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Positron Reaction Microscopy
Studies with positrons and positronium, the short-lived atom made of an electron and its antimatter counterpart the positron, range from fundamental tests of annihilation and investigations of events in the galactic centre (where it is estimated that approximately 93% of all annihilation events occur through the decay of Ps) to practical applications such as characterization of microscopic voids in materials as well as medical diagnostics and therapy. Much has been learnt, in the past few decades, about the interactions of positrons and Ps with atoms and molecules thanks to the ability of performing controlled reactions with these projectiles, of analyzing final-state products with improved sensitivity together with increasingly sophisticated theoretical descriptions. The aim of this proposal has been to advance this knowledge and explore particular aspects of fundamental antimatter-matter interactions. Of particular interest here have been reactions resulting in fragmentation of atoms and molecules as driven by positrons. In order to study these, a major instrumental development has been undertaken consisting of a 'positron reaction microscope'. It has required the development of a brightness-enhanced positron-beam, a cold gas jet and a spectrometer. This work has been supplemented by investigations of ionizing collisions involving excitation of the target and/or the positronium atom formed as well as positron-induced fragmentation of molecular targets. In particular, cross sections for ionization excitation of CO2 and N2 by positron impact have been measured for the first time and found to exceed those for electron impact factors of 3-5, the enhancement arising primarily via positronium formation. The cross sections for the formation of positronium in the 2P state in collisions of positrons with He, Ar, and Xe atoms have been determined by measuring coincidences between the remnant ion and the Lyman-alpha photon from positronium. The maximum fractional contributions to the total Ps formation cross sections have been found to increase progressively in going from He to Xe. The energy distributions of positrons forward scattered from H2O have been measured in coincidence with the remnant ions (H2O+, OH+ and H+) . The maxima of the energy distributions associated with the production of OH+ and H+ are about 5-10 times smaller than that for (HO+)-O-2.
1 Researchers
1 External Collaborators
  • Prof Akos Kover
    Hungarian Academy of Sciences - Hungary
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