Compositions – Inorganic luminescent compositions – Compositions containing halogen; e.g. – halides and oxyhalides
Reexamination Certificate
2000-06-26
2002-10-01
Koslow, C. Melissa (Department: 1755)
Compositions
Inorganic luminescent compositions
Compositions containing halogen; e.g., halides and oxyhalides
C501S152000, C250S36100C, C250S363040, C250S483100, C250S378000, C250S297000, C250S297000, C250S297000
Reexamination Certificate
active
06458295
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, &ggr; 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 phosphors include a phosphor having a good luminous efficiency, a phosphor having a shorter afterglow (a time required for light to attenuate to {fraction (1/10)} after X-ray irradiation is stopped) is required depending on its application. Specifically, large afterglow of scintillators used for detecting X-rays is particularly problematic in X-ray CT applications, for example, because it makes information-carrying signals indistinct in the time-axis direction. Very small afterglow is therefore required for scintillator material. However, the above-mentioned conventional rare-earth-system phosphors do not satisfy such requirement in afterglow even though they are high in luminous efficiency.
Although YAG-system phosphors (Y
3
(Al,Ga)
5
O
12
) have been also known as a phosphor for electron beams (Applied Physics Letters, Jul. 15, 1967), these phosphors have low X-ray stopping power and can not be practiced in an X-ray detector.
With regard to photodetectors, the peak response wavelength of PIN photodiodes, which is currently used as photodetectors in radiation detectors employed in X-ray CT and the like, is in the red region. In order to improve detection efficiency, phosphors having good wavelength matching with the PIN photodiodes are demanded.
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. 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 very small afterglow and 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 regarding rare-earth element oxide phosphors having Ce as luminous component and, discovering as a result that a phosphor having Gd
3
Al
5−y
Ga
y
O
12
as base material and Ce as an activator (luminous component) has high luminous efficiency and markedly low afterglow, they arrived at present invention. The inventors also conducted an intense study regarding a process for manufacturing the phosphor. As a result, they found that a phosphor having markedly high luminous efficiency can be obtained when potassium compounds are used as flux components for baking starting materials therewith to make scintillator powder.
Specifically, the phosphor of the present invention is a phosphor represented by the general formula
(Gd
1−z−x
L
z
Ce
x
)
3
Al
5−y
Ga
y
O
12
where L represents La or Y, and x, y and z are values falling in the ranges of 0≦z<0.2, 0.0005≦x≦0.02, 0<y<5.
The phosphor of the present invention is a phosphor represented by the above-described general formula and containing a very small amount of potassium.
The phosphor of the present invention is a phosphor represented by the above-described general formula and obtainable by sintering the press-molded starting materials, or by baking starting materials together with a flux component to make scintillator powder and sintering the scintillator powder after the scintillator powder is press molded.
The phosphor of the present invention includes Gd
3
Al
5−y
Ga
y
O
12
as base material and Ce as an activator (luminous component). It absorbs radiation such as X-rays and gamma rays, exhibits yellowish emission due to Ce ion. When such a 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 1.6 times or more than that of the CdWO
4
currently widely used as a scintillator for X-ray CT.
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 Gd (gadolinium) can be replaced with the element La (lanthanum) and/or the element Y (yttrium). 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.
By using Al (aluminum) together with Ga (gallium), 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−y) and Ga (y) is 5 to (Gd+L+Ce)=3 in atomic ratio, and y satisfies 0<y<5, preferably 1.7<y<3.3, more preferably 2≦y≦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 1.5 times or more than that of the CdWO
4
can be obtained.
Ce (Cerium) is an element that serves as an activator (luminous component) in the phosphor of the present invention. The Ce content (x) in (Gd+L+Ce) for generating Ce emission is 0.0005 or greater, prefe
Doi Motomichi
Kobiki Takaaki
Miura Ichiro
Sato Makoto
Yamada Hiromichi
Hitachi Medical Corporation
Koslow C. Melissa
Pennie & Edmonds LLP
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