Angular correlation studies of positron annihilation in the noble gases

Abstract

Twdimensional angular correlation distributions have been measured for the first time for positron and positronium annihilation in lowdensity helium, neon, argon, krypton and xenon, and in a range of helium-xenon mixtures. The full widths of the (cylindrically averaged) free positron components are in reasonable agreement with theoretical values, The intensity of the mixed-state positronium (Ps) component (mean vacuum lifetime 9.7 ns in a magnetic field of 0.8 T) in helium is consistent with the Ps fraction previously d e t a i n e d in lifetime measurements; the widths of the Ps component in helium. neon and argon are consistent with a Ps-atom scattering cross Section below 6.8 eV in all three gases of approximately (9-0.5E) nu:, where E is the Ps energy in eV.

Fragmentation of Positronium in Collision with He Atoms

“…The determination of cross sections for Ps scattering are not just difficult for experimentalists, due to the very small projectile fluxes involved, but also for theorists, due to both the projectile and the target being composite objects. The problem is highlighted in the case of the Ps-He total cross section, where a discrepancy of a factor of 5 is found among the results of different theories [16 -19] and different experiments [4][5][6]14,15].…”

“…Information on Ps collisions at room temperature may be obtained by studying its thermalization in a gas and measuring its lifetime [3], the Doppler shift of the energies of the annihilation quanta [4], or the deviation from collinearity between them [5,6]. At higher energies, cross sections can be measured directly using a Ps beam [7] produced by neutralizing a monoenergetic e beam in a gaseous target [8][9][10].…”

Positron and positronium scattering

“…Experimental data: triangle, Canter et al [18]; cross, Coleman et al [19]; square, Nagashima et al [20,21]; circle, Skalsey et al [22]. the frozen target results for He [16], Ne and Ar [17] with cross sections deduced from annihilation measurements at very low energies [18]- [22].…”

The quenching of ortho ‐positronium

“…Since it has a direct effect on the lifetime of o-Ps, the positron lifetime technique is widely used, but this method is not capable of distinguishing between the different quenching processes. Since the quenching leads to 2γ annihilation instead of 3γ, measurements of the momentum distribution by the angular correlation of the annihilation radiation (ACAR) [7,8], Doppler broadening of the annihilation radiation (DBAR) [9], or the age-momentum correlation (AMOC) technique [10] are useful. Effective use of a magnetic field may also be made.…”