In a method for producing precursor compounds suitable for use in the production of europium doped cesium halide photostimulable phosphors, at least one of the steps essentially makes use of an ammonium halide salt. The housing has a gas inlet for delivery of gas to the fluid within the frustum shaped housing at a position adjacent to the fluid outlet opening as the fluid moves from inlet to fluid outlet opening. The invention applies more particularly to direct injection diesel engines.