Positronium-like positron states in He bubbles in 600 MeV proton-irradiated Al

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Abstract
Aluminium samples containing He bubbles produced by 600 MeV proton irradiation at temperatures in the range 655-728 K have been investigated by positron annihilation. Both lifetime and angular correlation (AC) measurements have been made. Measurement temperatures range from 10 to 300 K. A very narrow component (FWHM approximately=1.6 mrad) is present in the AC spectra. This component is interpreted to be due to a positronium-like positron state in the He bubbles. The width of the narrow component indicates that the state is not free positronium (Ps) in the bubble interior by a Ps-like state at the bubble surface. The corresponding He bubble component in lifetime spectra has a lifetime value of 450-480 ps. The intensities of this lifetime component and of the narrow AC component indicate that only about half of the positrons trapped in bubbles contribute to the narrow AC component. It is suggested that this could be due to different momentum distributions of the annihilating electron-positron pair parallel and perpendicular to the bubble surface. The Ps-like state is attributed to the presence of impurity atoms, possibly Na, at the surfaces of the bubbles. In addition, positron trapping at dislocations in the samples is discussed.
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