Energy dependence of the positronium formation cross-section in argon

The positronium (Ps) formation cross-section, Q(Ps), for positron-argon interactions has been measured for incident positron energies from threshold to 60 eV, in an attempt to resolve the apparent discrepancy between earlier experimental results. Q(Ps) was found to vary smoothly with positron energy between 15 and 30 eV, in qualitative agreement with earlier results using methods involving the measurement of positron neutralization (as in the current experiment) and in disagreement with the double-peak energy dependence reported by Laricchia et al (2002 J. Phys. B: At. Mol. Opt. Phys. 35 2525-40), who used a positron-ion coincidence detection method. Possible reasons for this discrepancy, including Ps fragmentation and excited-state Ps formation, are discussed.