E. Piasecki

Particle identification method in the CsI(Tl) scintillator used for the CHIMERA 4π detector

Abstract The charged particle identification obtained by the analysis of signals coming from the CsI(Tl) detectors of the CHIMERA 4 π heavy-ion detector is presented. A simple double-gate integration method, with the use of the cyclotron radiofrequency as reference time, results in low thresholds for isotopic particle identification. The dependence of the identification quality on the gate generation timing is discussed. Isotopic identification of light ions up to Beryllium is clearly seen. For the first time also the identification of Z =5 particles is observed. The identification of neutrons interacting with CsI(Tl) by ( n , α ) and ( n , γ ) reactions is also discussed.

Saturation of the thermal energy deposited in Au and Th nuclei by Ar projectiles between 27 and 77 MeV/u

Abstract Multiplicities of neutrons and light charged particles associated with central collisions have been measured in the energy range 27–77 MeV/u for the systems 40Ar+197Au, 232Th. The experiments demonstrate the occurrence of a saturation of the thermal energy deposited in the system around 650 MeV, corresponding to a constant internal temperature close to 5 MeV.

Mass and charge identification of fragments detected with the Chimera Silicon-CsI(Tl) telescopes

Abstract Mass and charge identification of charged products detected with Silicon–CsI(Tl) telescopes of the Chimera apparatus are presented. An identification function, based on the Bethe–Bloch formula, is used to fit empirical correlations between Δ E and E ADC readings, in order to determine, event by event, the atomic and mass numbers of the detected charged reaction products prior to energy calibration.

Mass and nuclear charge yields for 237Np(2nth,f) at different fission fragment kinetic energies

Abstract The recoil mass separator LOHENGRIN of the Laue-Langevin Institute Grenoble has been used to measure for the first time, the yields of light fission fragments from the fissioning system: 239 93 Np; this odd-Z nucleus is formed after double thermal neutron capture in a 239 93 Np target. The mass distributions were measured for different kinetic energies between 92 and 115.5 MeV, but the nuclear charge distributions were determined only up to 112 MeV. These distributions are compared to the distributions obtained for the even-even system 240 94 Pu. At high kinetic energy, the mass distribution shows a prominent peak around mass number A L = 106. These cold fragmentations are discussed in terms of a calculation based on a scission point model extrapolated to the cold fission case. As expected for an odd- Z fissioning nucleus, the nuclear charge distributions do not reveal any odd-even effect. The global neutron odd-even effect is found to be (8.1 ± 1.5)%. A simple model has been used to show that most of the neutron odd-even effect results from prompt neutron evaporation from the fragments.

Isotope correlations as a probe for freeze-out characterization: central124Sn+64Ni,112Sn+58Ni collisions

Abstract 124 Sn+ 64 Ni and 112 Sn+ 58 Ni reactions at 35 AMeV incident energy were studied with the forward part of CHIMERA multi-detector. The most central collisions were selected by means of a multidimensional analysis. The characteristics of the source formed in the central collisions, as size, temperature and volume, were inspected. The measured isotopes of light fragments (3 ⩽ Z ⩽ 8) were used to examine isotope yield ratios that provide information on the free neutron to proton densities.

Evolution of fission and competing decay mechanisms in 40Ar-induced reactions on Au, Th at 27–77 MeV/u of incident energy

Abstract Fission fragments, heavy reaction residues and the multiplicity of all simultaneously evaporated neutrons are observed from the reactions 40 Ar + Au, Th over a wide range of incident 40 Ar energy from 27 to 77 MeV/u. The inclusive neutron-multiplicity distributions prove the invariance of energy dissipation with incident energy in these reactions: peripheral collisions with low dissipation and central collisions with high dissipation contribute invariably about half of the reaction cross section. Two fragment correlations and fragment angular distributions show that fission is, for energies in excess of 35 MeV/u, progressively escorted by the emission of lighter complex particles, while the importance of fission generally fades away in favour of very heavy-residue production and, to a smaller extent yet in the considered energy range, eventually also in favour of true multifragmentation.

DYNAMICAL EVOLUTION OF THE 197Au + 197Au SYSTEM AT 15 MeV/NUCLEON

Collisions of a very heavy 197Au + 197Au system have been studied at an energy of 15 MeV/nucleon with the aim to investigate a possible change of the energy dissipation mechanism from one-body dissipation (presumably dominating at low excitation energies) to two-body dissipation. The experiment was carried out in 4π geometry using the CHIMERA multidetector array. A class of ternary events satisfying nearly complete balance of mass numbers was selected. Preliminary results suggest that the selected ternary events represent mostly prompt ternary partitions of the colliding system, in which the lightest fragment is quite massive. In the light of theoretical predictions of Cârjan, Sierk and Nix this result may imply that the observed ternary reactions are driven by the two-body dissipation mechanism already at 15 MeV/nucleon.

Angular distributions of light charged particles from 252Cf fission in the ranges 0–46° and 134–180°

Abstract Alpha particles, tritons, deuterons and protons accompanying 252 Cf fission were registered in coincidence with both fission fragments by means of a system containing two-dimensional position-sensitive silicon detectors. Angular distributions, kinetic energy spectra of light charged particles as well as mass distributions of fission fragments in coincidence with light charged particles were measured. The experimental results are compared with some theoretical models.

Investigation of polar emission in 252Cf and 235U + nth fission☆

Abstract The energy distributions and relative intensities of protons, deuterons, tritons and α-particles emitted along the fission axis during spontaneous fission of 252 Cf were measured simultaneously with both fission fragment energies. The absolute intensity of particles, the mass distribution of fragments, the total kinetic energy and total excitation energy of both fragments were subsequently deduced from the experimental data. Statistical model calculations based on a hypothesis that the polar particles are evaporated from fission fragments have been performed for 252 Cf and 236 U fission. Although some experimental results agree remarkably well with the evaporation hypothesis, the considered model cannot describe many features of the polar emission phenomenon.

Emission of complex fragments in the reaction Ar + Au at 44 and 77 A · MeV

Abstract Complex-fragment emission from the 44 and 77 A · MeV 40 Ar + 197 Au reaction has been investigated. Equilibrium and non-equilibrium components have been identified which are discussed in terms of statistical emission from the hot target-like fragment and of a possible persistence of a deep-inelastic process.