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The annihilation of positrons in helium Lee, Gregory Frank

Abstract

Lifetime techniques have been used to investigate the annihilation of positrons in helium as a function of electric field and density. Positronium formation has also been examined as a function of electric field. Results obtained for zero electric field yielded a linear dependence of the direct annihilation rate on density of (0.7214 ± .0082)D x 10⁶ sec⁻¹ amagat⁻¹, with no evidence of non-linearity. This corresponds to a Zeff of 3.628 ± .041. Assuming the free ortho-positronium annihilation rate of 7.2 x 10⁶ sec⁻¹ a value of (0.0991 ± .0067) x 10⁶ sec amagat⁻¹ was obtained for the ortho-positronium quenching rate. A dependence of the direct annihilation rate on applied electric field was observed and compared with several current theories describing the positron-helium interaction. The predictions of the theory of Drachman (1968) are shown to be in closest agreement. Measurement of the amount of positronium formed as a function of electric field disagrees with previous results and based on the present work a value for the momentum-transfer cross-section of positrons in helium at 17.7 ev is found to be at least .135π a₀² .

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