The energy dependence of the positronium formation cross section in helium

Abstract

The total cross section for positronium formation in helium for positrons of energies E from threshold (17.8 eV) to 250 eV has been measured using a gas cell transmission method. The results indicate that the cross section above E=60 eV is satisfactorily described by 0.5e-0.02E pi a02, tending to zero at energies close to 200 eV. The approximate description of the cross section at intermediate energies, falling as E-2.5, is consistent with a number of theoretical predictions but is in conflict with earlier experiments, which show an E-1 behaviour. The implications of these measurements are discussed, with particular reference to the observed merging of the total cross sections for positron- and electron-helium scattering at intermediate energies.

A Self-Consistent Model for Positronium Formation from Helium Atoms

“…Diana et al [28] Fromme et al [29] Fornari et al [30] Overton et al [31] Murtagh et al [32] (b) Our results differ significantly from the CBA data at 100 eV incident energy. Below (above) 30…”

“…Consequently, the total cross sections obtained from the two formalisms converge at high energies. Figure 2 compares the total positronium-formation cross sections obtained from our formalism with the experimental measurements of Diana et al [28], Fromme et al [29], Fornari et al [30], Overton et al [31] and Murtagh et al [32] and also with the results of the CBA with Z T = 1.6875, 2 and √ −2ǫ i [52] and of the two-center convergent close-coupling (CCC) formalism [18]. As Fig.…”

“…Present Results CBA with Z T =2 [52] CBA with Z T =1.6875 [52] CBA with Z T =(-2 ε i ) 1/2 [52] Fojon et al [38] Bransden et al [39] Igarashi and Toshima [40] Campbell et al [41] Fornari et al [30] Overton et al [31] FIG. 3.…”

“…Figure 3 compares various theoretical approaches with experiment. Cross sections calculated by the CBA with Z T = 1.6875, 2 and √ −2ǫ i [52], the correct-boundary first-order Born (B1B) [38] formalism, the distorted-wave Born (DW) approximation [39], the continuum distorted wave (CDW) approximation [40] and the close-coupling method (CC) [41] are compared with the experiments of Fornari et al [30] and Overton et al [31]. The CBA, CDW, and CC agree well with the measurments in the depicted energy range.…”

“…3. Total cross sections, compared with the results of the CBA [52], B1B [38], DW [39], CDW [40] and CC [41] procedures and with the experimental data from Fornari et al [30] and Overton et al [31]. …”