Abstract
The efficiency of tungsten meshes and thin foils for moderation of fast positrons from 22Na has been investigated in transmission geometry and a fair agreement has been found with previous experimental results where directly comparable. For foils, the dependence on material thickness is found to be similar to the prediction of the Vehanen-Mäkinen diffusion model; however, the magnitude is 5–10 times lower. A broad consensus is observed between experiment and the results of a three-dimensional model developed in this work. For a given thickness, meshes are found to be generally better than foils by around a factor of 10 with a maximum efficiency (∼10−3) comparable to that achieved with thin single crystal foils, in accord with previous measurements and the results of the present model.
Highlights
IntroductionModerators are a crucial component for the production of quasi-monoenergetic positron beams, reducing the energy of bþ particles emitted from radioactive decay (or pair production) down to a few electron volts (e.g., Schultz and Lynn 1988, Coleman et al 2000)
The efficiency of tungsten meshes and thin foils for moderation of fast positrons from 22Na has been investigated in transmission geometry and a fair agreement has been found with previous experimental results where directly comparable
Moderators are a crucial component for the production of quasi-monoenergetic positron beams, reducing the energy of bþ particles emitted from radioactive decay down to a few electron volts (e.g., Schultz and Lynn 1988, Coleman et al 2000)
Summary
Moderators are a crucial component for the production of quasi-monoenergetic positron beams, reducing the energy of bþ particles emitted from radioactive decay (or pair production) down to a few electron volts (e.g., Schultz and Lynn 1988, Coleman et al 2000). Significantly higher efficiencies (m $ 10À3) have more recently been obtained from stacks of meshes which had been electropolished (Saito et al 2002) or etched (Weng et al 2004) The latter authors found the efficiency to depend on the annealing pre-treatments, the duration of the etching and the number of the folding layers. Stimulated by these observations, and in the course of the development of a new electrostatic beamline at UCL (K€over et al 2014), we have investigated the variation of m for W foils (both single- and poly-crystals) and meshes with material thickness and annealing pre-treatments. A model has been developed to allow for three-dimensional (3D) diffusion of the thermalized positrons and its results are compared with present and other available measurements
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