The Mechanism of Slow Component Suppression in Lanthanum Doped Barium Fluoride Crystal
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ABSTRACT The electronic structures of pure BaF 2 crystal and lanthanum doped BaF 2 crystal have been calculated in a self-consistent molecular-cluster model. The cluster is embedded in the crystal lattice and the entire system treated iteratively in the Hartree-Fock-Slater local-density theory. As lanthanum doped BaF 2 is concerned, the obtained results revealed that the F1- which is introduced by the lanthanum may contribute to the suppression of the slow component in the scintillation light of BaF2 crystal. 1. INTRODUCTION Barium Fluoride (BaF 2) is a scintillating crystal, having two fast emission components at 195 nm and 220 nm with a decay time less than 1 ns and a slow emission component at 310 nm with a decay time of 600 nsl1-2]. The fast components are particularly well suited for use at high count rates, but the slow component can cause serious problems with pileup at rates above 106 Ilz per readout elements. The slow component of the luminescence in pure BaF 2 crystal is caused by self-trapped excitons[3]. A self-trapped exciton is a metastable state of an electron bound at a self-trapped hole center[4-5]. The metastable state can either decay radiatively, which yields the luminescence at 310 nm, or decay by thermally activated dissociation of the electron from the self-trapped hole center. Schotanus et.al.[6] found that adding a small amount of lanthanum to the pure crystal can significantly suppress the slow component without reducing the fast components. The mechanism for this quenching was thought to be the La3+ ions in BaF 2 increase the 3 dissociation rate of the self-trapped excitons[61 and/or the La + ions act as electron trapping centers[7]. In this paper we used Hartree-Fock-Slater local-density discrete variational method (HFSXa-DVM) to calculate the electronic structures of pure BaF 2 crystal and lanthanum doped BaF 2 crystal, and revealed that the F7- which is introduced by the lanthanum may contribute to the decrease of the slow component of the lanthanum doped BaF2 crystal. 2. CALCULATIONAL METHOD The lll'S-X,-I)VMj81 1]has been used quite successfully to describe the electronic structures of molecules and solids. The one-electron Hamiltonian may be written as (in Hartree a.u.) 2 :VH. cu(r) + Vzr) = - 1+ o•l + +E+
Z,,
ft[--p(r')dr' _-3a[3p(r) ] I/3
(1)
where T, Vc,0 , and V., are kinetic energy, Coulomb and exchange-correlation potential, respectively. R, and Z, are the position and nuclear charge of the vt•hi atom. p is the molecular charge density. 393 Mat. Res. Soc. Symp. Proc. Vol. 348. 01994 Materials Research Society
The molecular orbitals (MO) %P, are expanded in a linear combination of orbitals (p1 (r) 'I'(r) =
Z e;(r)Cj.
(2)
where the pej(r) used in our calculation are the linear combinations of single site orbitals (SSO)[121 i.e.
vj(r) = WjW•n u.(r,))Y,(iv)
(3)
where the product of the radial function and the real spherical harmonic, unI(r )Ylm(f•), is the SSO centered on the vth nucleus with the principle quantum number n and spherical harmonic quantum numbers 1 and m
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