Positronium Scattering
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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.
refs. Garner et al, 1996 to 1998
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Positronium Beam Production
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        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
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Positronium Scattering
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        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
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A Rare Gas Solid Moderator
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        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.
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