Mean free path

About: Mean free path is a research topic. Over the lifetime, 4412 publications have been published within this topic receiving 114418 citations.

Dependency of Energy-, Position- and Depth of Interaction Resolution on Scintillation Crystal Coating and Geometry

Abstract: Options for optimizing the energy and spatial resolution of gamma-ray imaging detectors based on thick, monolithic crystals shaped like flat-topped pyramids were studied. Monte Carlo simulations were made of the scintillation light transport for evaluating the effect of four parameters on the energy resolution, the spatial resolutions, and the depth of interaction (DOI) resolution of the gamma-ray imaging detector. These four parameters are: the reflectivity of the surface coating; the scatter mean free path; the absorption mean free path of the scintillation light; and the angle that defines the inclination of the sides of the pyramidal frustum. In real detectors, the values for the mean free paths for optical photons are normally not known. We estimated these by comparing MC simulations of detector resolutions to measurements for three gamma-ray imaging detectors with LYSO and LSO from different suppliers and with different surface coatings and geometries. The gamma-ray imaging detector measures the energy, centroids, and depth of interaction of the gamma-ray. DOI enhanced charge dividing readouts were used to measure the depth of interaction.

An investigation into the structural features and thermal conductivity of silicon nanoparticles using molecular dynamics simulations

Abstract: Th es tructural features and thermal conductivity of silicon nanoparticles of diameter 2–12 nm are studied in a series of molecular dynamics simulations based on the Stilling–Weber (SW) potential model. The results show that the cohesive energy of the particles increases monotonically with an increasing particle size and is independent of the temperature. It is found that particles with a diameter of 2 nm have a heavily reconstructed geometry which generates lattice imperfections. The thermal conductivity of the nanoscale silicon particles increases linearly with their diameter and is two orders of magnitude lower than that of bulk silicon. The low thermal conductivity of the smallest nanoparticles is thought to be the result of particle boundary and lattice imperfections produced during fabrication, which reduce the phonon mean free path (MFP). Finally, it is found that the influence of the temperature on the thermal conductivity decreases significantly as the temperature increases. Again, this is thought to be the result of a reduced phonon MFP at elevated temperatures. (Some figures in this article are in colour only in the electronic version)

A study of the propagation of solar energetic protons in the inner heliosphere

Abstract: Eight solar energetic proton events observed in the inner heliosphere on the spacecraft Helios 1 or 2 are studied. Particle pitch angle diffusion coefficients and mean free paths are derived from a power spectral analysis of the turbulence of the magnetic field during the time of the events. Quasi-linear theory of wave-particle interaction is applied strictly throughout in combination with the slab model of fluctuations. Several points receive special attention: (1) the large difference in scattering conditions from one event to the other, which requires an individual analysis of each event; (2) the selection of the exact stretch of magnetic field data to be analyzed and the influence of this choice on the result; (3) the exclusion of data periods that contain discontinuities and other nonoscillatory structures; and (4) short-scale variations of the overall field fluctuation level, which can be considerable. As a consequence, the mean free paths of the particles are derived from time series of the varying momentary power density spectra as an appropriate average of the scattering conditions (as an average mean free path). These results from the magnetic field spectra are then compared with results of fits of propagation models to the particle time-intensity and time-anisotropy profiles observed at the same time. Of eight such comparisons, half of the cases exhibit for the first time an approximate agreement between the results of the two approaches, particle analysis and magnetic field analysis. The others, however, still show the discrepancy that is cited in the literature. Among the events with good agreement are the two extreme cases in the whole Helios data set, the event of April 11, 1978, Helios 2, with a mean free path from quasi-linear theory of 0.01 AU, and the events of June 7, 1980, Helios 1, with a mean free path from quasi-linear theory of about 0.8 AU at a rigidity of 0.1 GV. This shows not only that quasi-linear theory in a wave and slab model framework is indeed capable of producing correct results under certain conditions if an appropriate case-by-case analysis is conducted, but also that these cases can encompass a wide range of turbulence strengths. Several possibilities for the source of the discrepancy that remains for the other cases, which the theory and the assumptions applied in this work obviously do not explain, are discussed. However, at the present time no definite solution to the problem is obvious.

Thermodynamic variational method for liquid alloys with chemical short-range order.

Abstract: We present a first-principles study of chemical short-range ordering in liquid (s,p)-bonded alloys. Our approach is based on an optimized pseudopotential technique for the construction of the interatomic potentials and a thermodynamic variational technique based on the Gibbs-Bogoliubov inequality and hard-sphere Yukawa reference potentials (we use the analytical solution of the mean spherical solution for the equal-diameter case). The analysis of the redistribution of the valence electrons upon alloying allows us to elucidate the electronic origin of the ordering potential. In the case of a moderately strong ordering interaction, the application of the Gibbs-Bogoliubov variational technique yields a reasonably accurate prediction of the structure factors and of the thermodynamic excess functions. For very strong ordering potentials, a free minimization of the variational upper bound to the exact free energy gives unrealistic results. This is a consequence of the complete decoupling of number-density and concentration fluctuations in the mean-spherical approximation to the equal-diameter hard-sphere Yukawa mixture. We find that realistic solutions may be found by imposing the condition that the exact and the reference-system ordering potentials be the same at the mean effective atomic diameter. This constrained minimization of the variational free energy yields good results for the structure factors, but rather bad ones for the thermodynamic excess functions. We are able to show that this is due to a neglect of the finite electronic mean free path of the electrons in those concentration regions where it is comparable to the mean interatomic distance.