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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.

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This last component appears to be compatible with an evaporation from an equilibrated source at normal density, as described by the statistical code Gemini at the appropriate excitation energy. On the contrary, the midvelocity emission presents remarkable differences for what concerns both the dependence of the multiplicities on the energy deposited in the midvelocity region and the isotopic composition of the emitted light charged particles.

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The light output, α/β ratio, and pulse shape have been investigated at −25 • C with PbWO 4 crystal scintillators undoped, and doped by F, Eu, Mo, Gd and S. The fast 0.01 − 0.06 µs and middle 0.1 − 0.5 µs components of scintillation decay were observed for all the samples. Slow components of scintillation signal with the decay times 1 − 3 µs and 13 − 28 µs with the total intensity up to ≈ 50% have been recognized for several samples doped by Molybdenum. We found some indications of a pulse-shape discrimination between α particles and γ quanta with PbWO 4 (Mo doped) crystal scintillators.

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...

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