C. Signorini

Heavy-ion fusion below the Coulomb barrier: The systems 28,30Si + 58,62,64Ni and 32,34,36S + 58,64Ni

Abstract Fusion cross-sections for the 12 systems 28,30 Si + 58,62,64 Ni and 32,34,36 S + 58,64 Ni were measured at many energies from well below up to about 1.5 times the Coluomb barriers. The data show large sub-barrier fusion probabilities with strong isotopic dependences. The results of coupled-channel calculations involving the lowest 2 + and 3 − levels of both colliding nuclei and the 2-neutron transfer channel are compared with the measured excitation functions. Indications for the importance of the transfer channels are found. Signatures of system-dependent effects of the positive Q -value transfer channels are evidenced, also from a systematic intercomparison of the data.

Fluctuations in the excitation functions of dissipative heavy ion collisions

Abstract Excitation functions have been measured for different charge products of the 28 Si + 64 Ni reaction in the range 120–126.75 MeV in 250 keV steps (lab). Dissipative cross sections have shown fluctuations with coherence energies in the range 200–800 keV (CM). The formation of a dinuclear intermediate system is suggested.

Prolate and oblate 10+ isomers in 140Sm78 and 142Gd78

Abstract The N = 78 isotones 140Sm and 142Gd have been studied with α-particle- and HI-induced reactions. In both nuclei two close-lying 10+ isomers at ∼ 3.2 MeV have been established, and from the feeding E2 cascades they are characterized as the prolate ν h 11 2 −2 and the oblate π h 11 2 2 excitations. A third E2 cascade feeding into the 142Gd 8+ state is interpreted as the continuation of the collective ground state sequence that might assume the properties of a perfect deformed rotor for spins above 10.

Neutron and proton transfer in 32,36S+ 58,64Ni around the Coulomb barrier

Abstract Quasi-elastic transfer cross sections have been measured for the systems 32,36 S+ 58,64 Ni at three energies around the Coulomb barrier. Q -value distributions have been extracted for each of the neutron and charged particle transfer channels. The influence of the transfer channels on the sub-barrier fusion cross section is discussed, and the importance of the proton stripping in 32 S + 64 Ni is pointed out.

Prompt dipole radiation in fusion reactions

Abstract The prompt γ-ray emission is investigated in the 16 A MeV energy region by means of the 36,40Ar + 96,92Zr fusion reactions leading to a compound nucleus in the vicinity of 132Ce. The dynamical nature of this radiation is confirmed. We show that the prompt γ radiation has an angular distribution pattern consistent with a dipole oscillation along the symmetry axis of the dinuclear system. The data are compared with calculations based on a collective bremsstrahlung analysis of the reaction dynamics.

Cross sections and mean angular momenta for 64Ni + 92,96Zr fusion near and below the Coulomb barrier

Abstract Fusion cross sections and mean angular momenta have been measured for 64 Ni + 92,96 Zr around the Coulomb barrier. The results are presented and systematically compared with the predictions of a schematic coupled-channel model, demonstrating the need of more refined calculations including higher-order coupling effects, and/or other degrees of freedom, beyond the usual inelastic and quasi-elastic transfer channels.

Fusion and breakup at the barrier with unstable nuclei

Abstract Present experiments related to fusion and breakup processes in the energy region immediately above the Coulomb barrier are reviewed and discussed. The most of the material utilised concerns very recent experimental work not yet published, still subject to possible slight changes. These data involve skin, skin-like and halo nuclei with weakly bound outer nucleons. Skin nuclei: 1) the results on 4,6 He + 209 Bi represent a rather unsolved puzzle for their theoretical interpretation; 2) the data on 27,29,31 Al + 197 Au sub-barrier fusion do not show big singularities and suggest once more the influence of neutron transfer well expected for skin-like nuclei. Halo nuclei: 1) the system 9,10,11 Be + 209 Bi after a careful analysis, indicates that the contribution of the breakup-fusion process, under a certain schematization of the breakup, has only less than 25% probability; 2) the system 9,11 Li + Si indicates the existence of fast breakup neutrons in forward direction in coincidence with the evaporation residues originating from the fusion. Some of these data point out that new phenomena are slowly showing up with skin and halo nuclei.

Identification of theN=79147Er nucleus throughγ-recoil coincidences

Abstracthe so far unknown nucleus147Er has been identified by means of γ- recoil and γ — γ coincidences through the reaction92Mo(58Ni,2pn) at a bombarding energy of 260 MeV. The level scheme has been obtained up to 5.0 MeV of excitation energy and shows collective excitations built on the h11/2 intruder orbital.

Parity violation in the γ-decay of polarized 93Tc nuclei in the 172− isomeric state

We report on a determination of the parity nonconserving (PNC) matrix element in the bound parity doublet 17−2 − 17+2 of 93Tc. The experiment was carried out at the GSI, Darmstadt and LNL, Legnaro laboratories. The recoil-mass-separated radioactive beam of 93Tc nuclei in the 17−2 isomer, following a fusion-evaporation reaction, was polarized by the tilted-foil method and the resulting 0°–180° γ asymmetry with respect to the induced polarization direction was measured by two large-volume Ge detectors. The measured γ asymmetry of 3-σ significance, Aγ = 8.4(2.7) · 10−4, yields a matrix element of |〈17−2|Hpnc|17+2〉| = 0.59(19)(25) meV. This experimental result is c microscopic calculations based on the DDH “best value” interaction for the nuclear weak Hamiltonian. We discuss our results and their significance with respect to the existing data regarding PNC effects in bound nuclear systems.

Study of superdeformed bands in nuclei withA ∼ 150 by heavy-ion-γ coincidences

Superdeformed (SD) bands in152Dy,151Dy and151Tb have been populated via the 5n, 6n and 5np evaporation channels, respectively, with the33S+124Sn reaction at 160 and 170 MeV bombarding energies. Population intensities are in good agreement with existing data for SD bands in151, 152Dy and SD yrast band in151Tb. The excited “twin” SD band in151Tb with the sameγ-transitions as the band in152Dy is populated about 5 times more strongly than by the 6n evaporation channel. This might be explained in terms of competition between proton andγ emission out of an intermediate, excited superdeformed configuration of152Dy.