1. Field of the Invention
The present invention relates to a method for producing a photostimulable phosphor used in a radiological image conversion panel. Specifically, the present invention relates to a method for producing a rare earth activated, alkaline earth metal fluorohalide photostimulable phosphor which is expressed by a basic composition formula (Ba1xe2x88x92aMIIa)F(Br1xe2x88x92bIb):zLn.
2. Description of the Related Art
A radiological image recording and reproducing method which uses a photostimulable phosphor has been known as a method which can be used in place of a conventional radiography. This method uses a radiological image conversion panel containing a photostimulable phosphor (storable phosphor sheet). In this method, a photostimulable phosphor of a radiological image conversion panel absorbs radiation which has passed through a subject or been emitted from an object to be examined. Thereafter, the photostimulable phosphor is time serially excited by electromagnetic waves (excitation light) such as visible light rays or infrared rays, thereby allowing the radiation energy stored in the photostimulable phosphor to be released as fluorescence (stimulated emissions). An electric signal is obtained by photoelectrically reading the fluorescence, and the radiological image of the subject or the object to be examined is produced as a visible image based on the obtained electric signal. After the image, which was read but remains on the image conversion panel, is erased, the radiological image conversion panel is prepared for the next projection. Namely, the radiological image conversion panel can be used repeatedly.
As compared to a conventional radiography method using radiographic film in combination with an intensifying screen, the above radiographic recording and reproduction method has the advantage of being able to provide radiological images having ample amounts of information while receiving much less exposure to radiation. Further, in conventional radiography, the radiographic film is consumed each time projection is carried out. In contrast, since the radiological image conversion panel can be repeatedly used in the radiological image conversion recording and reproducing method, it is beneficial from the view points of conservation of resources and economic efficiency.
The photostimulable phosphor is a phosphor which generates stimulated emissions when irradiated with excitation light subsequent to being irradiated with radiation. In practice, a phosphor which generates stimulated emissions in a wavelength range of 300 to 500 nm by irradiating an excitation light in a wavelength range of 400 to 900 nm is generally used. An example of a photostimulable phosphor which has been conventionally used in the radiological image conversion panel is a rare earth activated, alkaline earth metal fluorohalide phosphor.
The rare earth activated, alkaline earth metal fluorohalide phosphor is generally produced by the following method. First, a mixture of phosphor raw materials (a phosphor precursor) is produced by a dry process in which the phosphor raw materials are uniformly mixed in a dry state, or a wet process in which the phosphor raw materials are uniformly mixed together in a slurry state and then dried. Next, the obtained mixture of the phosphor raw materials (the phosphor precursor) is calcined ordinarily at a temperature near a melting point of a host crystal (Ba, FX, and the like) in a neutral or weak oxidizing atmosphere at substantially atmospheric pressure for several hours (calcination process). The obtained calcined product may be re-calcined if desired. The calcination process allows growth of the host crystal of the phosphor, and at the same time, diffuses activator elements (Eu and the like) in the host crystal. Further, an F+-halogen center which serves as a central source of photostimulation is also generated. Accordingly, the calcination process is an important process which influences the light emission characteristics of the phosphor. After the calcination process, the obtained phosphor is subjected to washing, classification, and the like, as necessary.
The rare earth activated, alkaline earth metal fluorohalide phosphor is generally produced as described above. However, various characteristics, for example, sensitivity and erasability, of the rare earth activated, alkaline earth metal fluorohalide phosphor change greatly depending on the production method used. Various approaches have been made in order to solve this problem. Japanese Patent Application Laid-Open (JP-A) No. 8-231952 discloses one approach, which is to improve erasability of the rare earth activated, alkaline earth metal fluorohalide phosphor by annealing at a temperature lower than the calcination temperature (700xc2x0 C. to the melting point) and in the range of 400xc2x0 C. to 800xc2x0 C. (preferably 500xc2x0 C. to 750xc2x0 C).
In practice, however, the processing temperature preferable for a phosphor varies according to the composition (such as iodine content) of the phosphor. For example, the higher the iodine content, the lower the optimum processing temperature. More specifically, for example, when a BaFBrI:Eu phosphor precursor containing 50% of iodine is subjected to annealing (a heat treatment) at a temperature of 650xc2x0 C. or higher after calcination, a problem arises in that sensitivity greatly deteriorates. Moreover, various characteristics including sensitivity and erasability are greatly influenced not only by the annealing temperature and the concentration of oxygen but also by the amount of the phosphor to be processed and processing time. However, the relationship between these factors and the characteristics of the phosphor has been unclear.
Accordingly, an object of the present invention is to solve the conventional problems, namely, to provide a method for producing a photostimulable phosphor, by which method sensitivity and erasability can be obtained at a practical level, and improvements in image quality and a high throughput system can be realized.
The above object is achieved by the following means. The present invention provides a method for producing a rare earth activated, alkaline earth metal fluorohalide photostimulable phosphor which is expressed by a basic composition formula (Ba1xe2x88x92aMIIa)F(Br1xe2x88x92bIb):zLn, wherein MII is at least one alkaline earth metal selected from a group consisting of Sr and Ca; Ln is at least one rare earth element selected from a group consisting of Ce, Pr, Sm, Eu, Tb, Dy, Ho, Nd, Er, Tm, and Yb; and a, b, and z are numerical values of ranges 0xe2x89xa6a less than 1, 0xe2x89xa6bxe2x89xa61, and 0 less than zxe2x89xa60.2, respectively, the method comprising the steps of: calcining a precursor of the rare earth activated, alkaline earth metal fluorohalide photostimulable phosphor to obtain a calcined product; and annealing the calcined product so that the following annealing conditions are met: (1) Tmpxe2x88x92600xc2x0 C.xe2x89xa6Taxe2x89xa6Tmpxe2x88x92300xc2x0 C., wherein Ta is the annealing temperature (xc2x0 C.), and Tmp is the melting point (xc2x0 C.) of a host crystal; and (2) 10xe2x88x925xe2x89xa6Nxc3x97taxe2x89xa61, wherein ta is the processing time (in minutes), and N is the number of oxygen moles/the number of phosphor moles.