Search

Applying the HFB theory with the effective interaction D1S of Gogny, two kinds of “hyperheavy nuclei” were discovered: true “bubbles” with practically vanishing nuclear density in the central region of the nucleus, and “semi-bubbles” (“unsaturated nuclei”) with a reduced but finite density near the nuclear center. Semi-bubbles are found to be stable with regard to the emission of a neutron or a proton for nucleon numbers A and charge numbers Z in the ranges 292⪝A⪝750 and 120⪝Z⪝240, and true bubbles for 750⪝A⪝920 and 240⪝Z⪝280, respectively. For a limited number of nuclear species, a third type of hyperheavy nuclei with a finite, strongly reduced, halo-like central density (“internal halo nuclei”) is found. Coexistence of bubble and semi-bubble solutions for given nucleon and charge numbers is frequently obtained, the semi-bubbles being the ground states for A⪝780 and Z⪝240, and the true bubbles for the heavier nuclear species. The dominant disintegration mode of the semi-bubbles is α-decay and of the true bubbles is fission. The α-lifetimes of semi-bubbles range from several seconds down to ≃10−6 s, according to a simple semi-phenomenological formula. The fission lifetimes vary strongly as functions of the neutron and proton numbers ranging between years (for semi-bubbles) and microseconds or less (for true bubbles). [Copyright &y& Elsevier]

Abstract: We formulate a microscopic theory of the decay of a compound nucleus through fission which generalizes earlier microscopic approaches of fission dynamics performed in the framework of the adiabatic hypothesis. It is based on the constrained Hartree–Fock–Bogoliubov procedure and the Generator Coordinate Method, and requires an effective nucleon–nucleon interaction as the only input quantity. The basic assumption is that the slow evolution of the nuclear shape must be treated explicitly, whereas the rapidly time-dependent intrinsic excitations can be treated by statistical approximations. More precisely, we introduce a “reference density” which represents the slow evolution of the nuclear shape by a reduced density matrix and the state of intrinsic excitations by a canonical distribution at each given shape of the nucleus. The shape of the nuclear density distribution is described by parameters (“generator coordinates”q), not by “superabundant” degrees of freedom introduced in addition to the complete set of nucleonic degrees of freedom. We first derive a rigorous equation of motion for the reference density and, subsequently, simplify this equation on the basis of the “Markov approximation”. The temperature T which appears in the canonical distribution is determined by the requirement that, at each time t, the reference density should correctly reproduce the mean excitation energy at given values of the shape parameters q. The resulting equation for the “local” temperature must be solved together with the equations of motion obtained for the reduced density matrix . [Copyright &y& Elsevier]

Low energy fission is described using a microscopic and time-dependent approach. Fission dynamics is derived from the evolution of a collective wave function in the restricted space generated by the quadrupole and octupole collective variables. Propagation is made by use of the Time Dependent Generator Coordinate Method with a basis set of Hartree Fock Bogoliubov states obtained from self-consistent calculations with the Gogny force. Theoretical fragment mass and kinetic energy distributions in actinide fission are presented. First calculations with selected initial states indicate that the main features of experimental mass distributions in 238U are reproduced. The peak-to-valley ratio appears to significantly depend on the parity of the initial state chosen. [Copyright &y& Elsevier]

Abstract: We review the most important experimental results from the first three years of nucleus–nucleus collision studies at RHIC, with emphasis on results from the STAR experiment, and we assess their interpretation and comparison to theory. The theory-experiment comparison suggests that central Au + Au collisions at RHIC produce dense, rapidly thermalizing matter characterized by: (1) initial energy densities above the critical values predicted by lattice QCD for establishment of a quark–gluon plasma (QGP); (2) nearly ideal fluid flow, marked by constituent interactions of very short mean free path, established most probably at a stage preceding hadron formation; and (3) opacity to jets. Many of the observations are consistent with models incorporating QGP formation in the early collision stages, and have not found ready explanation in a hadronic framework. However, the measurements themselves do not yet establish unequivocal evidence for a transition to this new form of matter. The theoretical treatment of the collision evolution, despite impressive successes, invokes a suite of distinct models, degrees of freedom and assumptions of as yet unknown quantitative consequence. We pose a set of important open questions, and suggest additional measurements, at least some of which should be addressed in order to establish a compelling basis to conclude definitively that thermalized, deconfined quark–gluon matter has been produced at RHIC. [Copyright &y& Elsevier]