Color centers in topaz: comparison between neutron and gamma irradiation

Neutron- and gamma irradiation-induced paramagnetic centers in natural colorless topaz from four different Brazilian localities were studied by electron paramagnetic resonance (EPR) and optical absorption as a function of irradiation dose and thermal treatment. Gamma irradiation doses were applied up to 1,000 kGy with a dose rate of 15 kGy h(-1). For the neutron irradiation experiments, a neutron flux of 4 x 10(12) cm(-2) s(-1) was used with an integrated flux of up to 1 x10(18) cm(-2). From the experiments, it is concluded that brownish colors are induced by gamma-rays and may be associated with a single broad isotropic EPR line with g = 2.015(2). Both the EPR line and the related optical absorption band at 460 nm (2.7 eV) are lost during thermal treatments between 150 and 200 degrees C. Fast neutrons create the paramagnetic peroxyl radicals O-2 (-) and the paramagnetic O- hole centers. The O- centers have the same thermal stability as the optical absorption band at 620 nm (2.0 eV). It is confirmed that the absorption due to the O- center is responsible for the blue color in topaz. Both color centers and their absorption bands are discussed in the context of O- bound small polarons.