Positron kinetics in an idealized PET environment

The kinetic theory of non-relativistic positrons in an idealized positron emission tomography PET environment is developed by solving the Boltzmann equation, allowing for coherent and incoherent elastic, inelastic, ionizing and annihilating collisions through positronium formation. An analytic expression is obtained for the positronium formation rate, as a function of distance from a spherical source, in terms of the solutions of the general kinetic eigenvalue problem. Numerical estimates of the positron range - a fundamental limitation on the accuracy of PET, are given for positrons in a model of liquid water, a surrogate for human tissue. Comparisons are made with the 'gas-phase' assumption used in current models in which coherent scattering is suppressed. Our results show that this assumption leads to an error of the order of a factor of approximately 2, emphasizing the need to accurately account for the structure of the medium in PET simulations

Influence of the dynamical image potential on the rainbows in ion channeling through short carbon nanotubes

We investigate the influence of the dynamic polarization of the carbon valence electrons on the angular distributions of protons channeled through short (11,9) single-wall carbon nanotubes at speeds of 3 and 5 a.u. (corresponding to the proton energies of 0.223 and 0.621 MeV), with the nanotube length varied from 0.1 to 0.3 mu m. The dynamic image force on protons is calculated by means of a two-dimensional hydrodynamic model for the nanotubes dielectric response, whereas the repulsive interaction with the nanotubes cylindrical wall is modeled by a continuum potential based on the Doyle-Turner interatomic potential. The angular distributions of channeled protons are generated by a computer simulation method using the numerical solution of the proton equations of motion in the transverse plane. Our analysis shows that the inclusion of the image interaction causes qualitative changes in the proton deflection function, giving rise to a number of rainbow maxima in the corresponding angular distribution. We propose that observations of those rainbow maxima could be used to deduce detailed information on the relevant interaction potentials, and consequently to probe the electron distribution inside carbon nanotubes

Spectroscopic and quantum mechanical investigation of N,N '-bisarylmalonamides: solvent and structural effects

The UV absorption spectra of ten N,N'-bisarylmalonamides have been recorded in the range 200-400 nm in a set of selected solvents. The solute-solvent interactions have been analyzed on the basis of the linear solvation energy relationship (LSER) concept proposed by Kamlet and Taft. The effects of substituents on the absorption spectra have been interpreted by correlating absorption frequencies with Hammett substituent constants. Furthermore, the experimental findings have been interpreted using the DFT CAM-B3LYP/6-311+G(d,p) method. Electronic energies have been calculated using the same method in combination with the implicit solvation model (conductor-like polarizable continuum model, CPCM) as well as with the explicit addition of two molecules of solvent