In a method of preparing a storage phosphor or a scintillator layer on a support by vapor depositing from a crucible unit in a vapor deposition apparatus, while heating as phosphor or scintillator precursor raw materials a matrix component and an activator component or a precursor component thereof, said crucible unit comprises a bottom and surrounding side walls as a container for the said phosphor or scintillator precursor raw materials present in said crucible, said crucible is provided with an internal lid with perforations (5) and said crucible unit further comprises a chimney as part of the said crucible unit and a slit allowing molten, liquefied phosphor or scintillator precursor raw materials to escape in vaporized form under reduced pressure from said crucible unit in order to become deposited as a phosphor or scintillator layer onto said support; and at least one heating means (1) in the chimney (2) is positioned under a heat shield with a slit (3) and a slot outlet (3′), covering thereby said crucible unit and making part of said chimney (2), so that said heating means (1) cannot be observed when looking into the vaporization unit through said slot outlet (3′) from any point in the plane of the said support present as a vapor deposition target in the said vapor deposition apparatus and, while vaporizing said phosphor or scintillator precursor raw materials, a vapor cloud escapes from said slot outlet (3′) in the direction of the said support so that the ratio of the longest radius of the said vapor cloud versus the radius perpendicular thereto, when projected onto the phosphor or scintillator plate or panel from whatever an intersection through the said vapor cloud between slot outlet (3′) and support is at least 1.3, said intersection being taken parallel with the said support.