Peculiarity of the thermoluminescence of ruby

The thermoluminescence (TL) of ruby has some unique peculiarities, including the single TL peak after irradiation at room temperature, compared to four or five peaks for pure sapphire, monotonicity despite high peak temperature, a two-stage afterglow, and the rise in luminescence at constant temperatures some days after termination of irradiation. All these peculiarities can be explained in terms of the transition of Cr⁴⁺ ions which appear in the crystal after irradiation into a big radius state about 50 lattice in radius at increased temperature. The activation energy of the transition is 0.2 eV.