Doppler-broadening measurements of positronium thermalization in gases

Abstract

The formation and subsequent thermalization of positronium ͑Ps͒ produced at a few eV in gases are investigated using time-resolved Doppler-broadening measurements of the annihilation photons. A static magnetic field quenches the Ps enabling Doppler energy measurements from 25 to 70 ns after the Ps is formed. Varying the gas density permits a significant range of the thermalization process to be observed. Seven different gases are studied, He, Ne, Ar, H 2 , N 2 , isobutane, and neopentane. A classical elastic scattering model fits all the gas data reasonably well. For each gas, an elastic scattering cross section and an average Ps formation energy are determined from the classical model fit. When comparisons can be made, these cross sections are often significantly smaller than most quantum-mechanical-theory predictions and most previous experimental results obtained using the angular correlation technique. Various systematic tests have been applied to the apparatus and the analysis, reinforcing the discrepancy with previous works.

Positronium–atom collisions

“…As discussed in [22], these low energy data, derived from annihilation measure- solid curve, present results with 9Ps9He approximation for S-wave and 7Ps7He approximation for P-wave; dash-dot curve, frozen target results with 9Ps1He approximation for S-wave and 7Ps1He approximation for P-wave; long-dashed curve, S-wave total cross section of Chiesa et al [21]; short-dashed curve, cross section of Basu et al [19]. Experiment: square, Nagashima et al [23]; up triangle, Canter et al [24]; down triangle, Rytsölä et al [25]; circle, Skalsey et al [26].…”

Positron and positronium scattering

“…(a) Ps(1s)+H(1s) total cross section [28]. (b) Ps(1s)+He(1 1 S) momentum transfer cross section [29]; experiment: square, Nagashima et al [20]; up triangle, Canter et al [18]; down triangle, Rytsölä et al [30]; circle, Skalsey et al [31]. Figure 5(a) compares frozen target calculations [16,17] with beam measurements of the total cross section for He [23][24][25] at impact energies of 10 eV and above.…”

New Precision Measurement of Hyperfine Splitting of Positronium

“…As mentioned in Ref. [26], rovibrational excitations of the i-C 4 H 10 molecule increase σ m of Ps with kinetic energy above 0.17 eV because i-C 4 H 10 has a vibrational level at 0.17 eV. The 'pick-off technique' [9,10,1], which can access o-Ps with lower energy than 0.17 eV, is a complementary method.…”

“…by DBS (Doppler Broadening Spectroscopy) technique [26] in the range of 0.15-1.52 eV Ps kinetic energy. However, the DBS result should not be applied for o-Ps whose kinetic energy is less than 0.17 eV since σ m depends on the kinetic energy of Ps.…”