Direct measurement of the lifetime of the neutral pion

The lifetime of the neutral pion (π⁰) has been directly measured. The lifetime has been found to be τ_π⁰ = (0.897±0.022±0.017) × 10^?16 s. The systematic error results from uncertainties in the momentum spectrum of the π⁰ used for the measurement. This measurement yields a decay width of the π⁰ of (7.34±0.18±0.11) eV. An axial anomaly calculation, which neglects the finite mass of the π⁰, predicts a value of 7.87 eV.     

Su un tipo di applicazioni fra varietà differenziabili con atlante omografico

Si caratterizzano dal punto di vista geometrico due tipi di trasformazioni fra varietà differenziabili con atlante omografico e se ne ricavano le equazioni.     

Chaotic phenomena of charged particles in crystal lattices

In this article, we have applied the methods of chaos theory to channeling phenomena of positive charged particles in crystal lattices. In particular, we studied the transition between two ordered types of motion; i.e., motion parallel to a crystal axis (axial channeling) and to a crystal plane (planar channeling), respectively. The transition between these two regimes turns out to occur through an angular range in which the particle motion is highly disordered and the region of phase space spanned by the particle is much larger than the one swept in the two ordered motions. We have evaluated the maximum Lyapunov exponent with the method put forward by Rosenstein et al. [Physica D 65, 117 (1993)] and by Kantz [Phys. Lett. A 185, 77 (1994)]. Moreover, we estimated the correlation dimension by using the Grassberger-Procaccia method. We found that at the transition the system exhibits a very complex behavior showing an exponential divergence of the trajectories corresponding to a positive Lyapunov exponent and a noninteger value of the correlation dimension. These results turn out to be linked to a physical interpretation. The Lyapunov exponents are in agreement with the model by Akhiezer et al. [Phys. Rep. 203, 289 (1991)], based on the equivalence between the ion motion along the crystal plane described as a "string of strings" and the "kicked" rotator. The nonintegral value of the correlation dimension can be explained by the nonconservation of transverse energy at the transition.     

Low energy ion-beam post hydrogenation of phosphor implanted amorphous silicon films

Glow-discharge-deposited intrinsic hydrogenated amorphous silicon films have been doped by P⁺ implantation in varying doses between 10¹⁶ and 2 × 10²¹ atoms/cm³ and annealed at 260°C. Subsequent hydrogenation of the samples produces a decrease in conductivity explained by a hydrogen-induced decrease of the electrically active fraction of the dopant.     

A comprehensive computer program for ion penetration in solids

Computer codes previously set up to investigate charged particles interaction in matter have been matched together to obtain a comprehensive program to simulate penetration phenomena occurring both in amorphous and along the most important crystal directions in silicon.

The model can be used to simulate implantation processes in silicon crystals coated with an amorphous or polycrystalline layer. The model is based on the binary collision approximation, and includes thermal vibrations and impact parameter dependence of the stopping power for the crystal ordered case. A mechanism to take into account the progressive increase of disorder in the lattice as a consequence of ion implantation is available.

Calculations were performed for a large variety of physical conditions, the most significant of which are reported to display the potentialities of the model.     

Analysis of light elements in superposed layers by Monte Carlo simulation of EELS spectra

A Monte Carlo code, previously set up to simulate electron energy loss spectra of carbon films on silicon at 100 kV, has been extended to the analysis, at 300 kV, of a Si/SiO₂/Si structure; the final goal is the determination of the oxygen concentration in SiO_x precipitates embedded in a Si matrix. The upgrading of the programme has required the introduction of relativistic kinematics and relativistic corrections to elastic and inelastic cross sections.The Si/SiO₂/Si samples have been prepared by CVD deposition of a 16 nm thick silicon film onto a silicon wafer covered with a 11 nm thick thermal oxide. The thickness of both films has been checked by transmission electron microscopy on cross sections. The EELS experiments have been performed on planar sections, in regions of different thickness; the EELS spectra have been acquired with a parallel 666 Gatan spectrometer, fitted to a Philips CM 30 TEM/STEM, operating at 300 kV. The stoichiometry of the SiO_x can be obtained by the ratioing of the areas under the OK and SiK edges, taking advantage of the possibility given by the Monte Carlo simulation to separate the background electrons from the one suffering the characteristic energy loss. The agreement between experiments and calculations in the case examined is satisfactory, so that the application of this procedure to SiO_x precipitates is promising.