The density of helium bubbles in implanted materials: Results from VUV absorption and EEL spectroscopy.

The novel application of vacuum ultra-violet absorption spectroscopy and electron energy loss spectroscopy to helium bubbles in metals is presented. These measurements, carried out on thin aluminum films containing different concentrations of helium and various bubble size distributions, were aimed at determining the density (and thus pressure) of helium in bubbles by observing the shift and broadening of the 1S-2P transition in the helium. The data coupled with a theoretical model developed by the authors indicate densities as high as 10**2**3 He cm** minus **3 for specimens containing small bubbles. Data are also presented on the effect that annealing and cooling have on these spectra. The annealing experiments give rise to fairly complex changes in absorption peak structure but with a general shift towards the unperturbed resonance line. The cooling experiment gives rise to a further shift and a narrowing of the absorption spectrum on cooling to 77 K which is tentatively identified as the liquid/solid transition in the helium. Finally, a fluorescence spectrum of an Al/He specimen excited with low energy electrons is presented.