Why does zinc sulfide have to be doped with silver to phosphoresce?

$$\ce{ZnS}$$ is a boring white powder and it doesnt glow in the dark, but if its doped with silver it starts to show some phosphorescent properties.How does silver force zinc sulfide to do that?

• I remember this question was asked by Birks about $30$ years ago at a symposium about the use of $\ce{ZnS(Ag)}$ as a detector of alpha particules in scintillation counting. I also remember he did not get any answer from the audience to his question. Birks was specialized in using this substance for counting the number of alpha particules emitted by a radioactive source. He had published plenty of publications about this domain. Apr 17 at 18:19
• Copper and manganese also can serve as phosphorescence activators in ZnS. It is not the ZnS that luminesces: it is the excited (by light) activator species. A similar case, among many, is ruby fluorescence. It is due to $Cr^{3+}$ ions in the sapphire (crystalline alumina) matrix.
– Ed V
Apr 17 at 18:24
• Why doesnt a $\ce{Cr^3+}$ solution have these properties then? What is special about the lattice it is doped in? Apr 17 at 18:39
• My understanding is it is remotely similar to semiconductors, where there is specific semiconductor matrix with specific doping atoms. But in the scintillator case, it is about stable excitation, instead of conductivity. Apr 17 at 18:43
• All you say is perfectly correct. Nothing to object. Nevertheless nobody knows why only ZnS has this property to such a high degree of phosphorescence. And nobody knows why silver, and no other atom from the periodic table can make it phosphoresce. This is the same for $\ce{NaI(Tl)}$. This doped substance gives a strong fluorescence when irradiated by gamma rays. Why NaI and not KI ? Why use Thallium as doping agent ? Nobody knows ! Apr 17 at 19:21