Coulomb Excitation of Multi-Phonon Levels of the Giant Dipole Resonance

A closed expression is obtained for the cross-section for Coulomb excitation of levels of the giant dipole resonance of given angular momentum and phonon number. Applications are made to the Goldhaber-Teller and Steinwedel-Jensen descriptions of the resonance, at non-relativistic and relativistic bombarding energies.Comment: 16 pages, 5 figure

Peripherality of breakup reactions

The sensitivity of elastic breakup to the interior of the projectile wave function is analyzed. Breakup calculations of loosely bound nuclei (8B and 11Be) are performed with two different descriptions of the projectile. The descriptions differ strongly in the interior of the wave function, but exhibit identical asymptotic properties, namely the same asymptotic normalization coefficient, and phase shifts. Breakup calculations are performed at intermediate energies (40-70 MeV/nucleon) on lead and carbon targets as well as at low energy (26 MeV) on a nickel target. No dependence on the projectile description is observed. This result confirms that breakup reactions are peripheral in the sense that they probe only the external part of the wave function. These measurements are thus not directly sensitive to the total normalization of the wave function, i.e. spectroscopic factor.Comment: Reviewed version accepted for publication in Phys. Rev. C; 1 new section (Sec. III E), 2 new figures (Figs. 3 and 5

Variabilidad morfológica, distribución espacial y abundancia de las especies de Helicostomella (Ciliophora: Tintinnina) en relación con las condiciones ambientales (Mar Argentino; 40-55ºS)

For the first time a taxonomic and ecological study of species belonging to Helicostomella was performed over a mesoscale spatial distribution by examining ca. 3000 loricae collected in Argentine shelf waters during the austral autumn. Microscopic and statistical analysis revealed that the general shape and the oral diameter remained practically constant in the whole area surveyed, despite a continuous length fluctuation of ca. 300 µm, which includes the entire range present in eight previously reported species. Consequently, we consider that the genus may be represented only by H. subulata, whose strong fluctuations in length (mostly attributed to an increase in the collar length), density and biomass seem to respond to temperature, food availability and front-related processes. In Buenos Aires coastal waters associated with a quasi permanent estuarine front (40-40.5°S), mixed conditions would favour moderate abundances of long loricae, whereas in northern (42.5-45.5°S) and southern (46-54.5°S) Patagonian waters, high (103 ind. L-1) and low (< 5 ind. L-1) densities of short loricae seem to be a consequence of stratification and encystment, respectively. The non-occurrence of the species at 41-42°S, together with a three-fold reduction in length-related parameters between specimens from Buenos Aires and Patagonian waters, suggests a disjunct distribution.Se realizo el primer estudio taxonómico y ecológico de mesoescala sobre las especies del genero Helicostomella, examinando ca. 3000 lorigas colectadas en el Mar Argentino durante el otoño austral. Los análisis microscópicos y estadísticos revelaron que la forma general de la loriga y su diámetro oral permanecen prácticamente constantes en toda el area analizada, mientras que su longitud presenta una variación de ca. 300 μm, incluyendo todo el rango correspondiente a 8 especies previamente reportadas. En consecuencia, el genero estaría representado por una única especie, H. subulata, cuyas variaciones en longitud (atribuidas principalmente a la longitud del cuello), densidad y biomasa responderían a la temperatura, disponibilidad y calidad del alimento y procesos frontales. En aguas costeras de Buenos Aires asociadas a un frente estuarino casi permanente (40-40.5°S), las condiciones de mezcla favorecerían el desarrollo de lorigas largas en abundancias moderadas, mientras que en aguas del Norte (42.5-45.5°S) y del Sur (46-54.5°S) de la Patagonia prevalecen lorigas cortas en densidades elevadas (103 ind. L-1) o muy bajas (< 5 ind. L-1) como consecuencia de procesos de estratificación y enquistamiento, respectivamente. La no detección de la especie entre 41-42ºS, junto con la reducción en 3 veces de los parámetros relacionados con la longitud entre especimenes de las aguas de Buenos Aires y las Patagonicas, sugiere una distribución separada entre ambas áreas

Long-Time Behavior of Velocity Autocorrelation Function for Interacting Particles in a Two-Dimensional Disordered System

The long-time behavior of the velocity autocorrelation function (VACF) is investigated by the molecular dynamics simulation of a two-dimensional system which has both a many-body interaction and a random potential. With strengthening the random potential by increasing the density of impurities, a crossover behavior of the VACF is observed from a positive tail, which is proportional to t^{-1}, to a negative tail, proportional to -t^{-2}. The latter tail exists even when the density of particles is the same order as the density of impurities. The behavior of the VACF in a nonequilibrium steady state is also studied. In the linear response regime the behavior is similar to that in the equilibrium state, whereas it changes drastically in the nonlinear response regime.Comment: 12 pages, 5 figure

Determining the number of clusters in a mixture by iterative model space refinement with application to free-swimming fish detection

We present a clustering algorithm for use when the number of clusters is unknown. We first show that the EM algorithm for mixture modeling can be considered as an alternating minimization between the data space and the model space. We then show how data cleaning can be performed by alternating between the data space and two model spaces. Finally, we develop a mixture model approach that iteratively refines the model spaces, beginning with a coarse model and selecting finer models as indicated by the consistent Akaike information criterion

A Consistent Boltzmann Algorithm

The direct simulation Monte Carlo method for the Boltzmann equation is modified by an additional displacement in the advection process and an enhanced collision rate in order to obtain the exact hard sphere equation of state at all densities. This leads to consistent thermodynamic and transport properties in the low density (Boltzmann) regime. At higher densities transport properties are comparable to the predictions of the Enskog model. The algorithm is faster than molecular dynamics at low and moderate densities and readily run on a parallel architectur

Cell Size Dependence of Transport Coefficients in Stochastic Particle Algorithms

Using the Green–Kubo theory, the dependence of the viscosity and thermal conductivity on cell size is obtained explicitly for stochastic particle methods such as direct simulation Monte Carlo (DSMC) and its generalization, the consistent Boltzmann algorithm (CBA). These analytical results confirm empirical observations that significant errors occur when the cell dimensions are larger than a mean free path

The Semiclassical Coulomb Interaction

The semiclassical Coulomb excitation interaction is at times expressed in the Lorentz gauge in terms of the electromagnetic fields and a contribution from the scalar electric potential. We point out that the potential term can make spurious contributions to excitation cross sections, especially when the the decay of excited states is taken into account. We show that, through an appropriate gauge transformation, the excitation interaction can be expressed in terms of the electromagnetic fields alone.Comment: 12 pages. Phys. Rev. C, Rapid Communication, in pres

Semiclassical treatment of fusion processes in collisions of weakly bound nuclei

We describe a semiclassical treatment of nuclear fusion reactions involving weakly bound nuclei. In this treatment, the complete fusion probabilities are approximated by products of two factors: a tunneling probability and the probability that the system is in its ground state at the strong absorption radius. We investigate the validity of the method in a schematic two-channel application, where the channels in the continuum are represented by a single resonant state. Comparisons with full coupled-channels calculations are performed. The agreement between semiclassical and quantal calculations isquite good, suggesting that the procedure may be extended to more sophisticated discretizations of the continuum.Comment: 11 pages, 5 figure