Structural and thermoluminescence properties of borate mineral ulexite

Abstract

Natural ulexite (NaCaB5O6(OH)6.5H2O) mineral is an industrially important boron compound. In this work, the thermoluminescence (TL) properties in ulexite after beta irradiation and subsequent annealing were thoroughly investigated for radiation dosimetry. It has been determined by polarized x-ray fluorescence (PEDXRF) analysis that natural ulexite mineral contains 42.11% of B2O3, 17.93% of CaO, and 4.27% of Na2O in its structure. After the appropriate sintering at 800 °C for 8 h, calcium borate (CaB2O4) and ulexite phases were found in its structure using x-ray diffraction (XRD) analysis. The CaB2O4 rich (CBR) powder samples were doped with different Cu concentrations of 0.01–7% wt, and the appropriate Cu concentration was determined as 1% wt considering the TL signal intensities. The TL glow curve of 1% wt Cu doped CBR sample exhibited five TL peaks at ∼74, 122, 169, 224, and 323 °C for a 2 °C/s heating rate. The activation energies of these peaks were evaluated as ∼0.80, 0.89, 1.08, 1.16, and 1.24 eV, respectively, and the frequency factors were found to be between ∼108 ‒ 1010 s−1. The phosphor showed a sublinear behavior with no saturation up to 100 Gy. Fading of the TL peak at 333 °C after β-ray irradiation (from 90Sr/90Y) and dark storage at room temperature (RT) was recorded as 5% at the end of the 4th week. Through this preliminary study, it can be proposed that ulexite might be a candidate for dosimetric applications if this natural material is further developed using different dopants and various methods.