The Radiation Effect Mechanism of Some Rare Earth Doped BaF 2 Crystal
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ABSTRACT In this paper, we used the Discrete Variational X,, (DV-Xc,) method to calculate the energy level of some rare earth doped BaF 2 crystals and indicated that the Eu, Dy and Yb elements will change their valence from +3 to +2 under irradiation and thus change the optical properties of the crystals. The results show that this radiation effect model presented agree well with experiments. 1. INTRODUCTION Barium fluoride (BaF 2) was discovered to be a fast inorganic scintillator in 198311]. Considerable attention has been attracted in the fields of nuclear physics, nuclear medicine and high energy physics due to its high density, short radiation length, non-hygroscopic nature and high detection efficiency for -y-rays. Hlowever, when BaF 2 is exposed to a radiation environment, the fluorescence output will decrease, thus the study of the radiation damage mechanism of BaF2 is quite important. The identification of specific causes of radiation damage in any material is an extremely difficult problem. This is because that the radiation damage is an inherently complex process and often depends on a number of different factors.
It is important, however, to distinguish
the radiation damage which is intrinsic to the material of interest from that caused by various type of inlhomogenities, such as impurities or defects. In the case of BaF 2 , there are several indications that the radiation damdage observed so far is due to either impurities or defects[2]. The saturation effect of irradiation indicates that there are a finite number of either impurities or defects which act as sites for color center formation. Once these traps are filled with charges liberated by the radiation, no further damage occurs. These color centers also exhibit well defined absorption bands, which are characteristic of particular impurities in the crystal. Of the impurities related, the rare earth elements need to be carefully investigated, because rare earth in the raw material can be incorporated into BaF 2 easily and then change the radiation damage properities of the crystal. In the work of G. Chen et.al.13], the radiation effect of rare earth doped barium fluoride crystals was investigated. They found that the elements, such as Eu, Dy and Yb, can eliminate color centres in visible range effectively. In this paper, we used the DV-Xc, method to calculate the energy level of rare earth doped BaF 2 crystals, and infer that the Eu, Dy and Yb elements will eliminate color centers caused by irradiation by changing their valence from +3 to +2 under irradiation.
441 Mat. Res. Soc. Symp. Proc. Vol. 348. 01994 Materials Research Society
2. COMPUTATIONAL METHOD We begin with the non-relativistic Hiamiltonian for a system consisting of electrons and nuclei. Omitting small spin-dependent terms, the Hamiltonian has the form:
ft = f, +flN+
N
(1)
where the electronic term is:
ft
Iri --T rjl
-V. +
the nuclear term is: 72
e2
1
h 2
2ri
(2)
ZIZje2
1
and the electron-nucleus interation is:
"11,N
(4)
--2
-
By using the Born-Oppenheimer ap
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