Radiant energy – Invisible radiant energy responsive electric signalling – With or including a luminophor
Reexamination Certificate
2000-07-12
2003-01-07
Hannaher, Constantine (Department: 2878)
Radiant energy
Invisible radiant energy responsive electric signalling
With or including a luminophor
Reexamination Certificate
active
06504156
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to ceramic scintillator material converting radiation such as X-rays into visible light and a manufacturing method thereof, and a radiation detector therewith and a radiation inspection apparatus therewith.
2. Description of the Related Art
In the field of medical diagnosis and industrial nondestructive inspection, inspections using a radiation inspection apparatus such as an X-ray computed tomography apparatus (hereinafter referred to as X-CT apparatus) are in practice. An X-CT apparatus is constituted of an X-ray tube (X-ray source) emitting a fan-shaped X-ray beam and an X-ray detector in which plural X-ray detecting elements are arranged, both of which being disposed facing to each other with a sectional plane of an object as a center. In the X-CT apparatus, while circulating an X-ray tube with respect to an object, a fan-shaped X-ray beam from an X-ray tube is illuminated on the object, X-rays transmitted through the object being detected by the X-ray detector to obtain X-ray absorption data. Thereafter, a computer analyzes the X-ray absorption data to reconstruct a tomogram.
For a radiation detector of the aforementioned X-CT apparatus, detector elements such as solid state scintillator are widely in use. In the radiation detector using the solid state scintillator, due to easiness in downsizing the detector element to increase number of channels, resolution of the X-CT apparatus can be readily improved.
The scintillator, when excited by radiation such as X-rays, emits electromagnetic waves in the wavelengths of visible light or near visible light. As solid-state materials having such scintillation characteristics, single crystals such as NaI, CsI and CdWO
4
, polycrystalline materials (ceramics) such as BaFCl: Eu, LaOBr: Tb, CsI: Tl, CaWO
4
and CdWO
4
(cf. Japanese Patent Publication (KOKOKU) No. SHO 59-45022 and so on official gazette), polycrystalline materials (ceramics) of rare earth oxides having cubic crystal structure such as (Gd, Y)
2
O
3
:Eu (cf. Japanese Patent Laid-open Application (KOKAI) No. SHO 59-27283 official gazette and so on) and polycrystalline materials (ceramics) of rare earth oxysulfide such as Gd
2
O
2
S:Pr (cf. Japanese Patent Laid-open Application (KOKAI) No. SHO 58-204088 official gazette and so on) are known.
Among various kinds of solid-state scintillators such as mentioned above, ceramics of rare earth oxysulfide phosphors in particular, being high in emission efficiency, are suitable for scintillators. Accordingly, a combination of a rare earth oxysulfide ceramic scintillator and a photodiode is coming into wide use as a radiation detector.
The ceramic scintillator materials (phosphor ceramics) like this can be obtained by molding rare earth oxysulfide powder into an appropriate shape, followed by sintering. From the obtained sintered body, planar slabs in disk plate shape or rectangular plate shape are cut out, first. Next, scintillator chips of rectangular bar are cut out from the slabs, followed by slicing each of these scintillator chips into a plurality of segments. A detector element is constituted of a scintillator block in which for instance plural segments are integrated.
Now, as to rare earth oxysulfide phosphor ceramics, in order to improve transparency (light transmittance), sintering properties or the like, various kinds of inventions have been proposed. For instance, Japanese Patent Laid-open Application (KOKAI) No. HEI 7-188655 official gazette discloses that, by reducing contents of impurities in the phosphor ceramics such as Gd
2
O
2
S:Pr or the like, in particular by reducing a content of phosphate group (PO
4
) therein down to 100 ppm or less, light output of the scintillator can be improved.
Further, in Japanese Patent Publication (KOKOKU) No. HEI 5-16756 official gazette, rare earth oxysulfide powder is mixed with fluorides such as LiF, Li
2
GeF
6
and NaBF
4
as sintering aide, followed by sintering the mixture with a hot isostatic press (HIP), thereby obtaining highly densified phosphor ceramics. Here, through densification of the phosphor ceramics, light output of the scintillator is improved.
As mentioned above, as to the transparency and sintering properties of the rare earth oxysulfide phosphor ceramics, so far there have been various kinds of proposals. However, in a recent X-CT apparatus, downsizing of the detector elements is demanded due to higher resolution (multi-channel), and downsizing/lengthening of the detector elements is further demanded due to multi-section tomography. Due to these, new problems are occurring.
That is, due to the tendency of downsizing of the detector element, it becomes necessary to process the phosphor ceramics obtained through the sintering step into scintillator chips of a size of for instance such as a width of 1 mm or less, a length of 20 to 40 mm and a depth of 2 to 3 mm. The scintillator chips of such a size, due to the phosphor ceramics being the polycrystalline body, are liable to cause breaking and chipping during processing and assembling the detectors. Thereby, yield of the ceramic scintillators is deteriorated.
For such points, as described in for instance Japanese Patent Publication (KOKOKU) No. HEI 5-16756 official gazette, densification of the phosphor ceramics is to a certain degree effective. However, in the phosphor ceramics disclosed in the foregoing official gazette, due to the addition of a fluoride as a sintering aide, the sintering aide remains as impurities in the phosphor ceramics to result in deterioration of emission characteristics. This lowers the sensitivity of the ceramic scintillator. Further, in the above official gazette, due to the activity of the sintering aide, part of grains grows in pillar. However, in the phosphor ceramics having such a sintered texture, due to the smaller grain size of other than pillar-shaped grains, sufficient strength can not be obtained. In addition, the light output (sensitivity) also is disadvantageous.
Further, in a trend toward higher resolution of the X-CT apparatus, if artifacts (pseudo-image) would appear when reconstructing a sectional image through computer processing of the X-ray intensities after transmission of an object, this would cause severe problems. The artifact is often caused by local nonuniformity of the sensitivity of the ceramic scintillators. Since appearing of the artifacts is detrimental to medical diagnosis and nondestructive inspection, the ceramic scintillators are demanded to have further uniform sensitivity distribution to cope with the trend toward higher resolution of the CT apparatus.
In making the sensitivity of the ceramic scintillator uniform, in addition to making the properties of each scintillator chip uniform, it is effective to constitute one channel with the plural segments cut out of one scintillator chip. However, the existing phosphor ceramics are liable to break and tip when processing into chips. Accordingly, there is a limit in the length of one scintillator chip. That is, though a longer scintillator chip is demanded, there is a limit in lengthening the existing scintillator chip.
In particular, in the X-CT apparatus for multi-section tomography, one channel is constituted of plural segments. Accordingly, number of segments sliced out of one scintillator chip is necessary to be increased. However, since the scintillator chip cut out of the existing phosphor ceramics can not cope with such a demand, one channel is constituted of segments sliced out of a plurality of scintillator chips.
SUMMARY OF THE INVENTION
Accordingly, an object of the present invention is to provide a ceramic scintillator material that, while maintaining excellent light output, has sufficient mechanical strength capable of coping with downsizing of a detector, and a method for manufacturing thereof. In more specific, the present object is to provide a ceramic scintillator material having mechanical strength capable of putting a long length scintillator chip to practical use. Another object of the present invention i
Fukuta Yukihiro
Oyaizu Eiji
Saito Akihisa
Takahara Takeshi
Tamatani Masaaki
Hannaher Constantine
Kabushiki Kaisha Toshiba
Lee Shun
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