Effect of redox reactions on the thermoluminescence characteristics of Cu-doped NaLi2PO4 phosphors

Abstract

A Cu-doped NaLi₂PO₄ phosphor material was successfully synthesized through the high-temperature solid state diffusion method. It was mainly doped with Cu₂Cl₂ and CuCl₂ salts for impurities in the form of Cu⁺ and Cu²⁺, respectively. Formation of the material in the single phase of the phosphor material was confirmed by powder XRD. Morphological and compositional characterization was done using XPS, SEM and EDS techniques. The materials were annealed in reducing, (10% H₂ in Ar) and CO/CO₂ (by burning charcoal in a closed system), as well as in oxidizing (air) atmospheres at different temperatures. ESR and PL studies were conducted for studying redox reactions due to annealing and its effect on TL characteristics. It is known that the impurity Cu could exist in Cu²⁺, Cu⁺ and Cu⁰ forms. The material was doped with two different salts (Cu₂Cl₂ and CuCl₂) as sources of the impurities in two different forms i.e., Cu⁺ and Cu²⁺, however, it was found that it gets incorporated in both the forms inside the material. Also, annealing in different atmospheres not only changed their ionic states but also affected the sensitivity of these phosphors. It was observed that at ∼10 Gy, NaLi₂PO₄:Cu(II) is around 3.3 times, 3.0 times and almost equally sensitive than commercially available TLD-900 phosphor on annealing in air, 10% H₂ in Ar and CO/CO₂ at 400, 400 and 800 °C, respectively. However, NaLi₂PO₄:Cu(I) becomes 1.8 times sensitive after annealing in CO/CO₂ at 800 °C as compared to TLD-900. With high sensitivity, both the materials NaLi₂PO₄:Cu(II) and NaLi₂PO₄:Cu(I) are good candidates for radiation dosimetry with a wide dose response (mGy–5.0 kGy).