[en] An explicity expression for the effective critical exponent of the susceptibility is derived according to a method which is alternative to the renormalization group approach and known as the skeleton graph expansion. The results are significantly different from those obtained from the nonlinear renormalization group equations. A check of the model with the experimental data for the CrBr3 susceptibility is satisfactory

[en] It is shown that a simple application of the Feynman-Kac formula makes it possible to derive two inequivalent representations of the generating function of the Malthus-Verhulst model as an average over stochastic processes. One representation is the known Poisson one, while the other is new and is denoted as the generalized branching representation. The formalism to obtain closed expression for the generating function is exploited. The results are compared with computer implementations of the model. Generalizations to different models are hinted

[en] In this work the CATHARE V1.3 code was applied to two Numerical Benchmark Exercises proposed in the aim to test the behaviour of thermalhydraulic codes against some spurious numerical effects. In the analysis of the first exercise, which is related to a two-phase flow along a convergent-divergent nozzle, it was seen that the CATHARE code well predicts the expected physical trends in good agreement with the majority of other codes. The application of the code to the second exercise, concerning the water packing phenomenon, showed the presence of this numerical effect. From a sensitivity analysis on time step and mesh size, it was seen that the amplitude of the pressure spikes can be reduced increasing time step and decreasing the space increments

[en] Density and pairing responses are studied for the Holstein molecular crystal model. The charge density wave (CDW) and the superconductivity (SC) instabilities of the normal phase are investigated. Analytical results are obtained for small electron-phonon coupling in a simple cubic lattice in the limit of large dimensionality (d = ∞). Near half-filling the dominant instability is a CDW, due to nesting at q = (π, π, ..). For larger deviations from half-filling the CDW becomes incommensurate in a narrow filling range and finally disappears. Then the SC instability dominates. (orig.)

[en] We show how to characterize the temporal evolution of a single qubit interacting with a thermal bath by correlation functions of the bath under quite general hypotheses. The aim is to show how the qubit decoherence time depends on the degree of correlation of the bath. We explicitly study some spin-bath models where every spin of the environment interacts with the qubit either directly or by the mediation of a harmonic oscillator. In both cases we find an increase of the decoherence time as soon as a symmetry breaking occurs in the environment. In the latter case a possible experimental setup is discussed

[en] Ferric ion diffusion is one of the main problems that still restrains the dosimetric application of Fricke-agarose gels. In this work, we model this process within finite length gel samples. The temporal evolution of the ion concentration as a function of the initial concentration is derived by solving Fick's second law in two dimensions with boundary reflections. The influence of ion concentration gradient, elapsed time, diffusion coefficient and spatial resolution is studied. Due to the main drawbacks of MRI for studying these systems, i.e. high cost and acquisition time often non-negligible compared to diffusion time, we also investigate the possibility of using a microdensitometer. The application of this technique for Fricke gel dosimetry is proposed here for the first time. The estimate of the ion diffusion coefficient is in a very agreement with those reported in literature

No abstract available

[en] In Fricke-agarose gels, an accurate determination of the spatial dose distribution is hindered by the diffusion of ferric ions. In this work, a model was developed to describe the diffusion process within gel samples of finite length and, thus, permit the reconstruction of the initial spatial distribution of the ferric ions. The temporal evolution of the ion concentration as a function of the initial concentration is derived by solving Fick's second law of diffusion in two dimensions with boundary reflections. The model was applied to magnetic resonance imaging data acquired at high spatial resolution (0.3 mm) and was found to describe accurately the observed diffusion effects. (authors)

[en] Ferrous-sulphate infused gels, or 'Fricke gels', encounter great interest in the field of radiation dosimetry, due to their potential for 3D radiation dose mapping. Typically, magnetic resonance (MR) relaxation rates are determined in these systems in order to derive the absorbed dose. However, when large concentration gradients are present, diffusion effects before and during the MR imaging may not be negligible. In these cases, optical techniques may represent a viable alternative. This paper describes research aimed at measuring 3D dose distributions in a Fricke-xylenol orange gel by measuring optical density with a CCD camera. This method is inexpensive and fast. A series of early experiments is described, in which optical density profiles were measured with a commercial microdensitometer for film dosimetry. The light box of the device was modified to work at 567 nm, close to the maximum absorbance of the ferric ion-xylenol orange complex. Under these conditions, the gel shows linearity with dose and high sensitivity. (authors)

[en] Cygnus X-1 (Cyg X-1) is the archetypal black hole binary system in our Galaxy. We report the main results of an extensive search for transient gamma-ray emission from Cygnus X-1 carried out in the energy range 100 MeV-3 GeV by the AGILE satellite, during the period 2007 July-2009 October. The total exposure time is about 300 days, during which the source was in the 'hard' X-ray spectral state. We divided the observing intervals in 2-4 week periods, and searched for transient and persistent emission. We report an episode of significant transient gamma-ray emission detected on 2009 October 16 in a position compatible with Cyg X-1 optical position. This episode, which occurred during a hard spectral state of Cyg X-1, shows that a 1-2 day time variable emission above 100 MeV can be produced during hard spectral states, having important theoretical implications for current Comptonization models for Cyg X-1 and other microquasars. Except for this one short timescale episode, no significant gamma-ray emission was detected by AGILE. By integrating all available data, we obtain a 2σ upper limit for the total integrated flux of F _γ,U.L. = 3 x 10^-8 ph cm^-2 s^-1 in the energy range 100 MeV-3 GeV. We then clearly establish the existence of a spectral cutoff in the energy range 1-100 MeV that applies to the typical hard state outside the flaring period and that confirms the historically known spectral cutoff above 1 MeV.