Spinodal multifragmentation in nuclear physics is reviewed. Considering first spinodal instability within the general framework of thermodynamics, we discuss the intimate relationship between ...first-order phase-transitions and convexity anomalies in the thermodynamic potentials, clarify the relationship between mechanical and chemical instability in two-component systems, and also address finite systems. Then we analyze the onset of spinodal fragmentation by various linear-response methods. Using the Landau theory of collective modes in bulk matter as a starting point, we first review the application of mean-field methods for the identification of the unstable collective modes and the determination of their structure and the associated dispersion relations yielding their growth rates. Subsequently, the corresponding results for finite nuclei are addressed and, within the random-phase approximation, we establish the connection between unstable modes in dilute systems and giant resonances in hot nuclei. Then we turn to the temporal evolution of the unstable systems, discussing first how the dynamics changes its character from being initially linear towards being chaotic and then considering the growth of initially agitated instabilities within the framework of one-body dynamics. We review especially the body of work relating to the Boltzmann–Langevin model, in which the stochastic part of the residual two-body collisions provides a well-defined noise that may agitate the collective modes. We seek to assess the utility of various approximate treatments, including brownian one-body dynamics, and discuss the many possible refinements of the basic treatment. After these primarily formal or idealized studies, we turn to the applications to nuclear multifragmentation and review the various investigations of whether the bulk of the collision zone becomes spinodally unstable. Fragmentation studies with both many-body and stochastic one-body models are discussed and we address the emerging topic of isospin fractionation. We then make contact with experimental data which indicates that the spinodal region is being entered under suitable conditions and we discuss in particular recent results on multifragment size correlations that appear to present signals of spinodal fragmentation. It is demonstrated how various aspects of the data can be understood both qualitatively and quantitatively within the stochastic one-body framework, thus strongly suggesting that nuclear spinodal fragmentation indeed occurs. We finally outline perspectives for further advances on the topic and make connections to current progress on related issues.
The p((11)Li, (9)Li)t reaction has been studied for the first time at an incident energy of 3A MeV at the new ISAC-2 facility at TRIUMF. An active target detector MAYA, built at GANIL, was used for ...the measurement. The differential cross sections have been determined for transitions to the (9)Li ground and first excited states in a wide range of scattering angles. Multistep transfer calculations using different (11)Li model wave functions show that wave functions with strong correlations between the halo neutrons are the most successful in reproducing the observation.
The Super-FRS project at GSI Geissel, H.; Weick, H.; Winkler, M. ...
Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms,
05/2003, Letnik:
204
Journal Article, Conference Proceeding
Recenzirano
The GSI projectile fragment separator FRS has demonstrated with many pioneering experiments the research potential of in-flight separators at relativistic energies. Although the present facility has ...contributed much to the progress in the field of nuclear structure physics, major improvements are desirable in the future. The characteristics of the proposed next-generation facility at GSI, the Super-FRS, will be presented and compared to other projects. The Super-FRS is a large-acceptance superconducting fragment separator followed by different experimental branches including a combination with a new storage-cooler ring system. This system consists of a collector ring (CR) and a new experimental storage ring (NESR) which allow precision mass and lifetime measurements as well as in-ring reaction studies. The NESR can be operated in combination with an electron ring to measure electron scattering with exotic nuclei. This electron heavy-ion collider will open up new fields for nuclear structure research.
The distribution of the largest fragment is studied in different regions of the lattice gas model phase diagram. We show that first- and second-order transitions can be clearly distinguished in the ...grancanonical ensemble, while signals typical of a continuous transition are seen inside the coexistence region if a mass conservation constraint is applied. Some possible implications of these findings for heavy-ion multifragmentation experiments are discussed.
First-order phase transitions in finite systems can be defined through the bimodality of the distribution of the order parameter. This definition is equivalent to the one based on the inverted ...curvature of the thermodynamic potential. Moreover we show that it is in a one-to-one correspondence with the Yang–Lee theorem in the thermodynamic limit. Bimodality is a necessary and sufficient condition for zeroes of the partition sum in the control intensive variable complex plane to be distributed on a line perpendicular to the real axis with a uniform density, scaling like the number of particles.
The excitation of the giant dipole resonance induced by fusion reaction is studied with N/Z asymmetry in the entrance channel. The time dependent Hartree-Fock solution exhibits a strong dipole ...vibration which can be associated with a giant vibration along the main axis of the deformed compound nucleus. This dipole motion appears to be nonlinearly coupled to the shape oscillation, leading to a strong modulation of its frequency. These phenomena can be detected in the gamma-ray emission from hot compound nuclei.
The low-lying spectroscopy of 6He was investigated via the 2-neutron transfer reaction p (He 8 , t) with the 8He beam delivered by the SPIRAL facility at 15.4 A MeV . The light charged particles ...produced by the direct reactions were measured using the MUST2 Si-strip telescope array. Above the known 2 + state, two new resonances were observed: at E a = 2.6 +/- 0.3 MeV (width Phi = 1.6 +/- 0.4 MeV) and at 5.3 +/- 0.3 MeV with Phi = 2 +/- 1 MeV . Through the analysis of the angular distributions, they correspond to a 2 + state and to an L = 1 state, respectively. These new states, challenging the nuclear theories, could be used as benchmarks for checking the microscopic inputs of the newly improved structure models, and should trigger development of models including the treatments of both core excitation and continuum coupling effects.
The N = 28 shell closure has been investigated via the 46Ar(d,p)47Ar transfer reaction in inverse kinematics. Energies and spectroscopic factors of the neutron p(3/2), p(1/2), and f(5/2) states in ...47Ar were determined and compared to those of the 49Ca isotone. We deduced a reduction of the N = 28 gap by 330(90) keV and spin-orbit weakenings of approximately 10(2) and 45(10)% for the f and p states, respectively. Such large variations for the f and p spin-orbit splittings could be accounted for by the proton-neutron tensor force and by the density dependence of the spin-orbit interaction, respectively. This contrasts with the picture of the spin-orbit interaction as a surface term only.
Classical mechanics and time dependent Hartree-Fock (TDHF) calculations of heavy ions collisions are performed to study the rotation of a deformed nucleus in the Coulomb field of its partner. This ...reorientation is shown to be independent of the charges and relative energy of the partners. It only depends upon the deformations and inertias. TDHF calculations predict an increase by 30% of the induced rotation due to quantum effects while the nuclear contribution seems negligible. This reorientation modifies strongly the fusion cross section around the barrier for light deformed nuclei on heavy collision partners. For such nuclei a hindrance of the sub-barrier fusion is predicted.
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