Compositions – Inorganic luminescent compositions – Compositions containing halogen; e.g. – halides and oxyhalides
Reexamination Certificate
2000-08-09
2002-07-16
Koslow, C. Melissa (Department: 1755)
Compositions
Inorganic luminescent compositions
Compositions containing halogen; e.g., halides and oxyhalides
Reexamination Certificate
active
06419853
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of preparing a rare earth activated, alkaline earth metal fluorohalide based phosphor which can be used for a radiation image conversion panel which is favorably used in the medical field and the like.
2. Description of the Related Art
As a method used in place of a conventional radiography, there has been known a radiation image recording and reproducing method which uses a stimulable phosphor as disclosed, for example, in Japanese Patent Application Laid-Open (JP-A) No. 55-12145. This method uses a radiation image conversion panel containing a stimulable phosphor (i.e., accumulative phosphor sheet). In this method, a stimulable phosphor of a radiation image conversion panel absorbs radiation transmitted through a subject or emitted from an object to be examined. Thereafter, the stimulable phosphor is excited over time by an electromagnetic wave (excitation light) such as a visible ray or an infrared ray to thereby allow the radiation energy stored in the stimulable phosphor to be released as fluorescence (stimulated emission). An electric signal is obtained by photoelectrically reading the fluorescence, and the radiation image of the subject or the object to be examined is produced as a visible image on the basis of the obtained electrical signal. After the image remaining on the image conversion panel whose reading has been completed is erased, the radiation image conversion panel is ready for the next photographing. Namely, the radiation image conversion panel can be repeatedly used.
In accordance with the radiation image conversion recording and reproducing method, as compared with a case of using radiography in which a combination of a conventional radiographic film and an intensifying screen is used, there is an advantage in that it is possible to obtain a radiation image having a large amount of information with a much less exposure (radiation) dose. Further, in the conventional radiography, the radiographic film is used up each time photographing is carried out. In contrast, in the radiation image conversion method, since the radiation image conversion panel can be repeatedly used, it is beneficial from the viewpoints of conservation of resources and economic efficiency.
The stimulable phosphor generates stimulated emission when irradiated with excitation light subsequent to irradiation with radiation. In practice, generally used is a phosphor which generates stimulated emission in a wavelength range of 300 to 500 nm by an excitation light in a wavelength range of 400 to 900 nm being irradiated thereon. An example of the stimulable phosphor which has been conventionally used in the radiation image conversion panel is a rare earth activated, alkaline earth metal fluorohalide based phosphor. A radiation image conversion panel used in the radiation image recording and reproducing method is formed by, as a fundamental structure, a support and a stimulable phosphor layer which is provided on a surface of the support. However, the support is not necessarily needed if the stimulable phosphor layer is a self-supporting layer. The stimulable phosphor layer is generally formed by a stimulable phosphor and a binder which contains and supports the stimulable phosphor in a dispersed state. However, 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 an aggregate formed by vapor deposition or sintering thereof. Further, there has also been known a radiation image conversion panel which has a stimulable phosphor layer in which a polymeric substance is impregnated into the gaps in the aggregate of a stimulated phosphor. Whichever type of the stimulable phosphor layers is selected, the stimulable phosphor has a property of emitting a stimulated emission when the phosphor absorbs radiation such as X-rays and is then irradiated with excitation light. Therefore, the radiation transmitted through the subject or emitted from the object to be examined is absorbed in the stimulable phosphor layer of the radiation image conversion panel in proportion to the radiation dose (amount of emitted or transmitted radiation). A radiation image of the subject or the object to be examined is formed on the panel as an image in which radiation energy is stored. The image in which radiation energy is stored can be emitted as stimulated emission by irradiating excitation light thereon, and the image in which radiation energy is stored can be imaged by photoelectrically reading the stimulated emission and converting it into an electric signal.
The surface of the stimulable phosphor layer (i.e., the surface of the layer not facing the support) generally has a protective layer which is formed 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.
In practical use, the rare earth activated, alkaline earth metal fluorohalide based stimulable phosphor is an excellent stimulable phosphor because it has an excellent sensitivity and provides a radiographic reproduction image having a high level of sharpness when used for a radiation image conversion panel.
However, as the practical use of the radiation image recording and reproducing method has advanced, there has been a growing demand for a stimulable phosphor which has a higher stimulated emission luminance (i.e., higher sensitivity, higher sharpness, and the like).
An observation has been carried out, by using a scanning electron microscope (SEM), on the surface of a calcined product (unclassified) of the conventionally used rare earth activated, alkaline earth metal fluorohalide based phosphor (i.e., rare earth activated, alkaline earth metal fluorohalide based phosphor which is obtained by mixing at least particles of BaFBr phosphor material (hereinafter occasionally referred to as “BFB”) and particles of BaFI phosphor material (hereinafter occasionally referred to as “BFI”) and calcining the resulting mixture). As a result, it has been observed that BFB particles having small particle diameters are sintered around BFI particles having large particle diameters, thereby forming a number of large over-sintered particles. Formation of such over-sintered particles causes a problem in that the stimulated emission luminance decreases. Further, when a large number of these over-sintered particles are formed, another problem arises in that the yield of the resulting phosphor also decreases. Accordingly, the demand for a stimulable phosphor having a higher stimulated emission luminance can be satisfied by overcoming these problems.
SUMMARY OF THE INVENTION
The present invention accomplished the following objects by overcoming the conventional problems and satisfying the above-mentioned demand. Namely, an object of the present invention is to provide a method of preparing a rare earth activated, alkaline earth metal fluorohalide based stimulable phosphor by which a high yield of a rare earth activated, alkaline earth metal fluorohalide based stimulable phosphor having a high stimulated emission luminance can be obtained.
Means for overcoming the aforementioned problems is as follows.
There is provided a method of preparing a rare earth activated, alkaline earth metal fluorohalide based phosphor comprising the steps of: (a) providing a phosphor material, which includes BaFBr particles, and a phosphor material, which includes BaFI particles, wherein the average particle diameter of the BaFI particles is no more than four times the average particle diameter of the BaFBr particles; (b) mixing the phosphor materials together to thereby obtain a mixture of the phosphor materials; and (c) calcining the mixture of phosphor materials.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Hereinafter, the present invention will be described in detail.
The phosphor preparation method of the present invention includes a step of mixing phosphor materials, a calcining step, and other steps if necessary.
Birch & Stewart Kolasch & Birch, LLP
Fuji Photo Film Co. , Ltd.
Koslow C. Melissa
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