Characterization of optically-stimulated luminescence properties by NaCl:Eu²⁺ crystal and the thermal response

In this paper, we report optically-stimulated luminescence (OSL) properties of Eu²⁺-doped NaCl single crystal as a candidate phosphor for use in near real-time radiation measurement and dosimetry. The crystal was grown by the vertical Bridgman–Stockbarger method. The obtained crystal shows a strong photoluminescence (PL) band peaking at 430 nm due to the parity-allowed 5d-4f transitions of Eu²⁺. After irradiating with X-rays, it demonstrates OSL also at 430 nm due to the Eu²⁺ ion acting as an emission center while stimulating by light (500 ± 30 nm). Compared with a commercial BaFBr:Eu²⁺ OSL detector, the sensitivity to X-ray dose is lower by about one order of magnitude. In addition, while the crystal is reused multiple times, the OSL response signal increases despite the same given radiation dose. The OSL decay curve can be decomposed into three exponential decay functions with the lifetimes of 0.6, 6.4, and 120.5 s, and the decomposition analyses revealed that only the intensity of intermediate component increases with the number of reused cycles. Further analyses with thermally-stimulated luminescence (TSL) revealed that the intermediate OSL component is related to a TSL glow signal originated from trapping centers having the activation energies of ~0.93 eV and ~1.27 eV and the frequency factors of ~7.52 × 10⁸ s⁻¹ and ~6.29 × 10¹¹ s⁻¹, respectively.