Semi-insulating samples of zone-refined material were irradiated with 24GeV/c protons, to fluences of up to 1.64 x 1014/cm2. The transmitted and reflected optical spectra were accurately measured, for energies ranging from 0.6 to 1.4eV, in order to deduce (from the absorption coefficient) the concentrations of both neutral and ionized EL2 defects as a function of the proton fluence. Both of these concentrations were shown to increase linearly with the proton fluence. This behavior explained a marked decrease in the charge collection efficiency which was observed in proton-irradiated material.

R.Ferrini, M.Galli, G.Guizzetti, M.Patrini, F.Nava, C.Canali, P.Vanni: Applied Physics Letters, 1997, 71[21], 3084-6