When fast positrons from a radioactive source such as 58 Co or 22 Na are impinged on a suitably prepared moderator surface a measurable fraction (∼0.1%) is re‐emitted as a monoenergetic beam. Several groups have efforts underway to implement this phenomenon as a practical monoenergetic positron gun to be used for surface studies. To address the question that arises about the effects of the window of the capsule that contains the fast positron source, moderated positron intensities have been measured for windows constructed of several different materials of varying thicknesses. A semilog plot of the data approximates a universal curve, with all points on the same line regardless of material. A 58 Co source can be masked by windows as thick as 22 µm Al or 11 µm Ti without decresasing the moderated positron intensity by more than 35%. Since 22 Na has a similar fast β + spectrum we have the implication that window effects for sources of this material will be of the same degree.
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Hulett et al. (1980) studied this question.