Improvement of the Pulsed Low Energy Positron System (PLEPS) for complex problems in materials science

This thesis concerns the application and the improvement of the Pulsed Low Energy Positron System (PLEPS) at the high intensity positron source NEPOMUC at the Munich research reactor FRM-II. This system is used for the defect study in complex materials. Positrons are the ideal probe for non-destructive investigations of vacancy-like defects in matter. The combination of positron lifetime spectroscopy with a pulsed, monochromatic positron beam of variable energy conveys information on the type and the concentration of defects down to the sub-ppm range and their depth-profile with nm resolution. Defect structures in two materials were investigated with PLEPS for this thesis. First, we studied strontium titanate (STO), which is a material of great relevance in modern oxide electronics. The cation vacancies (strontium and titanium vacancies, VSr and VTi, respectively) were identified in STO films deposited by Pulsed Laser Deposition (PLD). It was also shown, that in commercially available STO substrates only titanium vacancies with a concentration of (1.26±0.16) ppm could be detected and that upon annealing in the same conditions as for the PLD procedure a 400 nm thick layer of titanium-oxygen divacancies VTi-O was introduced. The second investigation was performed in permanently densified silica glasses. In combination with XRD measurements the structure evolution upon densification was analyzed. In particular, it was demonstrated that the average inter-tetrahedral void radius measured with PLEPS permits to predict the shift of the first sharp diffraction peak of the static structure factor as a function of the density. In the second part of this work, from the experience gained with PLEPS in the course of this thesis, the limits of the apparatus were analyzed, measures to improve the quality of the positron lifetime spectra measured with PLEPS were identified and tested. Comprehensive simulations were performed to understand the structures in the background of the measured lifetime spectra and possible countermeasures were found. Modifications of the pulsing system allowed to a) improve the time resolution of PLEPS to about 250 ps, b) measure precisely positron lifetime longer than 3 ns, which enhances the capabibility of PLEPS for the determination of free volumes in polymer samples and membranes and c) get rid of disturbing structures in the background of the positron lifetime spectra. Thus, PLEPS in combination with the high intensity positron source NEPOMUC can be considered as the most productive pulsed positron beam for defect depth-profiling in materials currently available world-wide

Non-rigid image registration to reduce beam-induced blurring of cryo-electron microscopy images

Microscopic imaging and technolog

Correction: Alkoxy radicals generation: facile photocatalytic reduction of N-alkoxyazinium or azolium salts

Correction for 'Alkoxy radicals generation: facile photocatalytic reduction of N-alkoxyazinium or azolium salts' by Luca Capaldo et al., Chem. Commun., 2019, DOI: 10.1039/c9cc00035f

Environmental risk factors of airborne viral transmission: Humidity, Influenza and SARS-CoV-2 in the Netherlands

Objective: The relationship between specific humidity and influenza/SARS-CoV-2 in the Netherlands is evaluated over time and at regional level. Design: Parametric and non-parametric correlation coefficients are calculated to quantify the relationship between humidity and influenza, using five years of weekly data. Bayesian spatio-temporal models—with a Poisson and a Gaussian likelihood—are estimated to find the relationship between regional humidity and the daily cases of SARS-CoV-2 in the municipalities and provinces of the Netherlands. Results: An inverse (negative) relationship is observed between specific humidity and the incidence of influenza between 2015 and 2019. The space-time analysis indicates that an increase of specific humidity of one gram of water vapor per kilogram of air (1 g/kg) is related to a reduction of approximately 5% in the risk of COVID-19 infections. Conclusions: The increase in humidity during the outbreak of the SARS-CoV-2 in the Netherlands may have helped to reduce the risk of regional COVID-19 infections. Policies that lead to an increase in house- hold specific humidity to over 6g/Kg will help reduce the spread of respiratory viruses such as influenza and SARS-CoV-2.publishedVersio

Cancer and in general long-term ilnesses at workplaces

Studio commissionato dal Comitato Occupazione e Affari sociali del Parlamento europe