Thermoluminescence kinetics, time-dependent TL evolution and dosimetric performance of Tb3+-doped NaCa4(BO3)3: Effects of preheating and multi-trap structure

Abstract

Terbium-doped NaCa4(BO3)3 phosphors were synthesized by a microwave-assisted sol–gel combustion route and their thermoluminescence (TL) properties were investigated under β irradiation. The as-irradiated glow curve recorded at IRSL-TL 410 nm displays multiple peaks near 77, 188 and 264 ◦C, indicating the presence of trapping centers with different thermal stabilities. TM–Tstop measurements combined with Initial Rise (IR) analysis reveal a step-like evolution of activation energies from ~1.0 to ~1.7 eV, indicating a multi-trap system with a quasi- continuous distribution of trapping levels. Computerized glow-curve deconvolution (CGCD) further indicates that a relatively large number of overlapping components between ~1.0 and ~1.8 eV contribute before thermal treatment. After preheating, the glow curve can be reproduced using fewer and deeper components in the ~1.5–1.8 eV range. A controlled preheating step at 225 ◦C for 7 s acts as a thermal cleaning stage. This step suppresses shallow and intermediate traps near 80–190 ◦C while largely preserving the high-temperature peak around 260–270 ◦C. Under these optimized conditions, the isolated dosimetric peak exhibits a linear dose response from ~1.4 to at least 350 Gy. The response remains close to linear with a slight sublinear behavior (b ≈0.978), up to 500 Gy based on both peak height and integrated area. Heating-rate-dependent analysis indicates that instrumental temperature lag contributes to the apparent shift of the peak maximum; therefore, quantitative kinetic parameters were derived from the internally consistent uncorrected VHR analysis. Time- resolved storage experiments show a pronounced, non-monotonic TL evolution. The 270 ◦C peak intensity first increases by more than an order of magnitude with storage time at room temperature, followed by delayed fading at longer times. The peak temperature and shape remain essentially unchanged. This behavior is consistent with charge redistribution among shallow, metastable and deeper traps in a competitive multi-trap network. Overall, the results indicate that, when a standardized preheating protocol and controlled storage conditions are applied, Tb3+-doped NaCa4(BO3)3 provides a stable high-temperature TL peak with wide linearity and promising characteristics for medium-to high-dose radiation dosimetry.