Study of positron yield of W(100) single crystalline foil with in situ surface characterization

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Abstract
The slow positron yield of a 1 μm thick W(100) single crystalline foil moderator used in transmission mode geometry is measured as a function of successive heat treatments with and without annealing in partial pressure of oxygen. In situ surface characterization of the moderator foil is carried out using Auger spectroscopy. The yield gradually improves after each heat treatment leveling off beyond five treatments. The improvement in the yield does not seem to be correlated with low surface carbon content but rather due to removal of bulk carbon due to heat treatments. The reemitted positron flux reproducibly goes through a maximum within an hour after surface treatment and approaches an equilibrium level after about 12 h which is about 20% less than the maximum. The initial increase in the positron flux is correlated with an oxygen adsorption rate which is higher than for carbon immediately after surface treatment while the subsequent decrease in flux seems to correlate with a more gradual increase in contamination due to carbon adsorption with the oxygen contamination remaining fairly constant. Energy spectra of the reemitted positrons using a retarding field analyzer show a beam of FWHM∼0.25 eV with a low energy tail whose intensity remains at a minimum after surface treatment and during the approach to maximum flux, but whose intensity increases after the flux passes through a maximum. Preliminary surface preparation studies with annealing in partial pressure of hydrogen show evidence of an improved FWHM and a slight shift in positron work function.