Garner, Andrew James; (1997) Positronium beam production and scattering from gaseous targets. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Quasi-monoenergetic, energy-tunable beams of positronium (Ps) atoms can be produced by neutralising a positron beam in a gaseous target. Investigations into the efficiency for Ps beam production from several gases (He, Ar and H2) have been carried out across a range of energies and gas pressures. In each case optimal Ps beam production conditions have been deduced. The total cross-sections for Ps scattering from He, Ar, H2 and O 2 have also been determined at intermediate energies. These studies have shown that, of the gases studied, H2 is the most efficient positron to Ps beam conversion gas, by a factor of up to three times that of Ar or He, in the range 10 to 90 eV. At 120 eV Ar has been found to be more efficient than H2 by approximately 40%. Ps-gas total cross-sections, ai, have been measured for Ps energies between 10 and 110 eV, across several different Ps flight lengths (from 0.2 to 0.6 m) and hence solid angles (1.3×10 -3 to 10×10-3 sr). In all cases the cross-section initially rises rapidly with incident energy, reaching a broad maximum at -20-40 eV and followed by a slow decrease at higher energies. Of the gases studied, Ar possesses the highest cross-section, with a peak value of ax -20×10 -20 m2, followed by H2, σT -9×10 -20 m2, and then He, σT -5.3×10 -20 m2. Sample measurements for PS-O 2 scattering suggest a cross-section of a similar magnitude to that of Ar. The data are compared to available calculations and to other projectiles. Future work is also suggested.

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