An in situ field ion microscope study of irradiated tungsten I. Experimental results

The mobility of single self-interstitial atoms was studied in a series of in situ field ion microscope (FIM) experiments. High purity (≤ 1·5 × 10^–6 at. fr. impurity level) W specimens were irradiated with 20 kev W+ ions under ultra-high vacuum (~ 10^–10 torr) conditions at temperatures between 8°k and 18'k. The specimens were examined directly at the irradiation temperature employing the pulse field evaporation technique. The initial state of damage at 15°k consisted of depleted zones within 100 A of the irradiated surface, and a distribution of immobile self-interstitial atoms which were distributed throughout the bulk of the specimen, and which originated at the depleted zones. Direct observation of the FIM specimens during subsequent annealing experiments between 15°k and 120°k showed that these self-interstitial atoms underwent long-range migration (i.e. distances of ~ 200 Å) in Stage I, and were mobile at temperatures as low as ~28°k. Warming these same specimens to 298°k (~ middle of Stage II) followed by post-anneal pulse field evaporation experiments at 15°k showed that the initial concentration of self-interstitial atoms, which were distributed throughout the volume of the specimen, decreased by at least an order of magnitude as a result of this annealing treatment. Control experiments were performed to show that the imaging electric field had not caused extensive long-range stress-induced migration of these self-interstitial atoms. In addition, an extensive series of control experiments were carried out to show that the phenomena reported were not due to impurity interstitial atoms or surface artifacts. An analysis of the above experimental results is given in Part II [Scanlan, Styris and Seidman].