An abnormal property of luminescence of ZnS:Eu

2+

is reported.The excitation and emission spectra of ZnS:Eu

2+

are shown in Fig.1. Both are wide bands peaking at 400 nm and 550 nm respectively. This indicates that the luminescence is due to f-d transition of Eu

2+

.The afterglow of the yellow emission was measured.It was surprising to observe that the decay turns out to be tens of minutes long and follow a power law

I

≅

t

-n

with n=1.24 (Fig.2).Since f-d transition is an allowed one

the luminescence corresponding to it should have a very short lifetime

e. g. 1us. So it is believed that this long persistent afterglow must be due to the existence of some kind of electron traps

the origin and the nature of which are to be clarified.