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Development of a buffer gas trap for the confinement of positrons and study of positronium production in the GBAR experiment

Abstract : The GBAR experiment relies on the production of antihydrogen positive ions to achieve its goal of measuring the gravitational acceleration of antimatter at rest. The ANTION project, included in the GBAR enterprise, is responsible for the production of these antimatter ions. Moreover, it also aims to measure the cross section of antihydrogen production throughout the collision of antiprotons and positronium atoms, as well as the matter cross sections of hydrogen and the hydrogen negative ion. These experiments imply the formation of a very dense positronium cloud, thus a large amount of positrons will be implanted on a positron/positronium converter material. This thesis reports the construction of a three stage buffer gas trap with the goal of trapping and accumulating positrons for the ANTION project. The combination of the Penning-type trap with a LINAC source constitutes a unique experimental setup. The trap was commissioned and optimized and is now fully operational. Trapping protocols were studied and the effect of the buffer and cooling gases on the positron trapping rate and lifetime was assessed. In order to assist the cross section measurement of hydrogen, a GEANT4 simulation was developed. It evaluates the time and spatial evolution of the ortho-positronium atoms in a cavity, where hydrogen production will take place. It was estimated that 2.7 hydrogen atoms are produced for proton impact energy of ∼ 6 keV, according to the cross sections computed with the Coulomb-Born Approximation model, and 1.6 hydrogen atoms for a proton impact energy of ∼ 10 keV, according to the two-center convergent close-coupling method. The simulations also allow the estimation of the background associated with the positron and para-positronium decay. In addition, a suggestion is proposed to increase the number of positronium atoms in the cavity. In parallel, the positron moderation efficiency of a commercially available 4H-SiC epitaxial layer was studied. A 65% moderation efficiency was observed for kiloelectronvolt implanted positrons. This result can be of interest to slow positron physics experiments by improving the brightness of positron beams, and in particular to GBAR as it can potentially increase the efficiency of positron trapping.
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Amélia Mafalda Maia Leite. Development of a buffer gas trap for the confinement of positrons and study of positronium production in the GBAR experiment. Nuclear Experiment [nucl-ex]. Université Paris-Saclay, 2017. English. ⟨NNT : 2017SACLS380⟩. ⟨tel-01729186⟩

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