Characterization of deep energy level defects in α-Al2O3:C using thermally assisted OSL

Commercially available α-Al2O3:C powder was studied for deep energy level defects by a newly suggested method using thermally assisted optically stimulated luminescence (TA-OSL) phenomenon. The method involves simultaneous application of continuous wave optically stimulated luminescence (CW-OSL) as well as thermal stimulation up to 400 °C, using a linear heating rate of 4 K/s. By using this method, two well-defined peaks at 121 °C and 232 °C were observed. These TA-OSL peaks have been correlated to two different types of deeper defects which can be bleached at 650 °C and 900 °C respectively on thermal treatment. These deeper defects, having larger thermal trap depth and relatively lower photoionization cross-section at room temperature for stimulation with blue LED (470 nm), are stable up to 500 °C, so they can store absorbed dose information even if the sample is inadvertently exposed to light or temperature. As only a fraction of signal is bleached during TA-OSL readout, multiple readouts could be performed on an exposed sample using this technique. The dose vs TA-OSL response from deep traps of α-Al2O3:C was found to be linear up to 10 kGy, thus extending its application for high dose dosimetry. The value of thermally assisted energy (EA) associated with these traps in α-Al2O3:C has been determined to be 0.268 eV and 0.485 eV respectively and the corresponding values of photoionization cross-section at room temperature (25 °C), for optical stimulation with blue light (470 nm), are 5.82 × 10−20 and 3.70 × 10−22 cm2, respectively. The process of thermally assisted OSL has been formulated analytically as well as theoretically for describing the temperature dependence of optical cross-section and evaluation of thermally assisted energy associated with deep traps.

► A novel method of measuring the signal from deeper traps in Al2O3:C for its use in high dose dosimetry.
► Mathematical formulation of TA-OSL phenomenon.
► Extended dose linearity of Al2O3:C phosphor up to 10 kGy using TA-OSL of deeper traps, with choice of multiple readouts.
►  Discusses about the nature of deep traps in Al2O3:C.
► Determination of thermally assisted energy associated with the deeper traps and corresponding photoionization cross-section.