We have investigated the EPR spectra of LaOBr:Ce

3+

Tb

3+

powder at liquid helium temperature(4.2K).The EPR signals of three samples with differential concentrations of Ce

3+

and Tb

3+

have been obtained on the X-band at 4.2K.The measured results are as follows:ion sample1 2.3Ce

3+

g=2.440 g=2.457Tb

3+

g=3.110 g=3.125The EPR spectra of LaOBr:Ce

Tb powder are computed according to crystalline field theory.The crystalline field Hamiltonian of LaOBr:Ce

3+

Tb

3+

crystal is computed by means of point charge model.Using Stevens’“Operator Equivalents”method

we have obtained the crystalline field Hamiltonian operator:where Amn=-|e|γnm const are known as the “crystal field parameters”

and γnm are coefficients which are related to the crystal structure.The particular form of for LaOBr:Ce

3+

Tb

3+

is: Using the Hamiltonian operator

we have computed the energy level splitting of Ce

3+

and Tb

3+

ions in LaOBr crystal by perturbation theory at representations |LzSz| and |JJz| respectively.On this basis the values of g-factor of Ce

3+

and Tb

3+

ions are obtained:g=2.487 for Ce

3+

iong=3 for Tb

3+

ionAccording to the theory of Ibers ’et al.

through analyzing the line shape of powder EPR spectra.It is verified that the factor of crystal is almost equal to the that of LaOBr:Ce

3+

Tb

3+

powder.The calculated result agrees very well with the experimental result.It is thus affirmed that the dopant Ce

3+

and Tb

3+

ions substitute La3+ ions in host material and form luminescence centers.