Study of Giant Pairing Vibrations with neutron-rich nuclei

We investigate the possible signature of the presence of giant pairing states at excitation energy of about 10 MeV via two-particle transfer reactions induced by neutron-rich weakly-bound projectiles. Performing particle-particle RPA calculations on $^{208}$Pb and BCS+RPA calculations on $^{116}$Sn, we obtain the pairing strength distribution for two particles addition and removal modes. Estimates of two-particle transfer cross sections can be obtained in the framework of the 'macroscopic model'. The weak-binding nature of the projectile kinematically favours transitions to high-lying states. In the case of (~^6He, \~^4He) reaction we predict a population of the Giant Pairing Vibration with cross sections of the order of a millibarn, dominating over the mismatched transition to the ground state.Comment: Talk presented in occasion of the VII School-Semina r on Heavy Ion Physics hosted by the Flerov Laboratory (FLNR/JINR) Dubna, Russia from May 27 to June 2, 200

Superposed epoch study of ICME sub-structures near Earth and their effects on galactic cosmic rays

Interplanetary coronal mass ejections (ICMEs) are the interplanetary manifestations of solar eruptions. The overtaken solar wind forms a sheath of compressed plasma at the front of ICMEs. Magnetic clouds (MCs) are a subset of ICMEs with specific properties (e.g. the presence of a flux rope). When ICMEs pass near Earth, ground observations indicate that the flux of galactic cosmic rays (GCRs) decreases. The main aims of this paper are to find: common plasma and magnetic properties of different ICME sub-structures, and which ICME properties affect the flux of GCRs near Earth. We use a superposed epoch method applied to a large set of ICMEs observed \insitu\ by the spacecraft ACE, between 1998 and 2006. We also apply a superposed epoch analysis on GCRs time series observed with the McMurdo neutron monitors. We find that slow MCs at 1 AU have on average more massive sheaths. We conclude that it is because they are more effectively slowed down by drag during their travel from the Sun. Slow MCs also have a more symmetric magnetic field and sheaths expanding similarly as their following MC, while in contrast, fast MCs have an asymmetric magnetic profile and a compressing sheath in compression. In all types of MCs, we find that the proton density and the temperature, as well as the magnetic fluctuations can diffuse within the front of the MC due to 3D reconnection. Finally, we derive a quantitative model which describes the decrease of cosmic rays as a function of the amount of magnetic fluctuations and field strength. The obtained typical profiles of sheath/MC/GCR properties corresponding to slow, mid, and fast ICMEs, can be used for forecasting/modelling these events, and to better understand the transport of energetic particles in ICMEs. They are also useful for improving future operative space weather activities.Comment: 13 pages, 6 figures, paper accepted in A&

Expansion of magnetic clouds in the outer heliosphere

A large amount of magnetized plasma is frequently ejected from the Sun as coronal mass ejections (CMEs). Some of these ejections are detected in the solar wind as magnetic clouds (MCs) that have flux rope signatures. Magnetic clouds are structures that typically expand in the inner heliosphere. We derive the expansion properties of MCs in the outer heliosphere from one to five astronomical units to compare them with those in the inner heliosphere. We analyze MCs observed by the Ulysses spacecraft using insitu magnetic field and plasma measurements. The MC boundaries are defined in the MC frame after defining the MC axis with a minimum variance method applied only to the flux rope structure. As in the inner heliosphere, a large fraction of the velocity profile within MCs is close to a linear function of time. This is indicative of} a self-similar expansion and a MC size that locally follows a power-law of the solar distance with an exponent called zeta. We derive the value of zeta from the insitu velocity data. We analyze separately the non-perturbed MCs (cases showing a linear velocity profile almost for the full event), and perturbed MCs (cases showing a strongly distorted velocity profile). We find that non-perturbed MCs expand with a similar non-dimensional expansion rate (zeta=1.05+-0.34), i.e. slightly faster than at the solar distance and in the inner heliosphere (zeta=0.91+-0.23). The subset of perturbed MCs expands, as in the inner heliosphere, at a significantly lower rate and with a larger dispersion (zeta=0.28+-0.52) as expected from the temporal evolution found in numerical simulations. This local measure of the expansion also agrees with the distribution with distance of MC size,mean magnetic field, and plasma parameters. The MCs interacting with a strong field region, e.g. another MC, have the most variable expansion rate (ranging from compression to over-expansion)

Interplanetary Magnetic Field Guiding Relativistic Particles

The origin and the propagation of relativistic solar particles (0.5 to few Ge V) in the interplanetary medium remains a debated topic. These relativistic particles, detected at the Earth by neutron monitors have been previously accelerated close to the Sun and are guided by the interplanetary magnetic field (IMF) lines, connecting the acceleration site and the Earth. Usually, the nominal Parker spiral is considered for ensuring the magnetic connection to the Earth. However, in most GLEs the IMF is highly disturbed, and the active regions associated to the GLEs are not always located close to the solar footprint of the nominal Parker spiral. A possible explanation is that relativistic particles are propagating in transient magnetic structures, such as Interplanetary Coronal Mass Ejections (ICMEs). In order to check this interpretation, we studied in detail the interplanetary medium where the particles propagate for 10 GLEs of the last solar cycle. Using the magnetic field and the plasma parameter measurements (ACE/MAG and ACE/SWEPAM), we found widely different IMF configurations. In an independent approach we develop and apply an improved method of the velocity dispersion analysis to energetic protons measured by SoHO/ERNE. We determined the effective path length and the solar release time of protons from these data and also combined them with the neutron monitor data. We found that in most of the GLEs, protons propagate in transient magnetic structures. Moreover, the comparison between the interplanetary magnetic structure and the interplanetary length suggest that the timing of particle arrival at Earth is dominantly determined by the type of IMF in which high energetic particles are propagating. Finally we find that these energetic protons are not significantly scattered during their transport to Earth

Effects of finite width of excited states on heavy-ion sub-barrier fusion reactions

We discuss the effects of coupling of the relative motion to nuclear collective excitations which have a finite lifetime on heavy-ion fusion reactions at energies near and below the Coulomb barrier. Both spreading and escape widths are explicitly taken into account in the exit doorway model. The coupled-channels equations are numerically solved to show that the finite resonance width always hinders fusion cross sections at subbarrier energies irrespective of the relative importance between the spreading and the escape widths. We also show that the structure of fusion barrier distribution is smeared due to the spreading of the strength of the doorway state.Comment: 13 pages, 3 figures, Submitted to Physical Review

Role of Higher Multipole Excitations in the Electromagnetic Dissociation of One Neutron Halo Nuclei

We investigate the role of higher multipole excitations in the electromagnetic dissociation of one-neutron halo nuclei within two different theoretical models -- a finite range distorted wave Born approximation and another in a more analytical method with a finite range potential. We also show, within a simple picture, how the presence of a weakly bound state affects the breakup cross section.Comment: 8 pages, 9 figure

Evaluación del atributo de calidad del software "Aprendizaje" empleando un método multicriterio

La motivación de este trabajo es proponer la adopción de un modelo para la evaluación del atributo de calidad ‘Aprendizaje’ en el marco de aplicaciones web, tanto simples como complejas, permitiendo conocer de antemano cuán fácil y eficientemente podrán los usuarios realizar sus tareas habituales empleando una aplicación como soporte. Para la creación del modelo propuesto, se siguen los pasos indicados por el método LSP (Logic Scoring of Preference), un método multicriterio que permite la creación de modelos cuantitativos para la evaluación, optimización, comparación y selección de sistemas complejos. LSP es un método de decisión multicriterio y multiatributo que respalda la elaboración de modelos que pueden dar importancia diversa a los distintos atributos considerados, en este caso elementos en un árbol de requisitos de adecuación al atributo de calidad ‘Aprendizaje’, permitiendo elegir qué elementos tendrán una importancia más destacada en función de las necesidades del proyecto considerado. Por otro lado, se propone seguir directrices de normas reconocidas, como la International Organization for Standardization (ISO), como referencia para la creación del modelo, por ejemplo, los lineamientos da-dos en la norma ISO 9241-110: Ergonomics of Human-system Intereaction, la cual sirve de guía a organizaciones y desarrolladores para, entre otras cosas, formular e implementar estrategias de desarrollo a lo que dan en llamar ‘Learnability’ del software. En particular, en el presente trabajo, estas normas han servido de guía para seleccionar características y sub-características que hacen al modelo de evaluación del atributo ‘Aprendizaje’. Estas características han si-do aumentadas con otras propias, enriqueciendo así, y en base a otras propuestas analizadas, los requisitos considerados para la evaluación.Sociedad Argentina de Informática e Investigación Operativ

Evaluación del atributo aprendizaje en aplicaciones web

En este trabajo, presentamos los objetivos, lineamientos generales y resultados esperados de una línea de investigación sobre la creación de modelos de evaluación de la característica “Aprendizaje” en aplicaciones web. Dicha línea de investigación forma parte integral del desarrollo de modelos de evaluación de sistemas complejos. Considerando que la evaluación de la estructura y metodología de implementación del Aprendizaje de un sistema de software, que debería incluir los puntos necesarios para que el sistema pueda ser fácilmente aprendido, implica una evaluación de un sistema complejo, es que esta investigación tiene como objetivo la creación, puesta a punto y aplicación de diversos modelos que permitan obtener indicadores del nivel alcanzado en la implementación de medidas que contribuyan a facilitar el aprendizaje de aplicaciones web. Comprobar el atributo Aprendizaje es fundamentalmente valioso para aplicaciones y sistemas complejos a los que los usuarios acceden con frecuencia, no obstante saber cuán rápido los usuarios pueden prosperar en el uso de su interfaz es valioso incluso para sistemas objetivamente simples. En esta primera etapa, se aborda el estudio y la aplicación de la metodología para el desarrollo de dichos modelos de evaluación siguiendo los lineamientos del método Logic Score of Preference (LSP). Asimismo, se toma como referencia para la creación del modelo, estándares reconocidos como los de la International Organization for Standardization (ISO). Así, de acuerdo con la ISO/IEC 25010, el atributo Aprendizaje es uno de los seis componentes de calidad de la Usabilidad (o Capacidad de Uso); otros son la Inteligibilidad (Appropriateness Recognizability), Operatividad (Operability), Protección Frente a Errores de Usuarios (User Error Protection), Estética de la Interfaz de Usuario (User Interface Aesthetics) y la Accesibilidad (Accessibility), como puede observarse en la figura 1, en donde se muestran todas las dimensiones de la calidad del software de acuerdo a dicha norma. También se siguen los lineamientos dados en la norma ISO 9241-110: Ergonomics of Human-system Intereaction y la parte 210: Human-centred design for interactive systems de la ISO 9241-210. Ambas sirven de guía a las organizaciones para, entre otras cosas, formular e implementar estrategias de desarrollo a lo que dan en llamar ‘Learnability’ del software. Estos atributos pueden ser aumentados con desarrollos propios, enriqueciendo los requisitos considerados en la evaluación del Aprendizaje. Asimismo continuamos con el estudio de nuevas características a incluir en esta investigación con el fin de ajustar el modelo adecuadamente.Eje: Ingeniería de Software.Red de Universidades con Carreras en Informátic

On the Excitation of Double Giant Resonances in Heavy Ion Reactions

The interplay of nuclear and Coulomb processes in the inelastic excitation of single- and double-phonon giant resonances in heavy ion collisions is studied within a simple reaction model. Predominance of the Coulomb excitation mechanism on the population of the single-phonon and, on the contrary, predominance of the nuclear excitation for the double-phonon is evidenced. The effect of the spreading of the strength distribution of the giant resonances on the excitation process is analyzed, showing sizeable modifications in the case of Coulomb dominated processes.Comment: Accepten in Nuclear Physics A. 10 eps figures and source file in an uncompressed tar packag