Measurement of the depth distribution of light impurities in first-wall materials: He in Nb

The concentrations and depth profiles of implanted helium in niobium have been measured by a method demonstrated previously with hydrogen and lithium in copper. The three targets, bombarded at room temperature with 10 keV He+ at doses of 0.01, 0.16 and 0.98 C/cm2, were respectively: unblistered; covered with circular blisters; and marked with 'microrelief', without blisters. The corresponding doses retained in the metal were 0.0076, 0.039 and 0.052 C/cm2 (i.e. approximately 3 x 1022 He atoms/cm3) with a 10% normalization uncertainly. The profile shapes did not change much: in particular as the dose increased, an accumulation near the surface, which is receding by erosion (sputtering, blistering), was not observed. These results show that a mechanism of helium loss starts operating at a dose <=0.16 C/cm2, i.e. before the bursting of blisters (if they burst at all), and it is most effective near the surface. (Auth.)