Spectroscopic account of the point defects related to the sensitization of TL peaks beyond 220 °C in natural quartz

This study investigates the origin of point defects involved with a strong sensitization of the thermoluminescence (TL) signal of natural quartz between 220 and 400 °C. The sensitization was produced by the combined effect of high-dose of γ radiation and heat-treatments at moderate temperatures. The point defects related to silicon and oxygen vacancies and also with Al, Ge, Li, and OH impurities were assessed by infrared (IR) and electron paramagnetic resonance (EPR) spectroscopy in samples prepared from two single crystals with different sensitivities. The signal intensities of the point defects were measured in as-received, zeroed, γ-irradiated, sensitized and also after heat-treatments carried out up to 600 °C. The deconvolution of the TL curves showed that the major peaks appeared around 240 and 325 °C. The spectroscopic analysis showed the presence of several point defects identified as E'₁, [E'₁-GeO₄]⁰, [O₂^3-/Li⁺]⁰, [GeO₄/Li]⁰, [AlO₄]⁰, [AlO₄/H]⁰, [BO₄/H]⁰ and Li-dependent OH centers. The sensitization process was efficient to increase the number of electron traps by means of a stable coupling (up to 400 °C) between E'₁, [GeO₄]⁰ and Li⁺ ions dislodged from [AlO₄/Li]⁰ centers. It was concluded that the active electron traps responsible for the TL emission above 220 °C were [GeO₄/Li]⁰ and E'₁ centers whereas [AlO₄]⁰ was the main recombination center. The thermal behaviour observed for [E'₁-GeO₄]⁰ and [O₂³-/Li⁺]⁰ signals showed that the creation of these centers are connected each other. A sequence of reactions involving Li⁺ motion plus electron-hole recombination at localized transitions within ([GeO₄/Li]⁰, [AlO₄]⁰), (E'₁, [AlO₄/Li]^-) and ([E'₁-GeO₄]⁰, [O₂³-/Li⁺]⁰) defect pairs were proposed to explain the TL process in sensitized samples after the impartment of a given test-dose. The precursors for [E'₁-GeO₄]⁰ and [O₂³-/Li⁺]⁰ centers were assumed to compete with the active traps responsible for TL peaks found at 240 and 325 °C peaks.