Beginners Positrons Positronium
Recent Research
  • Research Highlights (2019-2010)


  • Positronium Scattering
    The Positronium Beam
            The positronium, Ps, beam at UCL is produced by the charge-exchange-reaction, e+ + A to Ps + A+, which converts a useful fraction of the positron beam into a collimated beam of Ps atoms. This beam can then be collided with a target in a second gas cell. The Ps atoms are detected using a number of different systems employing particle impact detectors and/or gamma-ray counters.
    Positronium Beam


    refs. Garner et al, 1996 to 1998


    Positronium Beam Production
            Until recently, argon was employed as a Ps production gas. In order to improve the Ps count-rate a study of the efficiency was carried out for three gases at a range of different pressures and energies.
            As shown the Ps beam production efficiency of H2 has been found to be higher than that of argon across a large range of pressures and energies investigated. At 30 eV the beam conversion efficiency of H2 increases linearly with pressure before reaching a plateau region. The plateau regions are caused when any incremental Ps creation, arising from increasing target pressure, is offset by increased collisional loss of the Ps atoms from the neutraliser gas. At the higher energies studied (60 and 90 eV) a plateau in the beam conversion efficiency is not reached when investigating H2. This suggests that the Ps-H2 total cross-section decreases beyond 30 eV and this has been confirmed by direct Ps-H2 cross-section measurements. As a result of this investigation the Ps flux has been improved by a factor of ~2.


    refs. Garner et al, 1996 to 1998


    Positronium Beam Production
    Positronium Scattering
    Ps-Gas Total Cross-Sections         The first direct measurements of Ps-gas total cross-sections, T, have recently been performed for He, Ar and H2. By varying the acceptance angle of the detector, it has been possible to evaluate and correct for forward scattering effects. The broad peak in T observed at the intermediate energies is usually attributed, by corresponding theoretical investigations, to the break-up of the Ps atom.


    refs. Garner et al, 1996 to 1998


    A Rare Gas Solid Moderator
            The most efficient positron moderators are rare-gas-solids. The UCL Ps beamline has been modified extensively in order to accommodate such a device. A two-stage cryostat has been installed to cool a radioactive source and cup arrangement to only 6 Kelvin. A gas, such as Ar, is then deposited on the surface of the source and cup where the positrons emitted from the source are moderated.
            Detailed studies to determine optimum operative conditions have been carried out. As a result of this work the positronium beam flux has been improved by over a factor of five.
    RGS Moderator