Blistering of tantalum foils during alpha particle bombardment

Helium has low solubility in metals and surface blistering will occur when the bombarding ion dose exceeds approx = 5 x 10 exp 21 ions/m exp 2 . In this experiment, three Ta foils, each 12.7 mu m thick, were stacked and then bombarded by 40 MeV alpha particles to a total dose of 2 x 10 exp 22 alpha /m exp 2 . Scanning electron microscopy (SEM) shows blisters similar to those that occurred with a 30 MeV alpha bombardment. The blister density ranges from 2.6 x 10 exp 10 -3 x 10 exp 11 m exp --2 , with prong lengths from 0.3-2 mu m. These results can be compared with the Kaminsky and Das results, in which three-pronged blisters were obtained on an Nb monocrystal irradiated with 0.5 MeV He exp + at 900 deg C. The irradiated foils were etched in a 30% HF, 70% HNO sub 3 solution to test the origin of the surface features. Some typical micrographs at different regions of the bombarded area are illustrated. The present results support the gas-driven model of blister formation due to the symmetry and appearance of blisters on both sides of the foils. The observed features may be related to other phenomena. The blister formation initially orders the voids to form a body-centered cubic superlattice. The voids grow by vacancy absorption, and the level volume swelling causes the voids to coalesce forming larger voids. Large lateral compressive stresses result that are relaxed by bowing the surface, forming blisters. In this experiment, blister development is due to displacement damage, local thermal spikes and the presence of oxygen in the residual gases in the vacuum. The formation of gas bubbles in the target does not seem to be feasible. The observed features may be attributed to void swelling that is large enough to deform the surface. Photomicrographs. 20 ref.--R.A.C.