In a method for vapor-depositing a substrate with a layer of a needle-shaped x-ray fluorescent material containing at least one alkali metal, alkali halogenide phases and an alkali halogenide are mixed in a vapor phase and are vapor-deposited on the substrate. A needle-shaped fluorescent material is thereby produced having the formula ( ( M ′ + ⁢ H ′ - ) a ⁢ ( M ′′ + ⁢ H ′′ - ) ( 1 - a ) ) k ⁢ : ⁢   ⁢ ( M x ′ + ⁢ S y z + ⁢ H x ′ - ⁢ H z * ⁢ y ′′′ - ) b ⁢ ( M x ′′ + ⁢ S y z + ⁢ H x ′′ - ⁢ H z * ⁢ y ′′′ - ) c ⁢ ( M x ′ + ⁢ S y z + ⁢ H x ′′ - ⁢ H z * ⁢ y ′′′ - ) d ⁢ ( M x ′′ + ⁢ S y z + ⁢ H x ′ - ⁢ H z * ⁢ y ′′′ - ) e wherein M+ is at least one metal ion from the group Na, K, Rb and Cs, H− is at least one halogenide from the group F, Cl, Br and I and Sz+ is at least one lanthanide ion from the group La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb or Lu.