Mechanism of phototransfer of thermoluminescence peaks in natural CaF2

Trapped charges related to thermoluminescence (TL) glow peaks appearing at high temperatures (⪆ 500 °C) can be transferred to empty trapping centres producing TL glow peaks at lower temperatures by incidence of UV photons. The phototransferred thermoluminescence (PTTL) produced in this way in a natural CaF2 sample is studied as a function of incident UV fluence and the intensity of the residual high-temperature TL. The intensity of the PTTL is found to be directly proportional to the residual TL (RTL). It is also directly proportional to the incident UV fluence except at high UV intensities. The spectra of all the PTTL glow peaks are identical to that of the RTL peak. This spectrum is interpreted as the emission from a Ce3+ substitutional impurity centre perturbed by an O2− impurity in the CaF2 lattice. A self-consistent model is proposed to explain the transfer of TL. The centre for the RTL peak is postulated to consist of Ce3+ and O2− substitutional ions (at Ca2+ and F− sites, respectively), adjacent to each other in the lattice. Irradiation converts Ce3+ to Ce2+ and O2− to O−. UV exposure transfers a hole from this centre to other empty trapping sites inducing PTTL glow peaks.