Energy transfer from Ce

3+

to Tb

3+

in various hosts has been normally observed.We report

in this paper

that the energy transfer in the reverse direction

i.e.from Tb

3+

to Ce

3+

in CaS: Ce

Tb phosphor.Excitation spectrum of 500nm emission of Ce

3+

in CaS:Ce consists of two bands.The band peaking at 280nm corresponds to the excitation host and that at 450nm relates to the 4f-5d transition of Ce

3+

.The excitation spectrum of 436nm emission of Tb

3+

in CaS:Tb consists of the 280nm band as well as a band at 320nm

which is a characteristic of Tb

3+

.For CaS:Ce

Tb

however

the excitation spectrum of Ce

3+

emission at 505nm also consists of the 320nm band.This indicates that the excitation energy may be transferred form Tb

3+

to Ce

3+

.Under the excitation of 320nm

CaS:Ce

Tb gives emission both from Ce

3+

and Tb

3+

and the intensity of Ce

3+

emission increases with the concentration of Tb

3+

.The enhancement of Ce

3+

luminescence by codoping of Tb

3+

can be as large as 3 times.The decay time τc of Ce

3+

luminescence in CaS:Ce is less than 1 Us.The decay of Tb

3+

in CaS:Tb consists of a fast decay component τT1=80μs and a slow one τT2=1400μs

the Tb

3+

concentration being 3×10

-5

mol.It can be seen that the decay of Ce

3+

emission in CaS: Ce

Tb has now a slow component

very similar to the decay of Tb

3+

luminescence.On the other hand

the decay of Tb

3+

emission depends on the concentration of Ce

3+

.All these results imply that the energy is non-radiatively transferred from Tb

3+

to Ce

3+

.Owing to the overlapping of the excitation spectrum of Ce

3+

emission in CaS:Ce and the emission spectrum of CaS:Tb

we believe that the energy transfer is a reasonable process.