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Abstract
The displacement threshold for the aluminium ion in sapphire has been determined by comparing measurements of the colour centre concentration induced by electron bombardment with values calculated from scattering theory. The value obtained is 24 eV. The main source of error in this is the value used for the oscillator strength of the centre; however, the rapid variation in the cross-section for displacement with energy causes the result to be rather insensitive to the measured value of colour centre concentration. Other features of the optical spectra are discussed, including an approximately linear background absorption and the appearance of F+ bands rather than the F band. A separate determination of the oxygen threshold, obtained by the more common technique of reducing the electron energy until no further centres are produced, is also described. The value obtained for E d is 79 ± 2 eV. A thin foil in the path of the accelerated electrons ensured that the samples were not also bombarded with energetic ions—a possible source of error in some previously reported results.