EPR spectroscopy in LiF:Mg,Cu,P thermoluminescent powder samples irradiated with high gamma doses

Significant changes of the glow curve shape of LiF:Mg,Cu,P have been observed when it is exposed to doses above 50 kGy. In order to clarify one of these changes, i.e., the appearance of a thermoluminescent (TL) peak (the so-called “B” peak) centered above 400 °C, this work investigated the changes induced by γ radiation in the electron paramagnetic resonance (EPR) spectra of LiF:Mg,Cu,P powder used for the production of the MCP-N dosimeters. To achieve this, aliquots were irradiated with 1, 100, 227 and 500 kGy of γ rays (60Co) and preheated at different temperatures up to 500 °C. EPR signals were measured at room and liquid nitrogen temperatures with an X-band spectrometer. Glow curves and TL emission spectra were measured for the same aliquots. EPR spectra were shown to originate mainly from unpaired electrons of Cu2+ ions, occurring in deformed octahedral sites, showing axially symmetric spectroscopic g-factors (g┴ = 2.1937; g║ = 2.0765). Two less intense signals with g-factors equal to 2.008 and 1.989, probably associated with different types of F centers, were observed after irradiation with doses ≥ 100 kGy. The analysis of EPR and TL intensities as a function of radiation doses and preheat temperatures suggested that the creation of the B peak is basically dependent on the electronic trapping states formed by Cu2+ and/or Cu+ ions and the F-related center responsible for the signal with g-factor equal to 1.989. The wavelength of the emitted light (peaked at ~ 360 nm) confirmed that recombination occurs along the same path for both the B peak and the dosimetric peak centered at 220 °C. In contrast to standard TL models, the position of the B peak shifted towards higher temperatures when the dose was increased. These results support the assumption that the origin of the B peak can be explained by localized transitions within spatially correlated defect clusters assembling Cu2+ and Mg2+ ions together with F-related centers.