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Annihilation of positrons in argon

University of British Columbia

The annihilation of positrons in Argon has been investigated as a function of Argon density and applied electric field using the technique of lifetime measurements. Lifetime spectra were analyzed using the maximum likelihood method of curve fitting. Results obtained at zero electric field yielded a linear dependence on density for the direct annihilation rate of (5.6 ± 0.1) x 10⁶sec⁻1 amagat⁻¹, with some evidence of non-linearity at densities greater than 10 amagats. The density dependence of the long-lived component of the time spectra indicated a zero density intercept of (7.2 ± 0.4) x 10⁻⁶sec⁻¹ in agreement with the theoretical value of the free orthopositronium annihilation rate (7-2. x 10⁶ sec⁻¹ ). In addition an orthopositronium quenching rate of (0.29 ± 0.04) x 10⁶ sec⁻¹ amagat⁻¹ was obtained from the linear dependence of the orthopositronium annihilation rate on density. The electric field dependence of the direct annihilation rate and orthopositronium formation have been measured and are used to provide an internally consistent picture of the behaviour of positrons in a gas under the influence of an applied electric field. Furthermore, these results have been compared with theoretical results for the direct annihilation rate obtained from one parameter representations of the effective positron-Argon atom interaction. It is shown that, while such potentials are successful in describing the low-energy elastic-scattering of electrons from noble gas atoms, they are inadequate for the case of positrons. However, consideration of the way in which the direct annihilation rate changes as a function of electric field leads to an upper limit of 15π₀² for the momentum-transfer cross-section for positrons in Argon at thermal energies. Such an estimate is shown to be independent of any assumption concerning the effective positron-Argon atom interaction.

Text

2011-10-26

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http://hdl.handle.net/2429/38320

Physics

University of British Columbia

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