THE ENERGY-DEPENDENCE OF THE POSITRONIUM FORMATION CROSS-SECTION IN HELIUM

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)pia0(2), 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 recent 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.