Compositions – Inorganic luminescent compositions – Compositions containing halogen; e.g. – halides and oxyhalides
Reexamination Certificate
2002-10-25
2004-03-16
Koslow, C. Melissa (Department: 1755)
Compositions
Inorganic luminescent compositions
Compositions containing halogen; e.g., halides and oxyhalides
C117S945000, C378S004000, C250S370090
Reexamination Certificate
active
06706213
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a rare-earth element oxide phosphor suitable for use in a radiation detector for detecting X-rays, y rays and the like and particularly for use in the radiation detector of an X-ray CT apparatus, a positron camera or the like. The present invention also relates to a radiation detector and an X-ray CT apparatus using the phosphor.
As the radiation detectors used in X-ray CT apparatuses and the like there have conventionally been used ones combining a xenon gas chamber or BGO (bismuth germanium oxide) single crystal and a photomultiplier tube or combining CsI:Tl single crystal or CdWO
4
single crystal and a photodiode. Properties generally required of a scintillator material used in a radiation detector include short afterglow, high luminous efficiency, high X-ray stopping power and chemical stability. The aforementioned single crystal phosphor, however, has variations in its characteristics and drawbacks in any of deliquescence, cleavage, afterglow (emission after X-ray irradiation is stopped) phenomenon, luminous efficiency and the like.
In recent years, however, rare-earth-system phosphors with high radiation-to-light conversion efficiencies have been developed as scintillators and radiation detectors combining such a phosphor with a photodiode have been put into practical use. Rare-earth phosphors consist of rare-earth element oxide or rare-earth element oxysulfide as base material and an activator as luminescence component. As a rare-earth element oxide phosphor, a phosphor including yttrium oxide or gadolinium oxide as base material has been proposed (Japanese Patent Publication No. 63(1988)-59436, Japanese Unexamined Patent Publication No.3 (1991)-50991, for example). As a rare-earth element oxysulfide phosphor, phosphors including Pr or Ce as the activator have been proposed (Japanese Patent Publication No. 60(1985)-4856).
Although these rare-earth-system phosphors include a phosphor having a good luminous efficiency, sufficient performance cannot always be achieved in such applications as X-ray CT which requires a phosphor having a very short afterglow time (a time required for light to attenuate to {fraction (1/10)} after X-ray irradiation is stopped).
For solving this problem, the inventors has developed and proposed phosphors including gadolinium (Gd), aluminum (Al) and gallium (Ga) as base material and cerium (Ce) as luminescence component, which has high X-ray stopping power and good wavelength matching with photodetectors (Japanese Patent Application No.9(1997)-355073, International Patent Application WO99/33934).
An object of the present invention is therefore to provide a phosphor with very short afterglow and high luminous efficiency that is particularly useful as a scintillator in X-ray CT and the like based on the above-mentioned GdAlGa:Ce phosphor. Another object of the present invention is to provide a radiation detector that is equipped with the phosphor and is high in detection efficiency. Another object of the present invention is to provide an X-ray CT apparatus that is equipped with a radiation detector with high luminous efficiency as a radiation detector and can provide high-resolution, high-quality tomographic images.
DISCLOSURE OF THE INVENTION
In order to achieve the foregoing objects, the inventors conducted an intense study aiming at improving properties such as luminous efficiency and afterglow of the GdAlGa:Ce phosphor including Ce as luminescence component and found as a result that luminous efficiency of the above-mentioned GdAlGa:Ce phosphor can be markedly improved by adding a certain amount of scandium (Sc) to the phosphor.
Specifically, the phosphor of the present invention is a phosphor represented by the general formula
(Gd
1-x-y
L
x
Ce
y
Sc
z
)
3
Al
5-d
Ga
d
O
12
where L represents La or Y, and x, y, z and d are values falling in the ranges of 0≦x<0.2, 0.0005≦y≦0.05, 0<z≦0.03, and 0<d<5.
The phosphor of the present invention is a phosphor represented by the general formula
(Gd
1-y-z
Ce
y
Sc
z
)
3
Al
5-d
Ga
d
O
12
where y, z and d are values falling in the ranges of 0.0005≦y<0.05, 0<z≦0.03, and 0<d<5.
The phosphor of the present invention includes Ce as an activator (luminescence component) and absorbs radiation such as X-rays and gamma rays, exhibits yellowish emission due to Ce ion. When the phosphor is used as a scintillator of a radiation detector, matching with the photodiode is relatively good and a luminous output can be obtained that is 1.8 times or more than that of the CdWO
4
currently widely used as a scintillator for X-ray CT. The luminous output is about 1.2 times that of the GdAlGa:Ce phosphor disclosed in Japanese Patent Application No.9(1997)-355073.
The phosphor is markedly low in afterglow since it contains Ce as luminous ion and its emission attenuates to 10% by about 220 ns (nano-seconds) after X-ray irradiation is stopped and to 2×10
−5
by about 30 ms. Generally phosphor afterglow includes primary afterglow and secondary afterglow (long-afterglow component. In X-ray CT, the secondary afterglow is problematic because information-carrying signals (X-ray) become indistinct in the time-axis direction. The phosphor is markedly low in the secondary afterglow (afterglow after 30 ms), i.e., 2×10
−5
, and therefore excellent in properties suitable for scintillators of X-ray CT.
In the phosphor of the present invention, part of the element gadolinium (Gd) can be replaced with the element lanthanum (La) and/or the element yttrium (Y). In this case, the phosphor remains markedly low in afterglow. However, the content of La or Y (ratio z replacing Gd) should be less than 0.2, preferably less than 0.1, since as the content increases, the luminous efficiency and X-ray stopping power are degraded. The luminous efficiency and X-ray stopping power can be maximized when La or Y is not included.
Scandium (Sc) markedly improves luminous efficiency of the GdAlGa:Ce phosphor when it is included therein. Specifically, the luminous efficiency becomes about 20% higher than that of the phosphor including no Sc. The effect of Sc can be obtained by adding even a very small amount of Sc. However, if the content (z) of Sc exceeds 0.03, the luminous efficiency lowers to the same level as that of the phosphor including no Sc. Accordingly, the content (z) should be 0.03 or less.
By using aluminum (Al) together with gallium (Ga), high luminous efficiency can be obtained. According to the inventors' investigation, it was found that when Gd-oxide-system phosphors containing Ce as luminous component include only one of Al and Ga, that is, base material is Gd
3
Al
5
O
12
or Gd
3
Ga
5
O
12
, they do not exhibit practical amount of emission contrary to YAG-system. However, once Al and Ga were coexistent in the phosphor, the phosphor becomes to exhibit emission and, in addition, have markedly low afterglow. The total content of Al (5-d) and Ga (d) is 5 to (Gd+L+Ce+Sc)=3 in atomic ratio, and d satisfies 0<d<5, preferably 1.7<d<3.3, more preferably 2≦d≦3. When the Al content and Ga content are within the range of from 1.7 to 3.3 respectively, a luminous output that is 1.8 times or more than that of the CdWO
4
can be obtained.
Ce is an element that serves as an activator (luminescence component) in the phosphor of the present invention. The Ce content for generating Ce emission (y) is 0.0005 or greater, preferably 0.001 or greater. The Ce content (y) is defined as 0.05 or less for applications requiring high luminous output because a luminous output 2 times that of CdWO
4
cannot be obtained when the Ce content (y) exceeds 0.05. Preferably, the Ce content (y) is defined as 0.02 or less, more preferably 0.015 or less.
The phosphor of the present invention may contain other elements inevitably included therein. For example, when Gd
2
O
3
is used as a starting material for manufacturing the phosphor of the present invention, Gd
2
O
3
having purity of 99.99% may include 5 wtppm or les
Kanai Tsuneyuki
Kobiki Takaaki
Miura Ichiro
Sato Makoto
Yamada Hiromichi
Antonelli Terry Stout & Kraus LLP
Hitachi Medical Corporation
Koslow C. Melissa
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