EXAFS STUDY OF THE LOCAL ENVIRONMENT OF THE THULIUM ION IN CASO4TM DOSIMETRIC PHOSPHORS

EXAFS (extended X ray absorption fine structure) spectroscopy has been used to study the local structural and chemical environment around th e thulium dopant in CaSO4 dosimetric phosphor obtained both commercial ly and fabricated in this laboratory. In the EXAFS technique, a tunabl e monochromatic X ray source is used to excite selectively photoelectr ons from an inner shell of the atomic species under investigation (in this case, the thulium L edges at 8648 eV). The coherent scattering of the outgoing photoelectron from the neighbouring atoms is then analys ed as a function of X ray beam energy to yield information on the site of the central atom. These experiments were performed at the X-11A be amline at the National Synchrotron Light Source. By comparing the EXAF S from standards such as Tm2O3, it can be shown that a diffusion metho d used for phosphor preparation was not effective for substituting Tm3 + onto Ca2+ sites and an acid distillation method starting from CaSO4: 2H2O was more successful. The phosphor samples where the Tm is shown t o be well incorporated onto Ca2+ cation sites are also those with the better TL dosimetry characteristics.