Temperature dependence of D atom adsorption on polycrystalline tungsten

Temperature dependence of D atom adsorption on polycrystalline tungsten was studied by in situ ion beam method Elastic Recoil Detection Analysis (ERDA). A new procedure named thermoadsorption (TA) was developed for this study, where the sample is first exposed to a deuterium atom beam at high temperature and then, while being continuously exposed to the atom beam, is slowly cooled down. H and D concentrations are determined during this cooling by ERDA. A stepwise increase of the surface areal density was observed starting from (1.2 +/- 0.3) x 10(15) D cm(-2) at sample temperature around 750 K, to (2.2 +/- 0.3) x 10(15) D cm(-2) when temperature was around 600-500 K and final increase to (6.8 +/- 0.6) x 10(15) D cm(-2) when sample temperature was below 440 K. From this, three individual binding states were identified for the studied polycrystalline tungsten. We present a numerical model adequate to our experimental procedure which was developed by taking into account all relevant surface processes. The binding energies for desorption/adsorption were derived by modeling the TA data with the numerical model and were determined to be: 1.05 + 0.06 eV, 1.7 + 0.08 eV and >2.2 eV. Isotope exchange at 485 K sample temperature was also measured and modeled. An estimate of the reflection coefficient for 0.2 eV hydrogen atoms on polycrystalline W was obtained from modeling the isotope exchange data and was determined to be 0.96 +/- 0.02. (C) 2013 Elsevier B. V. All rights reserved.