Compositions – Inorganic luminescent compositions – Compositions containing halogen; e.g. – halides and oxyhalides
Reexamination Certificate
2000-06-02
2002-03-26
Koslow, C. Melissa (Department: 1755)
Compositions
Inorganic luminescent compositions
Compositions containing halogen; e.g., halides and oxyhalides
Reexamination Certificate
active
06361714
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of preparing a stimulable phosphor which retains part of energy of irradiated radiation for a certain period of time and emits light by a specific electromagnetic wave. Specifically, the present invention relates to a method of preparing a barium fluorohalide based stimulable phosphor which exhibits good light emission performance and good erasability.
2. Description of the Related Art
Conventionally, there has been known a bivalent europium-activated barium fluorohalide phosphor (BaFX: Eu
2+
; wherein X is a halogen atom other than fluorine) which emits light (instantaneous emission of light) in ranges from a near-ultraviolet region to a blue light region by exicitation with radiation such as an X-ray, gamma-ray, electron beam, or ultraviolet beam. The phosphor is used as a phosphor for radiosensitization screens employed in radiography or the like.
Further, it has been recently found that, when the above-described phosphor is exposed to radiation such as an X-ray, gamma-ray, electron beam, or ultraviolet beam, and thereafter, excited with electromagnetic wave (excitation light) in wavelength ranges from a visible region to an infrared region, the phosphor emits light (stimulated emission) in ranges from a near-ultraviolet region to a blue light region. The above-described phosphor has been noted especially as a phosphor useful for a radiation image conversion technique, which is used in place of conventional radiography.
This radiation image conversion technique utilizes a radiation image conversion panel (an accummulative fluorescent sheet) in which a phosphor layer containing a stimulable (accelerated phosphorescent) phosphor is provided on a support. Radiation transmitted through an object or emitted from an object to be examined is absorbed by the stimulable phosphor on the panel. Thereafter, the stimulable phosphor is excited in accordance with a time series by an electromagnetic wave (excitation light) whose wavelength is in a range from a visible region to an infrared region. The radiation energy stored in the stimulable phosphor is thereby made to emerge as fluorescence (stimulated emission), and an electrical signal is obtained by photoelectrically reading the fluorescence. Finally, an image is produced based on the obtained electrical signal.
By using this radiation image conversion panel, an image can be produced at an accessible emission amount which is lower than that in conventional radiography. Moreover, an image obtained by using this panel can be processed by a computer. Therefore, a radiation image which contains much information can be obtained, and defective images can be corrected.
Among the above-mentioned bivalent europium-activated barium fluorohalide phosphors (BaFX: Eu
2+
), in particular, a phophor containing iodine as a part of halogen atom X has a high stimulated emission luminance. As the amount of iodine therein increases, the peak of the stimulated excitation spectrum, shifts toward the longer wavelength side. Accordingly, there has been proposed a method in which the phosphor is used in combination with a laser which emits light whose wavelength is in a red light region (for example, He—Ne laser), or a semiconductor laser which emits light whose wavelength is in a red light region or an infrared region, in accordance with a content amount of iodine.
The radiation image conversion panel itself generally deteriorates very little even with radiation or electromagnetic wave being irradiated thereto. Therefore, the radiation image conversion panel can be used repeatedly over a long period of time. Usually, the radiation energy stored in the panel is read by scanning a laser beam across the panel.
In practice, however, the stored radiation energy cannot be completely emitted from the panel only by using a scanning laser beam. Accordingly, in order to forcibly emit the radiation energy remaining in the panel, there has been proposed, as is disclosed in Japanese Patent Application Laid-Open (JP-A) No. 56-11392, a method in which remaining radiation energy is erased by irradiating light of stimulated emission in an excitation wavelength region onto the entire radiation image conversion panel after the reading.
However, a radiation image conversion panel using a stimulable phosphor which contains iodine has some drawbacks. Generally, as in the case in which the radiation energy stored in the general stimulable phosphor is erased, the remaining radiation energy cannot be sufficiently removed or erased from the panel only by irradiating light emitted from a daylight fluorescent lamp onto the panel in a short time (e.g., several seconds to several minutes). Further, a portion of the remaining radiation energy is recovered (i.e., an after-image emerges) with the passage of time after the erasing.
In a case in which a radiation image conversion panel is repeatedly used, the above-described insufficient erasability adversely affects the quality of a formed image. On the other hand, when the time for erasing is increased for the purpose of complete erasing, the total time required for reading and erasing in a reading device increases. As a result, the processing ability of the device itself deteriorates, and excessive heat is generated in an eraser within the device. These are not preferable in terms of durability of the device and power conservation.
The above-mentioned bivalent europium-activated barium fluorohalide phosphor is generally prepared by the following method.
First, a mixture of phosphor materials is prepared by a dry process in which the phosphor materials are homogeneously mixed in a dry state, or a wet process in which the phosphor materials are homogeneously mixed together in a slurry state and then dried.
Next, the obtained mixture of the phosphor material is fired (burned) ordinarily at a temperature near a melting point of a host crystal (Ba, FX, and the like) in a neutral to weakly oxidizing atmosphere at substantially atmospheric pressure for several hours (firing step). The obtained fired product may be further fired if desired. The firing step 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
+
-center which serves as a central source of stimulation is also generated. Accordingly, the firing step is an important step which exerts an influence upon the light emission characteristics of the phosphor.
After the firing step, the obtained phosphor is subjected to washing, classification, and the like, if necessary.
Further, JP-A Nos. 7-233369 and 10-195431 each disclose a method for preparing a rare earth-activated, alkaline earth metal fluorohalide based stimulable phosphor having a tetradecahedral structure (which hereinafter is simply referred to as a “tetradecahedron-structured phosphor” upon occasion) in which the particle shape and the particle aspect ratio are controlled.
In a radiation image conversion panel having a stimulable phosphor layer which contains the above-mentioned tetradecahedron-structured phosphor, the tetradecahedron-structured phosphor itself has no isotropy and is structured with a low directionality in the stimulable phosphor layer. Therefore, the transverse extension of the excitation light and the stimulated emission can be lessened, and the sharpness of a radiographic image formed on the radiation image conversion panel can be improved. The emission characteristics and sharpness of the tetradecahedron-structured phosphor obtained by the preparation methods disclosed in the above-mentioned publications are high. However, the amount of stimulated emission of this phosphor has not been sufficient for radiation conversion methods. Further, the phosphor does not have erasability sufficient for easy erasure. Therefore, there has been demand for further improvement in the amount of stimulated emission and erasability of the phosphors.
A method for improvement in the erasability of a stimulable phosphor is
Hasegawa Kazuhiro
Iwabuchi Yasuo
Takahashi Kenji
Umemoto Chiyuki
Birch & Stewart Kolasch & Birch, LLP
Fuji Photo Film Co. , Ltd.
Koslow C. Melissa
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