A study on radiation damage mechanism in PbWO4 scintillating crystals

A representative PbWO4 sample was annealed subsequently from 640 to 1040 degrees C in air and the optical absorption spectra were measured after each annealing step. It was found that the intrinsic color center absortion band peaking at 350 nm increased after annealing at lower temperature, then with increasing annealing temperature it decreased continuously until it was removed finally. Based on the analysis of intrinsic defects and corresponding charge compensation of PbWO4 crystals, the creation and transformation of the color center were discussed. This was caused by the diffusion of oxygen, entering into PWO crystals during the annealing process. It was supposed that a Pb3+ --> Pb4+ transformation probably occurred in the annealing process, it would lead to annihilation of Pb3+ centers. The UV radiation induced color center absorptions of PWO crystals annealed at different temperatures were measured and compared. It has been observed that the O- hole centers as the main product of the radiation damage of PWO crystals were produced predominantly from the transformation of the intrinsic Pb3+ hole centers. On the other hand, oxygen vacancies V-O as electron trap centers trapped the electrons during irradiatiion, thus stabilizing O- hole centers. When those two factors have been removed by means of annealing at high temperature in air (oxygen abundant atmosphere), the radiation hardness of PWO crystals can be effectively improved. While, the iron impurity in crystals is very harmful for the radiation hardness of PWO crystals.