Compositions – Inorganic luminescent compositions – Compositions containing halogen; e.g. – halides and oxyhalides
Reexamination Certificate
1999-12-23
2002-05-07
Koslow, C. Melissa (Department: 1755)
Compositions
Inorganic luminescent compositions
Compositions containing halogen; e.g., halides and oxyhalides
Reexamination Certificate
active
06383412
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a rare earth element-activated, alkaline earth metal fluorohalide based stimulable phosphor, a rare earth element-activated, alkaline earth metal fluorohalide based stimulable phosphor having a tetradecahedral structure in particular, a process for preparing the phosphor, and a radiographic image conversion panel using the phosphor.
2. Description of the Related Art
As a method which can replace conventional radiography, a radiographic image recording and reproducing method using a stimulable phosphor is known as described, for example, in Japanese Patent Application Laid-Open (JP-A) No. 55-12,145. This method utilizes a radiographic image conversion panel containing the stimulable phosphor (cumulative phosphor sheet) This method comprises the steps of absorbing radiation, which has passed through a subject or been emitted from an object to be examined, into the stimulable phosphor of the panel, then exciting the stimulable phosphor in accordance with a time series by an electromagnetic wave (exciting light) such as visible light, infrared light, or the like to thereby allow the radiation energy accumulated in the stimulable phosphor to emerge as fluorescent emission (stimulable emission), obtaining an electric signal by photoelectrically reading the fluorescence, and subsequently reproducing the radiographic image of the subject or object to be examined as a visible image based on the electric signal obtained. The panel, after being read, is subjected to the erasure of the remaining images so as to be made ready for subsequent photographing. That is, the radiographic image conversion panel can be used repeatedly.
The above-described radiographic image recording and reproducing method provides an advantage that radiographic images having abundant information can be obtained by a radiation dosage far smaller than that in conventional radiography using a combination of a radiographic film and sensitized paper. In addition, whereas a radiographic film is consumed for each photographing operation in conventional radiography, the above-described radiographic image recording and reproducing method makes it possible to use the radiographic image conversion panel repeatedly, thereby providing advantages in terms of the aspects of resource protection and economic efficiency.
The stimulable phosphor is a phosphor which emits stimulable emission when irradiated with exciting light subsequent to irradiation with radiation. In practical terms, generally used is a phosphor which emits stimulable emission in a wavelength range of from 300 to 500 nm by an exciting light in a wavelength range of from 400 to 900 nm. Examples of the stimulable phosphor hitherto used in a radiographic image conversion panel may include a rare earth element-activated alkaline earth metal fluorohalide based phosphor. The basic structure of the radiographic image conversion panel for use in the radiographic image recording and reproducing method is composed of a support and a stimulable phosphor layer provided on the support. However, the support is not necessarily needed if the stimulable phosphor layer is a self-supporting layer. Normally, the stimulable phosphor layer is composed of a stimulable phosphor and a binder which contains and maintains the substance in a state of a dispersion. Further, there has been known a stimulable phosphor layer which does not contain a binder and is composed solely of a stimulable phosphor in a state of a flocculated body formed by vapor deposition or sintering thereof. Furthermore, there has been known a radiographic image conversion panel having a stimulable phosphor layer in which a polymeric substance is impregnated into the gaps in the flocculated body of a stimulable phosphor. Whichever type is selected from these stimulable phosphor layers, the stimulable phosphor emits stimulable emission when irradiated with an exciting light subsequent to irradiation with radiation such as X-rays. Therefore, the radiation transmitted through a subject or emitted from an object to be examined causes the energy in an amount proportionate to the amount of the radiation to be absorbed in the stimulable phosphor layer of the radiographic image conversion panel so that a radiographic image of the subject or the object to be examined is formed as an image of accumulated radiation energy in the panel. This accumulated image can be released as a stimulable emission by the irradiation of the exciting light. Consequently, the photoelectric reading of this stimulable emission and the conversion thereof into an electric signal make it possible to convert the accumulated radiation energy image into a visible image.
The surface of stimulable phosphor layer (i.e., the surface of the layer not facing the support) has normally a protective layer composed of a polymer film or a vapor-deposited film of an inorganic substance to thereby protect the stimulable phosphor layer from chemical deterioration or physical impact.
Since the rare earth element-activated, alkaline earth metal fluorohalide based stimulable phosphor has an excellent sensitivity and provides a radiographic reproduction image having a high level of sharpness when the substance is used as a radiographic image conversion panel, it can be a practically excellent stimulable phosphor. However, as the practical use of radiographic image recording and reproduction method has advanced, there has been a growing demand for further advances in the stimulable phosphor. Because of this demand, after the examination of the grain shape of the rare earth element-activated, alkaline earth metal fluorohalide based stimulable phosphors hitherto utilized, the present inventors have found that these substances are made up of tabular grains. Conventionally known processes for the preparation of the rare earth element-activated, alkaline earth metal fluorohalide based stimulable phosphors comprise the steps of dry-blending or blending by suspending in a water-based medium the material compounds, which are an alkaline earth metal fluoride, an alkaline earth metal halide other than fluoride, a halide of a rare earth element, ammonium fluoride, and so on, firing these compounds, after the addition of a sintering preventing agent if necessary, and pulverizing the fired product. Accordingly, since the pulverizing step after the firing step is substantially essential to the conventional processes, most of the grains of the rare earth element-activated, alkaline earth metal fluorohalide based stimulable phosphors produced in the above-described manner were tabular (hereinafter referred to simply as “tabular phosphor” on occasion).
In a stimulable phosphor layer obtained by coating a mixture of the above-described tabular-grained phosphor and a binder resin solution on a support and drying the coating, the tabular-grained phosphor tends to be arranged such that the surface of the tabular-grained phosphor is parallel to the surface of the support, as illustrated in FIG.
1
. If a radiographic image is stored in a radiographic image conversion panel having a stimulable phosphor layer, in which the tabular-grained phosphor is arranged in the above-described manner, and thereafter the panel is irradiated with an exciting light, the exciting light and the stimulable emission generated tend to extend in a transverse direction (i.e., a direction parallel to the surface of the support) (refer to the horizontal arrow in FIG.
1
). This phenomenon presents a problem that the sharpness of radiographic reproduction images is liable to drop.
Based on the foregoing, in order to inhibit the drop in the sharpness of the radiographic reproduction images in the radiographic image recording and reproduction methods, an attempt to use a stimulable phosphor made up of approximately cubic grains has been proposed, as disclosed in JP-A No. 62-86,086. However, the reproducibility of the process for preparing the stimulable phosphor made up of approximately cubic grains disclosed above is not sufficient for i
Fuji Photo Film Co. , Ltd.
Koslow C. Melissa
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