THERMAL ANNEALING OF RADIATION DAMAGED TITANITE

Radiogenic impurities of 400 to 800 ppm U and Th in titanite, CaTiSiO5 , lead to moderate radiation damage (approximate to 1.5 x 10(18) alpha -decay events/g) and therefore to partial amorphization (approximate t o 30%). Powder X-ray diffraction on such damaged titanite from the Car diff locality in Canada shows that two modifications of the crystallin e material coexist. Both modifications are structurally beta phase but differ systematically in their lattice parameters and also in their c hemical composition. One modification exhibits strong particle size br oadening in X-ray diffraction patterns, whereas it is almost unstraine d with respect to fully annealed titanite. The other modification show s large strain broadening and increased specific volume (about 3%) due to a high concentration of defects. The unstrained modification consi sts of small nucleation centers in the damaged material, and it grows when the sample is annealed. At annealing temperatures above 823 K, th is modification dominates rapidly and replaces the strained titanite. The results of Rietveld refinement of the annealed samples and of the time evolution of isothermal annealing studies are discussed. The anal ysis of volume strain and of structural strain resulting from the peak profiles suggests a temperature-dependent activation energy for the r ecrystallization process, with E-A approximate to 380 kJ/mol at T > 87 3 K and E-A approximate to 500 kJ/mol at temperatures 773 K < T < 873 K.