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The characteristics of the fission step following a binary deep-inelastic interaction have been reconstructed for three-body events detected in the reactions 100 Mo + 100 Mo at 18.7A MeV and 120 Sn -f-120 Sn at 18.AA MeV. The observed anisotropy of the in-plane angular distributions points to the fast decay of a rotating (and strongly deformed) nuclear object formed at the end of the deep-inelastic interaction. The derived time scale of the process indicates that asymmetric divisions are faster than symmetric ones. PACS numbers: 25.70.Lm, 25.70.Gh Interest in nuclear fission in general and particularly in the determination of its characteristic time scale has been revived after a series of recent measurements of prescission neutrons (see Ref.[1], and references therein).As compared to particle emission, nuclear fission is expected to be a slower process, due to the complex change of collective degrees of freedom involved in it [2]. Experimentally, this expectation was repeatedly confirmed by the observation of isotropic in-plane angular distributions in a large number of fusion-fission reactions, as well as in the sequential fission decay of heavy reaction products [3]. This fact points to the nucleus undergoing, on the average, at least one full rotation before scission and sets a lower limit of several 10~2 1 s for the fission time scale. Recent studies on neutron emission in fusion-fission reactions have argued that the motion towards scission is highly viscous and leads to fission times of the order of 10 -20 s to 10 -19 s [1,4-6]. This time seems to decrease with increasing mass asymmetry, as reported in recent works based on the detection of neutrons [1,7,8] or light charged particles [9] in coincidence with heavy fragments.Some indications of short time scales (« 10~2 1 s) were obtained also from fragment correlations in sequential fissionlike decays following collisions at intermediate bombarding energies [10,11] and from "proximity" modulations of the relative velocity of sequential-fission fragments in the 129 Xe+ 122 Sn collision at 12.5,4 MeV [12]. In this latter case, anisotropic in-plane angular distributions were also observed, pointing to three-body events characterized by a preferential collinearity of the three fragments, with the lighter fission fragment roughly located in between the other two.This Letter presents for the first time evidence that the in-plane angular distribution of a fissionlike decay evolves from a "well behaved" isotropic shape for symmetric splits towards a strong anisotropy for the most asymmetric splits. Moreover, a novel and independent method of estimating the time scale of the process is suggested, based on the analysis of the shape of the in-plane angular distribution.Two symmetric systems were investigated, namely, 100 Mo + 100 Mo at 18.7A MeV and 120 Sn + 120 Sn at 18.4A MeV. Heavy fragments (A > 20) were detected with twelve position-sensitive gas detectors covering about 75% of the forward hemisphere [13,14]. Prom the measured velocity vectors, primary (pre-e...

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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