Recombination luminescence of Cu and/or Ag doped lithium tetraborate single crystals

Complex investigations of thermostimulated luminescence (TSL) and radioluminescence properties of Li₂B₄O₇ (LTB), LTB:Cu, LTB:Ag and LTB:Cu, Ag crystals suitable for tissue equivalent dosimeters were carried out in the temperature range 4.2-700 K. TSL, cathodoluminescence and X-ray excited luminescence spectra are compared to those measured under photoexcitation. The emission band at 4.6 eV in LTB:Ag is reliably related to Ag⁺ ions based on the comparison of the results of optical spectroscopy studies and first principle calculations. Energy transfer from the relaxed exited state of the Ag⁺ ion to the Cu⁺ ion in double-doped LTB:Cu, Ag crystals is demonstrated. Thermostimulated recombination of charge carriers in irradiated crystals is seen to take place mainly at oxygen sites at low temperatures and at impurity sites at high temperatures. For the first time, the appearance of the lowerature TSL peak at 90 K is assigned to ionic processes in LTB crystals. The appearance of pyroelectric flashes due to the lattice relaxation in the temperature region 90-240 K is demonstrated and their surface-related nature clarified. In accordance with EPR studies the dosimetric TSL peaks in copper and silver doped LTB crystals are attributed to thermally released electrons recombining with Cu²⁺ and Ag²⁺ centres.