|Orthopositronium annihilation and emission in mesostructured thin silica and silicalite-I films|
|L. Liszkay, , C. Corbel, M. Etienne, V. Valtchev, P. Desgardin, M.-F. Barthe, T. Ohdaira, R. Suzuki, P. Crivelli,g, A. Walcarius, P Perez|
Mesoporous silica layers and silicalite-I pure silica zeolite films were investigated using slow positrons. Detection of the 3γ annihilation fraction was used as a quick test to estimate the emission of orthopositronium (oPs) into vacuum. Positronium time-of-flight (TOF) spectrocopy, combined with Monte-Carlo simulation of the detection system was used to determine the energy of oPs emitted from the layers. Evidence for an efficient oPs emission was found in both the silicalite-I layer and in mesoporous films. A 3γ fraction in the range of 31-36 % was found in the layers with the highest oPs yield in each type of porous material, indicating that 40-50 % of the implanted positrons form positronium in the pore systems with very different pore sizes. Time-of-flight measurements showed that the energy of the orthopositronium emitted into vacuum is below 100 meV in the film with approximately 2 nm pores at 3 keV positron energy, indicating an efficient slowing down but no complete thermalization in the porous layer.