Tests of a diamond field-assisted positron moderator

Diagnostic studies have been performed on a 70 μm thick diamond film with a view to its development as a possible field-assisted moderator (FAM) for slow positron emission. The film was coated on one side with a 200 nm gold electrode and on the other with a similar thickness of gold in the form of a fine mesh (10 μm lines, 40 μm spaces) in the central part of the diamond surface and a solid edge for electrical contact. A potential difference of up to 300 V could be held across the film with a leakage current of ∼10 nA. Although positron re-emission was observed from the diamond surface, the re-emitted fraction was observed to decrease when a bias of 300 V was applied across the film. This negative result is interpreted in terms of the drifting of thermalised positrons to the gold mesh lines, a conclusion supported by Doppler broadening measurements. The potential for the development of a diamond-film-based positron beam tagger is discussed.