Ion-beam profiling of 3 He in tritium-exposed type 304l and type 21-6-9 stainless steels
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I.
INTRODUCTION
THISpaper reports the first use of high-energy ion beams to determine the depth distribution of the decay product 3He in austenitic stainless steels exposed to tritium. The method is based on detection of protons from the nuclear reaction 3He (d, p) 4He during bombardment with deuterons having energies in the range 0.3 to 2.0 MeV. Significant features of this analysis include a sensitivity to 3He better than one atomic part per million (appm), a depth resolution of about 0.5 p.m, and the capability of measuring concentrations at depths up to 10 p.m without sectioning of the specimen. (Deuterium can be depth profiled in solids in essentially the same way. J The nuclear reaction is then excited with a beam of 3He ions.) In general there are several reasons for examining 3He distributions in tritium-containing steels. One motivation comes from recent experimental results which indicate that the He may produce even more severe embrittlement than the parent tritium. 2 In those studies tensile specimens of various stainless steels were charged with either tritium or hydrogen, and some were then stored to permit tritium decay. Ultimately all were elongated to failure at room temperature. Both hydrogen isotopes embrittled the steels, but the ductility loss was substantially greater in the case of tritium charging followed by storage, presumably due to He-related degradation. 3 Another potentially important He effect is the strong trapping of hydrogen which has been observed in the presence of small He bubbles. This interaction was identified in other ion-beam experiments4 and is attributed to a chemisorption-like binding of hydrogen to the walls of the bubbles. Such trapping is expected to enhance tritium uptake by steels, a matter of particular concern in fusion-reactor applications. The 3He also provides an essentially immobile trace of the diffusing tritium. Although isolated atoms of He within an otherwise perfect fcc metal matrix are believed to migrate at room temperature, 5'6 this impurity is strongly immobilized by clustering and by trapping at lattice defects, 5-s the bind-
ing enthalpies ranging up to several electron volts. Consequently, on a scale greater than a few hundredths of a micron, He can be considered immobile at temperatures up to several hundred degrees Celsius in austenitic steels. Using the aforementioned nuclear-reaction analysis, this property can be exploited to measure tritium diffusion rates and solubilities and to characterize surface permeation barriers. Moreover, the high sensitivity and submicron depth resolution of the analysis allow it to be carried out even for relatively brief tritium exposures below room temperature. Another significant feature of this indirect detection is that it allows tritium behavior to be characterized in inaccessible environments, such as fusion reactors, through the analysis of materials after their removal. In this paper we first provide a detailed description of the method of analysis. Although basically the same procedure was used previou
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