Bleaching and thermal recovery of PL emissions in natural silica

We have investigated the bleaching of two photoluminescence (PL) emissions at 3.1 and 4.2 eV and the related growth of an electron spin resonance (ESR ) signal, consisting in a hyperfine doublet split by 11.8 LnT, in natural s ilica gamma-irradiated by low doses, up to 1 Mrad. These observations defin itely support the existence of a conversion mechanism, from optically activ e defects to paramagnetic ones. To further investigate this conversion proc ess and the stability of the gamma-induced paramagnetic centers, we perform ed PL and ESR measurements in samples that, after a gamma exposure at 1 Mra d dose, were thermally treated at various temperatures ranging from 330 to 430 K. We found that the intensities of PL and ESR signals recover their pr e-irradiation values, that is the PL bands intensities are completely resto red and the 11.8 mT doubler disappears at all. These features give evidence of a thermally activated back-conversion mechanism, from the paramagnetic center to the optically active one, able to restore completely the structur e of the latter defect before gamma-irradiation. (C) 2000 Elsevier Science B.V. All rights reserved.