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ABSTRACT Different impurities influence the radiation damage of barium fluoride (BaF2 ) crystal in different ways. Stable metal elements, such as alkali elements and alkali earth elements, have no effect on the radiation resistance. Most transtion metal elements are harmful to the radiation resistance. There are three kinds of effects of rare earth elements. Stable trivalence elements, such as Y, La and Lu, have no effect on the radiation resistance, while the elements which changes its valence from +3 to +4 under irradiation, such as Ce and Pr, are harmful. The elements which changes its valence from +3 to +2 under irradiation, such as Sm,Eu, Dy and Yb, are useful in eliminating color centers in visible range. Oxygen and hydroxyl ions which were introduced into the crystal from the raw material of BaF 2 or a low vacuum during crystal growing are very harmful to the radiation resistance. The formation of "Complex Color Centers" is considered as the cause of the radiation damage of BaF 2 crystal in this paper.

INTRODUCTION

Barium fluoride crystal has been actively studied as a fast and a high density scintillation material for electromagnetic calorimeters for several years[1-5]. However, the radiation damage of large BaF2 crystal prevents its use in a high radiation enviroment in high energy accelerators.The overall studies of R.Y.Zhu on the radiation damage of induced absorption BaF 2 crystal showed that the damage is radiation instead of the scintillation mechanism damage of the crystal[6,7]. The All damage shows saturation on both transmittance and light output. damage can be recovered by heat annealing or optical bleaching. It is clear that the damage of BaF2 crystal is caused by the formation of color centers, which cause self-absorption of the scintillation light. Some earlier studies also indentified the radiation damage of BaF 2 was In generaly, the caused by radiation induced color centers[8-13] . radiation induced damage of the crystal is caused by the impurities and For improving the radiation defects in the lattice of crystal. resistance of BaF2 , the effect of impurities on the radiation damage of the crystal needs to be investigated. This report summarizes our 435 Mat. Res. Soc. Symp. Proc. Vol. 348. 01994 Materials Research Society

studies in

years past [14-16].

EXPERIMENT Metal ions doping is of use for investigating their effects on the radiation damage of BaF 2 . For doped crystal samples, the raw material was doped with various metal fluorides. As a reference, a undoped crystal was grown in the same furnace at the same time. Therefore, except for the difference of the doping, these two samples were grown under the same conditions. These ingots were then cut and polished to 10 mm thick samples. Other samples for investigating the effects of oxygen ions were also prepared with this thickness.The transmission of these samples were measured before and after gamma-ray irradiation of 1 Mrad.

RESULTS Alkali Elements and Alkali Earth Elements The result shows that alkali elements are not harmful to