Zafar, Nazrene; (1990) An experimental study of thin foil positron moderators and positronium interactions in gases. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The use of thin single crystal tungsten, W(100), and nickel, Ni(100), foils as slow positron moderators in the transmission mode has been investigated. A simple annealing technique has been developed, employing resistive heating in low vacuum conditions of 6×10⁻² Torr, to treat the foils of thicknesses in the range 1000-18000Å. The maximum efficiencies obtained were (8.8±1.2)×10⁻⁴ from a 2000Å W(100) foil and (6.5±1.0)×10⁻⁴ from a 5000Å Ni(100) film in a system with a base pressure of 10⁻⁷ Torr. The considerations required to determine the efficiency, defined as the ratio of the slow positrons emitted to the number of fast positrons striking the foil, have been discussed in the context of the present arrangement. Comparisons show that the results obtained are a factor of 2.5-4.5 lower than calculated yields, however are in good agreement with values from experimental studies using more complex annealing procedures in higher vacuum conditions (≥10⁻⁸ Torr). The full width at half maximum of the energy distribution of the emitted positrons were found to be 1.7eV from W and 0.3eV from Ni compared to 2.8eV from W mesh. The timed positronium beam system developed by Laricchia et al (1988b) has been modified to allow the observation of n=2 state Ps formed in helium and argon gases. The excited state component of the positronium flux was quantified at incident positron energies of 16-52.5eV and found to compose, at maximum, 33% (He) and 42% (Ar) of the total positronium beam without correction for in-flight decay. Comparisons made with theoretical studies indicate that metastable S state positronium may constiuite a large proportion of the excited state flux (≈70%). Total positronium scattering cross-sections were extracted and the results obtained ranged from (4.5±0.8) to (7.6±0.8)×10⁻²⁰m² in Ar for positronium energies of 17-41eV and fluctuated about ≈3×10⁻²⁰m² in He in the regime 7-35eV from a minimum of (2.4±0.8) to a maximum of (3.5±1.3)×10⁻²⁰m². The assumptions and corrections required to determine these values have been detailed and comparisons have been made with theory and with cross-sections for hydrogen scattering from helium and argon. Possible extensions of both the slow positron moderator and the positronium scattering investigations have been discussed.

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