On the thermalization times of positrons in polyatomic gases

Abstract

The time taken for positrons from a radioactive source to reach a velocity distribution in the near-thermal range has been measured for mixtures of nitrogen gas with argon, and of methane with argon. An analysis of the shoulder breadths for the free positron components leads to slowing down times of 16 ns for nitrogen and 0.25 ns for methane at densities of 1 amagat. The long slowing time for nitrogen must occur during the last eV or so of slowing and the dominant mode of energy loss is attributable to rotational excitation of the molecules. For methane the slowing to the threshold for vibrational excitation must be extremely rapid, but there is no clearly established mechanism which could account for the remaining energy loss taking place so rapidly.

Positron Cooling and Annihilation in Noble Gases

“…is the Maxwell-Boltzmann distribution taken at T = 293 K. Comparisons between theory and experiment are focussed around the shoulder, which is somewhat insensitive to the initial distribution of the positrons [5,29] as a result of the bunching around the minimum in B(k). The traditional measure of the thermalisation time in positron-gas studies is the 'shoulder length' τ s , defined viaZ eff (τ s ) ≡Z eff − 0.1∆Z where ∆Z =Z eff −Z min , andZ min is the minimum ofZ eff (τ ) [39]. The calculated τ s are given in Table I along with experimental and previous theoretical results.…”

“…For all the noble gases the increase in Z eff (k) as k → 0 results in the evolution of Zeff (τ ) through a transient 'shoulder' region resulting from epithermal annihilation at k < k min [1][2][3] towards its steady-state thermal value Zeff ≡ ∞ 0 Z eff (k)f T (k)dk, where f T (k) is the Maxwell-Boltzmann distribution taken at T = 293 K. Comparisons between theory and experiment are focussed around the shoulder, which is somewhat insensitive to the initial distribution of the positrons [5,29] as a result of the bunching around the minimum in B(k). The traditional measure of the thermalisation time in positron-gas studies is the 'shoulder length' τ s , defined via Zeff (τ s ) ≡ Zeff − 0.1∆ Z where ∆ Z = Zeff − Zmin , and Zmin is the minimum of Zeff (τ ) [39]. The calculated τ s are given in Table I along with experimental and previous theoretical results.…”

The slowing down times of positrons emitted from selected β+ isotopes into metals

“…a source. In that case the SDT was well visible in the positron lifetime spectra and the experimental method for it determination was presented by Paul and Leung [17]. The data reported in Refs.…”

The behaviour of positron clouds in the single-particle regime under the influence of rotating wall electric fields

“…However, it has also been known for many years, again from positron lifetime measurements, that, among the molecular gases, nitrogen is the poorest positron cooler, with the lowest density-normalized cooling rate or the longest thermalization time [37][38][39][40]. Thus, if a rapid cooling is required, which is the case here when the rotating wall is used, a further gas must be inserted into the system.…”