Thermally assisted tunneling of charge carriers between trapping sites in calcite (CaCO₃) mineral

Thermally Stimulated Luminescence (TSL) mechanism of natural orange calcite mineral has been studied. The glow curve of X-ray irradiate sample has been studied in unannealed and 673 K annealed samples by a Computerized Glow Curve De-convolution (CGCD) technique. It has been observed that after exposure to X-ray, both the un-annealed and 673 K annealed samples show three distinct peaks at around 323, 360 and 409 K in the glow curves. Study on the effect of post-annealing treatment on the excited sample reveals that the 323 K peak can be thermally bleached by annealing at ∼340 K, however it is very significant that the 360 K TSL peak can't be bleached by increasing the post-annealing temperature up to 380 K. CGCD analysis reveals that the activation energies corresponding to the 323, 360 and 409 K peaks are found to be around 0.70, 0.60 and 1.30 eV respectively. The low activation energy peak observed at relatively high temperature and existence of the 360 K peak after postannealing the sample beyond its peak maximum temperature are due to thermally assisted tunneling of trapped charge from 1.30 to 0.70 eV trapping sites in calcite. This analysis leads to an important conclusion that though theoretically the glow peak observed at relatively high temperature should have high activation energy, but due to charge tunneling phenomenon between the trapping sites, glow peak observed at high temperature may have relatively low activation energy. (paper)