X-ray intensifying screens are generally used in conjunction with silver halide photographic films and serve to enhance the image formed on that film. Phosphors, which are the active component of X-ray screens, are legion in number and include the tungstates, the oxysulfides and oxybromides, among others.
Particularly efficient phosphors in an X-ray intensifying screen, are the tantalates taught in Brixner U.S. Pat. No. 4,225,623, the disclosure of which is incorporated herein by reference. These phosphors are based on yttrium, lutetium and gadolinium tantalates of the M' monoclinic structure. The host tantalate may be activated with niobium or with rare earths, such as terbium and thulium, for example, as well described in the aforementioned patent. Since these phosphors have a high X-ray stopping power, they are widely used in intensifying screens and the methods for their preparation include the mixing of ingredients followed by firing to form the phosphor crystal lattice.
During the firing step, it is often beneficial to use a flux which usually forms a partial liquid at the elevated firing temperatures. Thus, the flux can be thought of as a fluid in which the various phosphor precursors react to form the phosphor. These fluxes are generally alkali metal or alkaline earth salts, including Li.sub.2 SO.sub.4, pure LiCl, BaCl.sub.2, SrCl.sub.2 and mixtures of these salts, for example.
In the process of preparing tantalate phosphors, it has been found that alkali halides and combinations of alkali halides and alkaline earth halides are suitable for use as fluxes in increasing the reaction rate between oxides of yttrium and tantalum. A problem associated with the use of halides is their low melting points and the reactivity of these compounds with materials of construction used in furnaces and crucibles which contain the reaction mixture. The reaction between yttrium oxide and oxides of tantalum and niobium is best run at temperatures above 1200.degree. C. At these temperatures, lithium and sodium chlorides are within 250.degree. C. of their boiling points. Consequently, significant amounts of the chlorides are present as highly reactive gases in the furnace and the crucibles holding the reaction mixture. This condition can lead to shortened equipment lifetimes. Alkali sulfates are more stable in this regard but give phosphor with lower efficiency due to the effect of flux decomposition products. Lithium and sodium sulfates decompose, at the high reaction temperature used to obtain the phosphor, to lithium and sodium oxide, respectively, and oxides of sulfur. Lithium oxide is very reactive and can form lithium tantalate and/or lithium niobate in the phosphor oxide mixture. Lithium tantalate and niobate, however, are not as efficient as, for example, the yttrium tantalate compounds when used as X-ray phosphors. Thus, there is a need to find better flux systems for the preparation of X-ray intensifying phosphors.
The use of a rare earth oxide phosphor containing alkali metal silicates and germanates, is also known. In this particular case, however, the silicate is used as an integral mixture with the phosphor itself and it is reported that the brightness of the rare earth oxide phosphor is increased. The phosphors produced by this technique are not X-ray intensifying phosphors, but are red-emitting phosphors used in cathode ray tubes, for example.
It has been found that the effect of these decomposition products can be minimized by adding small amounts of a sequesterant such as alkali metal metasilicate, e.g., lithium metasilicate, etc., to the flux which reacts with the alkali oxide and produce lithium orthosilicate. It has also been found that tantalate phosphors with very high efficiency can be produced while minimizing effects of flux and decomposition products by combining the stable sulfates with the reactive halides in the presence of small amounts of a sequesterant compound.
The phosphors typically used in X-ray intensifying preparation are a crystalline material with a multifaceted, or polyhedral, shape. For the monoclinic M' tantalate phosphors in particular, polyhedral crystals are used exclusively in the prior art. X-ray intensifying screens utilizing a mixtures of various sized polyhedral phosphor particles are known in the art, as exemplified in Rabatin, U.S. Pat. No. 4,088,894.
It is also known to use mixtures of different polyhedral phosphors, as exemplified by Kano et. al., U.S. Pat. No. 4,042,527; Waller et. al., U.S. Pat. No. 4,011,455; Singh et. al., U.S. Pat. No. 3,621,340; Thornton, U.S. Pat. No. 3,670,194 and Patten, U.S. Pat. No. 4,387,141. These references all disclose mixtures of different polyhedral morphology phosphors for adjusting various components of light output based on competition for excitation source.
There is an ongoing need in the art to improve the resolution for a given phosphor without affecting the speed of the eventual X-ray intensifying screen formed with the phosphor. Screen speed is typically directly related to coating weight, whereby an increase in phosphor coating weight results in a higher speed screen. Unfortunately, as well known in the art, an increase in phosphor coating weight also increases light spread within the phosphor layer, thereby decreasing resolution.
It has now been found that an X-ray intensifying screen having improved resolution at a constant speed and coating weight results when a mixture of tabular and polyhedral shaped tantalate phosphor crystals are used therein.