Papers by Keyword: Ortho-Positronium

Abstract: The positron annihilation lifetime spectroscopy (PALS) was applied to investigate the properties of capsules composed of n-alkanes (filling material) and polymer (shell) in the broad range of pressures up to 450 MPa. These microcapsules aggregate into the grains having about 200 μm in diameter. Their properties were investigated as a function of pressure (p) at several selected temperatures: when the filling material is in liquid, rotator and solid phase. Pressure experiments were performed without gas access to the sample and in an argon atmosphere. Two o-Ps components were found, the longer-lived correspond to the filler material, and the shorter-lived one – to the shell. These components change with p; even a small pressure (6 MPa) reduces considerably the o-Ps lifetimes (τ). At 303 K the o-Ps lifetime in the core changes non-monotonically, and at 60 MPa τ is higher than at 20 MPa. The increase of pressure induces the phase changes in the filling material, and also produces the deformation of microcapsule aggregates and crash of small capsules at the grain boundary region. Internal structure of the microcapsules was observed by SEM.

Abstract: In this paper, we present the Monte-Carlo simulation that we developed and used to design and to interpret the data of a new kind of lifetime spectrometer especially constructed for the determination of the ortho-positronium (o-Ps) re-emission yield from thin mesoporous silica films. The main issue was to have the detection efficiency independent of the o-Ps re-emission energy. This was done in order to avoid a bias of the results related to the positron implantation energy. Furthermore, the simulation is used to calculate the detection efficiencies for annihilation events yielding 2 and 3 gamma photons allowing a precise evaluation of the absolute yield of o-Ps produced in the samples. Calculations were performed using the GEANT4 package. New classes were written to describe the different decay types related to the Ps formation in matter. Moreover, the propagation and interaction of the o-Ps reemitted in a vacuum cavity were also included in the simulation.