Lead Tungstate Crystal Electronic Structure And Optical Properties Of The Study

Since PbWO₄(PWO) crystal is of high density, short radiation length, fast decay and cheap price, it has been chosen as a scintillator for detectors at the large collider in CERN. However, the light output of PWO is low in room temperature and its irradiation hardness is variable with different samples. To increase light output and radiation hardness has attracted special interests. The PWO crystal can be easily colored by UV irradiation. The UV irradiated crystal exhibit 4 absorption bands peaking at 350nm, 420nm, 550nm and 680nm, respectively. The 420nm absorption band overlapped with the useful 420nm blue-luminescence band will significantly degrade the light yield. It is important to study the origin of the absorption bands for enhancing the scientillation properties of the crystal. The PWO is a typical non-stoichiometric crystal. There are lead vacancies V_pb^2- and oxygen vacancies V_o²⁺ existed in the PWO crystal. It is very important to study the physical properties of PWO with intrinsic defects and design the color center models.The thesis contains three parts, part I illustrates the experimental results (chapter 3), the part II illustrates the computer simulations of electronic structures and optical properties for the perfect PWO crystal and the PWO crystal contain intrinsic point defects (chapter 4, 5,6), the part III illustrates the origins of the absorption bands and the color center models (chapter 7).In the part I of the thesis, Cooperated with Laboratory of Functional Inorganic Materials, we finished the follow experiments, (1) The absorption spectra of polarized light of the sample with the crystal c-axis parallel to its surface have been measured. Subtracting the polarized light absorption spectrum with the electric vector E parallel to the c-axis by that one with the electric E perpendicular to the c-axis, the polarized light difference spectrum are obtained. The polarized light difference spectrum indicates that the 350nm band has two peaks and can be decomposed into two bands peaking at 330nm and 360nm, respectively. In order to determine the detailed structure of the 350nm band, annealing experiments in air condition of the as-grown crystal at different temperatures were performed. Difference spectra of the annealed crystal have been obtained by subtracting the absorption spectra of the crystal annealed at different annealing temperatures by that one of the as-grown crystal. The absorption spectra features also indicate that the 350nm-absorption band is composed of two bands peaking at 330nm and 360nm, respectively. The annealing properties of the 330nm band and the 360nm band are obviously different. We come to the conclusion that the 350nm band is a composed band and can be decomposed into two bands peaking at 360nm and 330nm, respectively. (2) The 340nm monochromatic light-irradiated crystal exhibits a strong band peaking at 420nm and a broad band in the range of 500-700nm almost in pairs along with a decreasing of the 350nm band. It is found that the 420nm and 500-700nm bands can be reduced, with a re-irradiation of the 411nm monochromatic light, while the...