new-generation 4π detector array for heavy-ion collisions with radioactive beams. This article summarizes the main results of the R&D phase, devoted to the search for significant improvements of the techniques for charge and mass identification of reaction products. This was obtained by means of a systematic study of the basic detection module, consisting of two transmission-mounted silicon detectors followed by a CsI(Tl) scintillator. Significant improvements in ΔE-E and pulse-shape techniques were obtained by controlling the doping homogeneity and the cutting angles of silicon and by putting severe constraints on thickness uniformity. Purposely designed digital electronics contributed to identification quality. The issue of possible degradation related to radiation damage of silicon was also addressed. The experimental activity was accompanied by studies on the physics governing signal evolution in silicon. The good identification quality obtained with the prototypes during the R&D phase, allowed us to investigate also some aspects of isospin physics, namely isospin transport and odd-even staggering. Now, after the conclusion of the R&D period, the FAZIA Collaboration has entered the demonstrator phase, with the aim of verifying the applicability of the devised solutions for the realization of a larger-scale experimental set-up.

HAL is a multidisciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L'archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

No abstract

Abstract. The12 C+ 12 C 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 to select a set of dissipative events which corresponds, to a large extent, to the statistical evaporation of highly excited 24 Mg. 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 of light nuclei above the threshold for particle emission. In particular, a fast increase of the level density 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.PACS numbers: 25.70.z, 24.60.Dr, 27.30.+t, 24.10.Pa NUCLEAR REACTIONS 12C(12C,X), E = 95 AMeV, Measured Fusion-evaporation reactions, Observed deviation from statistical behaviour.

Experimental results concerning the dynamical fission of quasiprojectiles in semiperipheral collisions for the system 80 Kr+ 48 Ca at 35 MeV/nucleon are presented. Data have been collected with four blocks of the FAZIA setup in the first physics experiment of the FAZIA Collaboration. The degree of isospin equilibration between the two fission fragments and its dependence on their charge asymmetry is investigated. The data are compared with the results of the AMD model coupled to GEMINI as an afterburner, in order to get hints about the timescale of the process.

Isospin mixing in the hot compound nucleus 80 Zr was studied by measuring and comparing the γ -ray emission from the fusion reactions 40 Ca + 40 Ca at E beam = 200 MeV and 37 Cl + 44 Ca at E beam = 153 MeV. The γ yield associated with the giant dipole resonance is found to be different in the two reactions because, in self-conjugate nuclei, the E1 selection rules forbid the decay between states with isospin I = 0. The degree of mixing is deduced from statistical-model analysis of the γ -ray spectrum emitted by the compound nucleus 80 Zr with the standard parameters deduced from the γ decay of the nucleus 81 Rb. The results are used to deduce the zero-temperature value, which is then compared with the latest predictions. The Coulomb spreading width is found to be independent of temperature.The issue of isospin impurity in nuclei has been a longstanding open problem in nuclear physics. In particular, its knowledge is interesting in connection with the properties of the isobaric analog states (IAS) and with the Fermi β decay of the N ≈ Z nuclei around the proton drip line. The evaluation of the isospin impurity provides an important correction to the Fermi-transition rates allowing the extraction, in a nucleusindependent way, of the up-down quark-mixing matrix element of the Cabibbo-Kobayashi-Maskawa matrix [1,2]. Concerning the IAS, they are known to have a narrow spreading width ↓ related to the isospin impurities [3,4] originating from the Coulomb interaction coupling them to states of different isospin.In general, the breaking of isospin symmetry can be observed using, as a magnifying lens, the decays which would be forbidden by the selection rules if isospin mixing was not to occur. This is the case of the neutron decay from the IAS [5] and of the E1 decay from self-conjugate nuclei [6].The giant dipole resonance (GDR), where the maximum strength of the E1 transitions is concentrated, is the ideal excitation mode where this selection rule of E1 decay can be fully exploited. This approach was employed to measure the E1 decay of the GDR in nuclei at a finite temperature produced with fusion-evaporation reactions [7][8][9][10][11]. Fusion-evaporation reactions allow the production of self-conjugate compound nuclei (CN) at high excitation energy which, in many cases, * Present address: CEA Saclay, F-91191 Gif-sur-Yvette, France. are far from the β-stability valley. The use of a self-conjugate projectile and target ensures that the CN produced in fusion reactions has isospin I = 0. Therefore, E1 emission associated with the decay of the GDR is hindered due to the fact that, if the isospin of the initial state is pure, only the less-numerous I = 1 final states can be reached in the decay [12]. Conversely, if the initial state is not pure in isospin but contains an admixture of I = 1 states, it can decay to the more numerous I = 0 final states. Thus, the first-step γ yield depends on the degree of isospin mixing of the CN. In addition, at a finite temperature one expects a partial restoration of the isospin symmetry b...

No abstract