Multilayer adsorption of uranium on tungsten

Abstract

The adsorption of uranium on tungsten field emitters has been studied for coverages of up to 12 monolayers for uranium deposited at 295°K and at pressures below 2 × 10 −10 Torr. A drop in work function from 3.60 to 3.38 eV was observed in the range 3 to 12 monolayers. This was attributed to growth of loose-packed α-uranium (100) faces on particular planes of the tungsten substrate. Further growth was observed on heating the deposit up to 900°K. Heating in the β-uranium temperature range (934–1042°K) resulted in a change in the regions in which preferred growth occurred. No evidence for nucleation in these regions was observed. Consideration of the surface topography revealed that the preferred growth regions correspond to surfaces on which the bulk uranium structure could be accommodated with only a small geometrical misfit. Mass spectrometric analyses of the evaporant flux from the evaporation sources were made, showing that the adsorbate contained no appreciable traces of W, UO or UO 2 and further that the evaporation rate remained constant for the duration of the experimental measurements.