  Helium-ion-induced swelling and blistering of single-crystal tungsten is
investigated using a Helium Ion Microscope for site-specific dose-controlled
irradiation (at 25 keV) with analysis by Helium Ion Microscopy, Atomic Force
Microscopy and Transmission Electron Microscopy (cross-sectioning by Focused
Ion Beam milling). Our measurements show that the blister cavity forms at the
depth of the helium peak and that nanobubbles coalesce to form nanocracks
within the envelope of the ion stopping range, causing swelling of the blister
shell. These results provide the first direct experimental evidence for the
interbubble fracture mechanism proposed in the framework of the gas pressure
model for blister formation.
