Precipitation in low energy helium irradiated molybdenum

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Abstract
The precipitation of low energy helium injected into molybdenum has been monitored by thermal helium desorption spectrometry (THDS). Precipitates nucleating at vacancies have been studied in the range from n = 1 to n = 2500 He per precipitate. The behaviour of the growing precipitates with respect to helium trapping, self-interstitial capture and emission, and helium release is discussed against the background of results of TEM work done for precipitates with n > 500 He. Evidence is found for (i) “trap mutation” at n = 10 He, i.e. formation of bound self-interstitials around the helium filled vacancy, and (ii) emission of at least one of these “mutation produced self-interstitials” (MPI) at n ∼ 12 He. For larger precipitates (n > 20 He) the emission of single MPI is not apparent. At n = 1000 signs of helium binding to MPI-loops punched out by the precipitates are observed in the spectra.