Papers by Author: Ulisse Gendotti

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.

Abstract: Positron annihilation gamma energy distribution, lifetime spectroscopy and time-of-flight method were used to study surfactant-templated mesoporous silica films deposited on glass. The lifetime depth profiling was correlated to Doppler broadening and 3γ annihilation fraction measurements to determine the annihilation characteristics inside the films. A set of consistent fingerprints for positronium annihilation, o-Ps reemission into vacuum, and pore size was directly determined. The lifetime measurements were performed in reflection mode with a specially designed lifetime spectrometer mounted on a slow positron beam system. The intensity of the 142 ns vacuum lifetime component was recorded as a function of the energy of the positron beam. In a film with high porosity a reemission efficiency of as high as 40 % was found at low positron energy. Positron lifetime in samples capped by a thin silica layer was used to determine the pore size. The energy of the reemitted o-Ps fraction was measured by a time-of-flight detector, mounted on the same system, allowing determination of both o-Ps re-emission efficiency and energy in the same sample. We demonstrate the potential of the simultaneous use of different positron annihilation techniques in the study of thin porous films.