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7 pags., 3 figs., 4 tabs. ; We present a range of cross section measurements for the low-energy scattering of positrons from pyridine, for incident positron energies of less than 20 eV, as well as the independent atom model with the screening corrected additivity rule including interference effects calculation, of positron scattering from pyridine, with dipole rotational excitations accounted for using the Born approximation. Comparisons are made between the experimental measurements and theoretical calculations. For the positronium formation cross section, we also compare with results from a recent empirical model. In general, quite good agreement is seen between the calculations and measurements although some discrepancies remain which may require further investigation. It is hoped that the present study will stimulate development of ab initio level theoretical methods to be applied to this important scattering system. ; The authors would like to acknowledge the Australian Research Council (ARC) Discovery Programmes for funding support (No. DP140102854) and Ross Tranter for technical support for the experimental apparatus. L.E.G., F.B., and G.G. also acknowledge partial financial support from the Spanish Ministry MINECO (No. FIS2016- 80440) and the European Union ITN-Marie Curie programme (No. ARGENT-608163). ; Peer Reviewed

"The models CPE and CPE4, which were successfully used in positron impact ionization studies, are applied to positron impact ionization of Li, Na and K. This work produces total cross sections which are in agreement with the existing theoretical papers and shows the necessity for experimental measurements of these processes. Keywords: Positron collisions; Ionization of Atoms "

This worked was support by the Portuguese Foundation for Science and Technology through the project POCTI/FAT/40924/2001 co-financed by European Union fund FEDER. ; In this work the depth of interfaces in multilayered structures was estimated. The fractions of positron annihilation as function of the implantation energy were estimated from an S-W plot and then converted into a function of the sample depth through the positron implantation profile in the multilayer system computed from a reduced positron profile. The results of this method in Ti/Al samples are comparable to those using the common analysis based on positron diffusion equations. The positron analyses results were compared with SIMS profiles for the same samples. ; authorsversion ; published

Proceeding of: 2010 IEEE Nuclear Science Symposium, Medical Imaging Conference and 17th Room Temperature Semiconductor Detector Workshop (IEEE), Knoxville, Tennessee, USA, October 30 - November 6, 2010 ; Positron range depends on the materials in which positron propagates and on positron emitter isotope. As positron range limits the spatial resolution of PET images, good quantitative estimates of it should be included in any realistic simulation of PET acquisitions. In this work we compare positron range estimates obtained with PeneloPET to previous available simulations and experimental data. PeneloPET was used to simulate the positron range of 18F, 11C, 13N, 15O, 68Ga and 82Rb in the following tissues: cortical bone, soft bone, skin, muscle, brain, water, adipose tissue and lung. The 3D and 1D annihilation Point Spread Functions (aPSF) were calculated for each isotope-material combination. We have studied with more detail the 3D aPSF (radial distributions) and the cumulative fraction of annihilation events. These aPSF distributions were also studied for non-uniform media. Results obtained were consistent with other results previously reported in the literature as well as with experimentally measured data. ; This work has been supported by MEC (FPA2007-62216), UCM (Grupos UCM, 910059), CPAN (Consolider- Ingenio 2010) CSPD-2007-00042, European Regional Development and ENTEPRASE grant, PSE-300000-2009-5, and the CENIT program project AMIT, Ministerio de Ciencia e Innovación, Spanish Government. Part of the calculations of this work were performed in the "Clúster de Cálculo de Alta Capacidad para Técnicas Físicas" funded in part by UCM and in part by UE under FEDER programme". ; Publicado

The impact of the interfacial contribution on overall properties increases with decreasing grain size of polycrystalline materials. It is well known that distribution and size of cluster defects are rather different in bulk than grain boundaries. In light of "bottom-up" approaches, a study at the atomic level determining the distribution of crystallographic defects could clarify their contribution to the macroscopic properties, and then differentiate materials for outstanding or precise applications. In this work, Positron Annihilation Spectroscopy (PAS) is used to characterize the distribution of defects within 8 mol% Y₂O₃-stabilized zirconia (8YSZ) structures prepared by sintering through three different thermal treatments, i.e. a conventional thermal cycle in air and N₂/H₂ atmosphere, and a fast firing cycle in air, which lead to average grain sizes < 260 nm. ; The authors would like to acknowledge the financial support from the Spanish Government through the projects MAT2009-14448-C02-01 and IPT-310000-2010-12. ; Publicado

A criação de pares elétron-pósitron é um dos vários mecanismos de interação do fóton com a matéria e o par pode ser produzido quando o fóton interage com um núcleo atômico ou com um elétron. As energias limiares do fóton kth para estas reações são respectivamente ~ 2m0c2 ou 4m0c2 (m0 é a massa de repouso do elétron e c é a velocidade da luz no vácuo). Um aspecto interessante da criação de par elétron-pósitron ocorre quando um fóton de energia muito baixa (k << m0c2) colide com um elétron ultra-relativístico (E >> m0c2). Nesta nota a cinemática da colisão entre um elétron e um fóton é revista e o aspecto particular e interessante da produção de tripleto quando elétrons ultra-relativísticos colidem com fótons de baixíssimas energias é brevemente descrito.

On montre que le temps de thermalisation d'un positron dans le plasma électronique d'un métal, calculé en prenant le potentiel effectif de la théorie de la réponse linéaire pour décrire l'interaction électron-positron, est nettement plus grand que celui calculé dans une approximation statique par Lee-Whiting.
Pour l'aluminium, le processus de thermalisation est à peine trois fois plus rapide que le processus d'annihilation. Il semble donc bien que l'on ne puisse considérer, comme c'est généralement admis, que, dans les métaux, le positron est toujours thermalisé avant son annihilation.

Microstructural characterization of stress states of alumina layers subjected to residual compressive stresses was performed by positron annihilation spectroscopy (PAS). Al₂O₃-t-ZrO₂ three layered samples were appropriately designed and processed by sequential slip casting with symmetric structure. The zirconia contents inside the core layer were selected in order to generate compressive stresses of variable intensity in the outer Al₂O₃ layers. PAS results highlighted a correlation between positron annihilation parameters and microstructural changes related to the generation of residual stresses. ; This work was supported by Spanish Government under Contract MAT2009-14448-C02-01 and IPT-310000-2010-012. ; Publicado

Positron annihilation spectroscopy (PAS), indentation, nanoindentation experiments and scanning electron microscopy (SEM) observations were performed on Al₂O₃-ZrO₂ laminates samples to assess the effect of residual stresses on their mechanical and microstructural properties. Layered samples were implemented by slip-casting, constituted by two thin Al₂O₃ external layers and an intermediate thick one, consisting of a mixture of Al₂O₃ and monoclinic ZrO₂ in the range 0-30 vol.%. In these systems residual tensile stresses fields were generated inside the external layers during cooling from the sintering temperature, by the expansion of the adjacent ZrO₂-containing layer. SEM observations showed the microstructural effects due to the level of tension related to the zirconia content. A correlation between the PAS parameters and the microstructural changes caused by the presence of residual stresses was found. Nanoindentation measurements were used to trace the sign and magnitude of the residual stress gradient across the interface between the layers. ; This work was supported by Spanish Government under Contract MAT2006-01038. The authors gratefully acknowledge the financial support from the Comunidad de Madrid and the Ministry of Education and Science of Spain, through the ESTRUMAT-CM (MAT/77) programs. ; Publicado

Pulmonary administration of drugs has recently gained attention because it exhibits numerous advantages compared to oral or intravenous administration. The administration of aerosols for inhalation to animals, however, remains a critical challenge and only a few methods of administration have been developed. Herein, we compare the regional distribution of aerosols in the lungs of wild type rats after pulmonary administration using three different methods: (a) The Penn-Century MicroSprayer® Aerosolizer; (b) an in-house designed aerosol generation system; and (c) the Aeroneb® Lab Micropump Nebulizer. Both the regional distribution and the fraction of aerosol deposited in the lungs were determined by means of radiolabelling of the aerosol followed by in vivo and ex vivo Positron Emission Tomography (PET) imaging and dissection and gamma counting. Endotracheal nebulisation using the PennCentury MicroSprayer resulted in >85% of the administered dose accumulated in the lungs, with a non-uniform distribution of the radioactivity in different lobes and a low animal-to-animal reproducibility. Administration using the in-house designed and the Aeroneb nebulizers resulted in a uniform distribution over the lungs, but only a small fraction of the nebulized activity (ca. 0.1%) was deposited in the lungs. ; This project has received funding from the European Union's Seventh Framework Programme for research, technological development and demonstration under grant agreement no 604434 (PneumoNP). This research was also supported by the Spanish Ministry of Economy and Competitiveness, project CTQ2017-87637-R. The authors would like to thank M. T. ten Kate (Erasmus MC, University Medical Center Rotterdam) for technical training regarding endotracheal insufflations.