In order to interpret the origin of two emission quenching concentrations and a sharp increase of afterglow at certain Sm ~(3+) doping concentration in Ba _2SnO _4:Sm ~(3+) phosphor, the Eu ~(3+) is used as a probe and it reveals that the Sm ~(3+) ions occupy Sn _4 sites at low Sm ~(3+) concentration and then the Ba _2 sites at a high concentration. The Sm ~(3+) centers at different sites lead to two quenching processes and the two quenching concentrations effect. The thermoluminescence reveals that the substitution of Sm ~(3+) for Ba _2 sites will induce a large amount of shallow hole traps and it should be mainly responsible for the sharp enhancement of the afterglow.
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