Positron lifetime measurements were performed on semi-insulating material in darkness at 20K, before and after illumination by light with a photon energy of 1.32eV. The EL2 concentrations were measured by infra-red absorption. After illumination, the positrons revealed a metastable vacancy whose trapping rate could be correlated with the total EL2 concentration. It was concluded that this vacancy was incorporated into EL2*, the metastable configuration of EL2. The present correlation permitted a value of 3.0 x 1016/s to be proposed for the positron trapping coefficient of EL2* at 20K.

C.Le Berre, C.Corbel, M.R.Brozel, S.Kuisma, K.Saarinen, P.Hautojärvi: Journal of Physics - Condensed Matter, 1994, 6[48], L759-63