Irradiation of Metals with Hydrogen and Helium Isotopes

Abstract

A PN-400 Van de Graaff accelerator has been upgraded to a nominal 700 kV. A stable proton beam has been extracted from the upgraded accelerator with a terminal voltage of 900 kV. Improvements to the vacuum system have greatly reduced target contamination. An optical microscope mounted on the target chamber enabled in-situ observation of blister formation on irradiated targets. The irradiation of metals with gaseous ions often produces small gas bubbles in the implanted region. At low temperatures (<0.3Tm) overpressured gas bubbles are commonly assumed to grow by punching out prismatic dislocation loops. The loop punching model for bubble growth is discussed. It is concluded that, at least for the case of 30 keV helium irradiation of copper, all the implanted gas can not be accommodated in the visible bubble array. A striking feature of the blistering produced by the irradiation of metals with hydrogen isotopes at 120K is the formation of a ring of blisters around the periphery of the irradiated area but separated from the central blistered region by a band of unblistered material. It is suggested that irradiation induced stresses play an important role in the formation of the blister ring. The blistering and flaking of metal surfaces following irradiation at 120K with hydrogen isotopes often exhibits two or more distinct planes of fracture at different depths below the surface. Models have been proposed to explain the different fracture planes. There are several mechanisms in particular which are likely to be important. High levels of compressive stress in the swollen implanted layer may inhibit the nucleation and early growth of bubbles in the depth region near the maxima in the damage and gas deposition profiles. The gradient, along a surface normal, in the lateral stress may assist the movement of gas deeper into the target. Early bubble growth may be enhanced in a region below the maximum in the gas deposition profile. Shear introduced by differential expansion, caused by a combination of radiation induced swelling and localised heating could be an important factor leading to fracture.