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

We have measured the concentrations and depth profiles of implanted helium in niobium 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/cm ⁺, 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/cm ² (i.e.≈3 × 10 ²² He atoms/cm ³) with a 10% normalization uncertainty. The profile shapes did not change much: in particular we did not observe, as the dose increased, an accumulation near the surface, which is receding by erosion (sputtering, blistering). These results show that a mechanism of helium loss starts operating at a dose ⩽0.16 C/cm ², i.e. before the bursting of blisters (if they burst at all), and it is most effective near the surface.