Low energy positrons from metal surfaces

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Abstract
There is reason to believe that slow positron spectroscopy will have value for surface analyses, but in order to develop practical methods it is necessary to devise monoenergetic sources of positrons that output usable intensities. This paper describes how this can be achieved by preparing certain metal surfaces to act as moderators for fast positrons from radioactive sources. After entering the moderator, the positrons are thermalized and a certain fraction are re‐emitted back into the vacuum. They can then be extracted by an electrostatic field and accelerated to any desired energy. Yields of low energy positrons were found to be generally large for certain metals having negative positron work functions (theoretically calculated). It is also necessary to minimize bulk defects, which act as positron traps. Tungsten and molybdenum are metals having negative work functions which can be easily prepared as effective moderators by simply heating in vacuum. This effects a cleaning by volatilizing oxides and anneals defects. The inertness of their surfaces towards oxidation allows them to maintain the negative work function effect which might otherwise be reduced by exposure to air and poor vacuum. These results have been utilized by the authors to design a monoenergetic positron gun to measure discrete energy losses in reflection spectra and threshold absorption effects. A Brandeis University group has used a tungsten moderator gun to make the first experimental measurements of low energy positron diffraction spectra.

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