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

Ab initio molecular dynamics of liquid water using embedded-fragment second-order many-body perturbation theory towards its accurate property prediction

A direct, simultaneous calculation of properties of a liquid using an ab initio electron-correlated theory has long been unthinkable. Here we present structural, dynamical, and response properties of liquid water calculated by ab initio molecular dynamics using the embedded-fragment spin-component-scaled second-order many-body perturbation method with the aug-cc-pVDZ basis set. This level of theory is chosen as it accurately and inexpensively reproduces the water dimer potential energy surface from the coupled-cluster singles, doubles, and noniterative triples with the augcc-pVQZ basis set, which is nearly exact. The calculated radial distribution function, self-diffusion coefficient, coordinate number, and dipole moment, as well as the infrared and Raman spectra are in excellent agreement with experimental results. The shapes and widths of the OH stretching bands in the infrared and Raman spectra and their isotropic-anisotropic Raman noncoincidence, which reflect the diverse local hydrogen-bond environment, are also reproduced computationally. The simulation also reveals intriguing dynamic features of the environment, which are difficult to probe experimentally, such as a surprisingly large fluctuation in the coordination number and the detailed mechanism by which the hydrogen donating water molecules move across the first and second shells, thereby causing this fluctuationopen

Swimming clusters in thallium-rich liquid caesium-thallium alloys

The purpose of the work presented here is to obtain structural information on thallium-rich caesium-thallium alloys by means of neutron diffraction. The alloys exhibit a long-range (>1 nm) superstructure. This range increases with the thallium content. The results are interpreted with the help of a 'swimming cluster' model in which units CsmTln are dissolved in liquid thallium. (C) 1999 Elsevier Science B.V. All rights reserved

Large clusters in liquid potassium-thallium alloys

Neutron diffraction measurements on liquid KTl were performed using Tl-205 isotope substitution. The measurement gives evidence for the occurrence of Tl-clusters with an average mutual distance of 0.96 nm. The structure in liquid KTl is compared with the crystal structures of KTl and K8Tl11, where resp. Tl-6 and Tl-11 clusters occur. To extend the search for analogies in the structure of liquid and solid alloys, we also measured liquid K8Tl11, K8Tl10Zn, K10Tl10Pd, and K18Tl20Au3, they all exhibit interesting crystal structures with large clusters (Tl-11, Tl10Zn, Tl10Pd and Tl9Au2). Indeed, some similarities are found between the structures in these liquid alloys and their solid counterparts