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Search Results: 1 - 10 of 304919 matches for " G. F. Bertsch "
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Nuclear pairing: basic phenomena revisited
G. F. Bertsch
Physics , 2012,
Abstract: I review the phenomena associated with pairing in nuclear physics, most prominently the ubiquitous presence of odd-even mass differences and the properties of the excitation spectra, very different for even-even and odd-A nuclei. There are also significant dynamical effects of pairing, visible in the inertias associated with nuclear rotation and large-amplitude shape deformation.
Nuclear fission as resonance-mediated conductance
G. F. Bertsch
Physics , 2014,
Abstract: For 75 years the theory of nuclear fission has been based on the existence of a collective coordinate associated with the nuclear shape, an assumption required by the Bohr-Wheeler formula as well as by the R-matrix theory of fission. We show that it is also possible to formulate the theory without the help of collective coordinates. In the new formulation, fission is facilitated by individual states in the barrier region rather than channels over the barrier. In a certain limit the theory reduces to a formula closely related to the formula for electronic conductance through resonant tunneling states. In contrast, conduction through channels gives rise to a staircase excitation function that is well-known in nanoscale electronics but has never been seen in nuclear fission.
Spin-triplet pairing in large nuclei
G. F. Bertsch,Y. Luo
Physics , 2009, DOI: 10.1103/PhysRevC.81.064320
Abstract: The nuclear pairing condensate is expected to change character from spin-singlet to spin-triplet when the nucleus is very large and the neutron and proton numbers $Z,N$ are equal. We investigate the transition between these two phases within the framework of the Hartree-Fock-Bogoliubov equations, using a zero-range interaction to generate the pairing. We confirm that extremely large nucleus would indeed favor triplet pairing condensates, with the Hamiltonian parameters taken from known systematics. The favored phase is found to depend on the specific orbitals at the Fermi energy. The smallest nuclei with a well-developed spin-triplet condensate are in the mass region A ~ 130-140.
Source Dimensions in Ultrarelativistic Heavy Ion Collisions
M. Herrmann,G. F. Bertsch
Physics , 1994, DOI: 10.1103/PhysRevC.51.328
Abstract: Recent experiments on pion correlations, interpreted as interferometric measurements of the collision zone, are compared with models that distinguish a prehadronic phase and a hadronic phase. The models include prehadronic longitudinal expansion, conversion to hadrons in local kinetic equilibrium, and rescattering of the produced hadrons. We find that the longitudinal and outward radii are surprisingly sensitive to the algorithm used for two-body collisions. The longitudinal radius measured in collisions of 200 GeV/u sulfur nuclei on a heavy target requires the existence of a prehadronic phase which converts to the hadronic phase at densities around 0.8-1.0 GeV/fm$^3$. The transverse radii cannot be reproduced without introducing more complex dynamics into the transverse expansion.
Variational approach to anharmonic collective motion
G. F. Bertsch,H. Feldmeier
Physics , 1996, DOI: 10.1103/PhysRevC.56.839
Abstract: We derive large-amplitude collective equations of motion from the variational principle for the time-dependent Schroedinger equation. These equations reduce to the well-known diabatic formulas for vibrational frequencies in the small amplitude limit. The finite amplitude expression allows departures from harmonic behavior of giant resonances to be simply estimated. The relative shift of the second phonon falls with nuclear mass A as A^(-4/3) in the three modes we consider: monopole, dipole, and quadrupole. Numerically the effect is very small in heavy nuclei, as was found with other approaches.
Optical response of small carbon clusters
K. Yabana,G. F. Bertsch
Physics , 1996, DOI: 10.1007/s004600050357
Abstract: We apply the time-dependent local density approximation (TDLDA) to calculate dipole excitations in small carbon clusters. A strong low-frequency mode is found which agrees well with observation for clusters C_n with n in the range 7-15. The size dependence of the mode may be understood simply as the classical resonance of electrons in a conducting needle. For a ring geometry, the lowest collective mode occurs at about twice the frequency of the collective mode in the linear chain, and this may also be understood in simple terms.
Suppression of isoscalar pairing
G. F. Bertsch,Simone Baroni
Physics , 2009,
Abstract: The short-range nuclear attraction is stronger in the isoscalar channel than in the isovector channel, as evidenced by the existence of the deuteron and not the dineutron. Nevertheless, apart from light N=Z nuclei, pairing is only seen in the isovector channel. This is explained by the effect of the strong spin-orbit splitting on the single-particle energies. A semiquantitative argument is presented treating the high-j orbitals at the Fermi surface as plane waves on a two-dimensional sheet.
Spin-rotation coupling in ferromagnetic clusters
G. F. Bertsch,V. Visuthikraisee
Physics , 1996,
Abstract: We examine the magnetic response of free clusters considering the spin direction and the cluster orientation as the only active degrees of freedom. The average magnetization in small fields approaches the Langevin value for paramagnets, depending on the degree to which the Hamiltonian preserves symmetries. Superparamagnetic behavior is not achievable within models considering only these degrees of freedom.
Numerical Convergence in Solving the Vlasov Equation
C. Jarzynski,G. F. Bertsch
Physics , 1995, DOI: 10.1103/PhysRevC.53.1028
Abstract: When the Vlasov equation is investigated numerically using the method of test particles, the particle-particle interactions that inevitably arise in the simulation (but are not present in the Vlasov equation itself) result in an accumulation of errors which eventually drive the collection of test particles toward a state of classical thermal equilibrium. We estimate the rate at which these errors accumulate.
Off-shell effects in heavy particle production
G. F. Bertsch,P. Danielewicz
Physics , 1995, DOI: 10.1016/0370-2693(95)01402-0
Abstract: Off-shell propagation of nucleons is neglected in one-body transport models of heavy-ion collisions, but it could be significant in processes that are limited by phase space, such as the threshold production of heavy particles. We estimate the relative magnitude of off-shell production to on-shell production of the N$^*$(1535) resonance in heavy ion collisions. In the region where the on-shell production is dominated by a~two-step mechanism with an intermediate $\Delta$, we find that the contribution of off-shell scattering between projectile and target nucleons is indeed small. Beyond the latter contribution, however, correlations in the initial wave function produce off-shell contributions which can exceed those of the on-shell $\Delta$ mechanism.
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