A facility upgrade at Texas A&;M University for accelerated radioactive beams

The Cyclotron Institute at Texas A&M University is carrying out an upgrade project which will lead to accelerated radioactive ion beams at intermediate energies. The project involves recommissioning a K150 cyclotron for acceleration of stable beams which will be used to produce radioactive ions. Both light-ion and heavy-ion guides will be used to stop and transport the radioactive ions to a charge breeding electron cyclotron resonance ion source. Following charge breeding, highly-charged ions will be injected into the K500 cyclotron, accelerated and then transported to existing equipment to carry out experiments.     

Neck dynamics

Intermediate-energy heavy-ion reactions produce a mid-rapidity region or neck, mostly in the semiperipheral collisions. Brief theory and experiment surveys are presented. General properties of the mid-rapidity zone are reviewed and discussed in the framework of reaction dynamics. Hierarchy effect, neutron enrichment, isospin diffusion are all new neck phenomena which are surveyed. The main neck observables are also examined, mainly in the context of the symmetry term of the nuclear equation of state.     

Physical preseparation: A powerful new method for transactinide chemists

In recent years, the concept of physical preseparation of single atoms was introduced into the field of transactinide chemistry. In this approach, the transactinide element of interest is isolated in a physical recoil separator and then extracted from this machine. The beam as well as the unwanted reaction products are strongly suppressed, allowing for the investigation of new chemical systems that were not accessible before. The most important aspects of the technique are discussed and the advantages for chemistry experiments with transactinides are presented, using some of the chemical studies that were performed with preseparated isotopes as examples.     

On the momentum correlations in the fragmentation of <Superscript>197</Superscript>Au + <Superscript>197</Superscript>Au reactions

We aim to present the formation of fragments using static as well as momentum-dependent interactions and larger nucleonic cross-sections. We shall also examine, in detail, the role of momentum correlations in these situations which are close to the fire-ball as well as to the spectator matter decay. The role of momentum correlations will be studied by imposing an additional cut in the momentum space. A clear need of a momentum cut is demonstrated in both fire-ball as well as spectator matter physics. The momentum-dependent interactions and larger cross-sections play a drastic role in peripheral reactions, whereas the role of momentum cut remains the same in these situations.     

Freeze-out volume in multifragmentation --Dynamical simulations

Stochastic mean-field simulations for multifragmenting sources at the same excitation energy per nucleon have been performed. The freeze-out volume, a concept which needs to be precisely defined in this dynamical approach, was shown to increase as a function of three parameters: freeze-out instant, fragment multiplicity and system size.     

Isotopic compositions and scalings

We review experimental and theoretical studies devoted to extract information on the behaviour of the symmetry energy, in density regions different from the normal value, with charge-asymmetric reactions at Fermi energies. In particular, we focus on the analysis of fragmentation reactions and isotopic properties of the reaction products. Results concerning “isoscaling” properties and the N/Z equilibration among the reaction partners in semi-peripheral reactions are also discussed.     

Prompt ternary break-up in 32S+59Co and 32S+63Cu reactions at 5.6 A · MeV

Final states indicating the presence in the reaction of three-body fragmentation processes has been observed in ³²S+⁵⁹Co and ³²S+⁶³Cu dissipative collisions at 5.6 A · MeV. Besides the already observed sequential binary process, data analysis reveals the presence of prompt ternary break-up of the composite system. Indications on the system configuration at the scission have been deduced by analyzing the event shape in the momentum phase space. The decay appears to occur in a collinear configuration, one of the produced fragments originating from the neck which connects the other two. In spite of the large energy dissipation, structure effects in the charge partition seem to affect part of events. Received: 3 July 1998 / Revised version: 15 October 1999     

Influence of momentum-dependent interactions on balance energy and mass dependence⋆

We aim to understand the role of momentum-dependent interactions in transverse flow as well as in its disappearance. For the present study, central collisions involving masses between 24 and 394 are considered. We find that the momentum-dependent interactions have different impact in lighter colliding nuclei compared to heavier colliding nuclei. In lighter nuclei, the contribution of the mean field towards flow is smaller compared to heavier nuclei where binary nucleon-nucleon collisions dominate the scene. The inclusion of momentum-dependent interactions also explains the energy of the vanishing flow in the ¹²C + ¹²C reaction which otherwise was not possible with the static hard equation of state. An excellent agreement of our theoretical attempt is found for balance energy with experimental data throughout the periodic table.     

Evolution of the fusion cross-section for light systems at intermediate energies

Ar + Ni and Ni + Ni collisions are investigated between 32 and around 100A MeV incident energy with the 4π multidetector INDRA. Fusion cross-sections are found to decrease from ∼ 180mb at 32A MeV to zero above 50A MeV. Other experimental results, for light systems, are compared. Moreover, theoretical works are discussed and fusion cross-sections, calculated from two dynamical simulations based on nuclear Boltzmann equation (Boltzmann-Nordheim-Vlasov and Landau-Vlasov models), are also compared to experimental results.     

Fusion-evaporation cross-sections for 48Ca + 154Sm near the Coulomb barrier

Measurements of fusion-evaporation cross-sections for the system ⁴⁸Ca + ¹⁵⁴Sm have been performed in the sub- and near-barrier energy range. Barrier-passing cross-sections have been obtained by adding recently measured capture-fission cross-sections at the same energies, and the barrier distribution for capture has been extracted. The data have been analyzed within a coupled-channel model, and a large subbarrier cross-section enhancement is observed, due to the ground-state prolate deformation of ¹⁵⁴Sm. The ⁴⁸Ca + ¹⁵⁴Sm capture cross-sections are compared to existing data on ¹⁶O + ¹⁸⁶W fusion, leading to the same CN, where a few higher-energy points have also been measured. The evaporation residue cross-sections for the two systems above the barrier indicate that complete fusion is inhibited for ⁴⁸Ca + ¹⁵⁴Sm by 40% in that energy region, with respect to ¹⁶O + ¹⁸⁶W.     

Jets of nuclear matter in He-A T inelastic collisions at 4.5 A GeV/c

The problem of the nuclear matter jets in nucleus-nucleus collisions at 4.5 A GeV/c is discussed. The global analysis of experimental data, namely the sphericity tensor, is used to evidence such jets. The experimental results are compared with those obtained in the same collisions for hydrodynamic flow and thrust. The experiments have been performed in the frame of the SKM 200 Collaboration from JINR Dubna. Received: 20 March 1997 / Revised version: 20 July 1997     

III. A stochastic two-stage reaction model for heavy-ion collisions

We propose a two-stage, stochastic model of heavy-ion reactions. Nucleons becoming participants by mean-field effects or by nucleon-nucleon interactions are transferred to definite final states, creating a PLF, a TLF, clusters, or escaping to continuum. Nucleon transfer probabilities are governed by state densities. In this way different hot particle sources are created which afterwards decay by particle emission. Received: 12 March 2001 / Accepted: 20 June 2001     

Pre-equilibrium dipole strength in charge asymmetric peripheral heavy-ion reactions

We report on the results obtained from the study of the ³²S + ⁶⁴Ni and ³²S + ⁵⁸Ni peripheral reactions at incident energies E _lab = 288 MeV and E _lab = 320 MeV, respectively. High-energy γ-rays were detected in an array of 8 seven-pack BaF₂ clusters. Coincidence with complex fragments detected in 12 three-stage telescopes ensured the selection of peripheral reaction events. All of the relevant reaction parameters were kept constant with the exception of the different initial dipole moment caused by the different entrance channel charge asymmetry. While for quasi-elastic events no N/Z effect was observed in the differential γ-ray multiplicities of the two reactions, for deep-inelastic events a larger dipole γ-ray emission occurs during the more N/Z asymmetric reaction. A theoretical interpretation based on a collective Bremsstrahlung analysis of the reaction dynamics is presented. Received: 26 September 2002 / Accepted: 13 November 2002 / Published online: 6 March 2003 RID="a" ID="a"e-mail: pierroutsakou@na.infn.it RID="b" ID="b"Present address: INFN, Laboratori Nazionali di Legnaro, Padova, Italy. Communicated by C. Signorini     

Pre-equilibrium dipole excitation and γ-ray fragment angular correlation in the 32S +74Ge reaction

We report on the results obtained from the study of the ³²S +⁷⁴Ge deep inelastic reaction at incident energy E= 320 MeV. High-energy γ-rays were detected in an array of 6 seven-pack BaF₂ clusters. Coincidence with complex fragments detected in 12 three-stage telescopes ensured the selection of peripheral reaction events. In order to investigate the pre-equilibrium dipole strength excitation two independent analyses were performed. In the first analysis the energy spectra of the γ-rays were evaluated in the statistical model framework while in the second one the γ-ray fragment angular correlation with respect to the nuclear spin vector of the composite system was studied. Both methods indicate the excitation of dipole strength in the highly deformed dinucleus and provide dipole resonance parameter sets that are in good agreement with each other. Received: 24 March 1999 / Revised version: 17 May 1999     

Emission order of light particles and light fragments from hot nuclei followed by binary fission

Emission orders of light charged particles and Li fragments from highly excited fissioning nuclei in the reaction of ⁴⁰Ar+¹⁹⁷Au at E _beam /A=25 MeV have been studied by measuring difference velocity distributions of two correlated particles at small relative angles in coincidence with two fission fragments. By comparing the data with three-body trajectory calculations, we found that high velocity deuterons are emitted prior to high velocity Li fragments but low velocity deuterons are emitted after low velocity Li fragments. On the other hand, no preferential emission was observed among light particles, such as protons and deuterons. Furthermore, the emission orders are found to depend only weakly on the mass asymmetry of fission fragments. Received: 3 June 1997 / Revised version: 23 July 1997     

Formation and decay of super-heavy systems

We investigate the formation and the decay of heavy systems which are above the fission barrier. By using a microscopic simulation of constrained molecular dynamics (CoMD) on Au + Au collision, we observe that composite states stay for a very long time before decaying by fission. Received: 13 December 2001 / Accepted: 19 March 2002     

Systematics of stopping and flow in Au+ Au collisions

Excitation functions of flow and stopping observables for the Au+ Au system at energies from 40 to 1500MeV per nucleon are presented. The systematics were obtained by merging the results of the INDRA and FOPI experiments, both performed at the GSI facility. The connection to the nuclear equation of state is discussed.     

Recent developments and on-line tests of uranium carbide targets for production of nuclides far from stability

The capacity of uranium carbide target materials of different structure and density for production of neutron-rich and heavy neutron-deficient nuclides have been investigated. The yields of Cs and Fr produced by a 1 GeV proton beam of the PNPI synchrocyclotron and release properties of different targets have been measured. Yields and release efficiencies of Cs and Fr produced from a high density UC target material and from low density UCx prepared by the ISOLDE method at IRIS in the collaboration with PARRNe group from Orsay are compared. The yields from ISOLDE original target are presented for comparison as well.