Phosphor, and radiation detector and X-ray CT unit each...

Details Phosphor, and radiation detector and X-ray CT unit each... Phosphor, and radiation detector and X-ray CT unit each...

2000-03-07

2002-01-22

Koslow, C. Melissa (Department: 1755)

Compositions

Inorganic luminescent compositions

Compositions containing halogen; e.g., halides and oxyhalides

C250S483100, C250S483100, C378S006000

Reexamination Certificate

active

06340436

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to a rare-earth element oxysulfide 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:T
1
single crystal or CdWO
4
single crystal and a photodiode. 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 include rare-earth element oxide phosphors having a matrix mainly of L
2
O
3
(L representing an element such as Y or Gd) and containing a small amount of activator (Japanese Unexamined Patent Publication No. 64 (1989)-38491, for example) and rare-earth element oxysulfides having a matrix of L
2
O
2
S and containing a small amount of activator. Although the former phosphors can be produced as ones of cubic crystal system and therefore have the advantage of excellent transparency, they have the drawback of being inferior to the latter phosphors in luminous efficiency.
In contrast, rare-earth element oxysulfide phosphors are high in luminous efficiency. Japanese Patent Publication No. 60(1985)-4856, for example, teaches Gd
2
O
2
S:Pr, Ce, F and Japanese Patent PublicationNo. 59(1984)-38280 teaches (Y, Gd, La or Lu)
2
O
2
S:Tb, Ce. The emission peaks of these phosphors differ depending on the activator. A phosphor using Pr as activator emits green light and a phosphor using Tb as activator emits blue or green light. Phosphors using Eu as activator emit red light and are used as color TV phosphors (Japanese Patent Publication No. 47(1972)-13243).
Properties generally required of a scintillator material used in a radiation detector include short afterglow, high emission efficiency, high X-ray stopping power and chemical stability. Large afterglow is particularly a problem 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.
Phosphor afterglow generally includes primary afterglow and secondary afterglow (long-afterglow component). The primary afterglow has a relatively short attenuation period (less than around 2 ms) but the secondary afterglow has a longer attenuation period that is particularly undesirable when the phosphor is used as a scintillator. When the secondary afterglow is large, information-carrying signals become indistinct in the time-axis direction. Secondary afterglow is thought to be caused by the contribution to emission of electrons and holes thermally released from traps formed by phosphor lattice defects. It can be reduced by reducing the number of defects becoming shallow traps or by adding another additive that essentially reduces the action of the shallow traps.
For example, in the case of the rare-earth element oxysulfide phosphor taught by Japanese Patent Publication No. 60(1985)-4856, whose luminous component is Pr, a phosphor capable of utilization as an X-ray CT scintillator is obtained by addition of Ce.
For medical diagnosis applications, however, a detector of still higher detector efficiency is desired in order to minimize the radiation dosage received by the human body while still securing excellent detector efficiency and high SN ratio. In addition, phosphors that use Pr or Tb activators have a problem of low overall detection efficiency of the radiation detector, despite high emission efficiency and short afterglow, since they emit blue or green light and therefore have poor wavelength matching with PIN photodiodes currently used as photodetectors in radiation detectors employed in X-ray CT and the like, owing to the fact that the PIN photodiode's peak response wavelength is in the red region.
An object of the present invention is therefore to overcome these problems of the prior art and to provide a phosphor with very short afterglow and high emission 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 exhibits excellent wavelength matching between the phosphor and the photodetector and is high in detection efficiency (luminous output). 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 emission 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 oxysulfide phosphors having Eu as the luminous component and, discovering as a result that a phosphor of high emission efficiency and greatly reduced secondary afterglow is obtained by adding prescribed components, they arrived at present invention.
Specifically, the phosphor of the present invention is a phosphor represented by the general formula
 (L
1-x-y-z-d
Eu
x
M
y
Ce
z
M′
d
)
2
O
2
S
where L is at least one element selected from the group consisting of Gd, La and Y, M is at least one element selected from the group consisting of Tb and Pr, and M′ is at least one element selected from the group consisting of Ca, Sr and Zn. In addition, x, y, z and d are values falling in the ranges of 0.001≦×≦0.06, 0<y≦12×10
−5
, 0<z≦12×10
−5
, and 0≦d≦2.5×10
−4
.
This phosphor is a rare-earth element oxysulfide phosphor having a matrix of L
2
O
2
S and containing Eu activator component. It absorbs radiation such as X-rays, gamma rays and nuclear radiation, exhibits Eu emission having peaks straddling 600nm, and exhibits numerous line emissions in the range of 450-700nm. When such a phosphor is used as the scintillator of a radiation detector, matching with the photodiode is extremely good and a luminous output can be obtained that is twice or more than that of the CdWO
4
currently widely used as a scintillator for X-ray CT.
Any of Gd, La and Y can be used as the element L. Although two or more of these elements can be used, the X-ray stopping power can be maximized by replacing the L position totally with Gd. The emission characteristics remain substantially the same, however, even if part of the Gd is replaced with La or Y.
Eu is an element that serves as an activator (luminous component) in the phosphor of the present invention. The Eu content for generating Eu emission (x:number of moles replacing 1 mole of element L) is preferably 0.001 or greater. The Eu content x is defined as 0.06 or less for applications requiring high luminous output because a luminous output twice that of CdWO
4
cannot be obtained when the Eu content x exceeds 0.06. More preferably, the Eu content x is defined as 0.002-0.03. About 2.5 times the luminous output of CdWO
4
can be obtained in this range.
Element M and Ce lower the afterglow of the phosphor of the present invention. As pointed out earlier, it is thought that shallow traps produced by phosphor lattice defects contribute to secondary afterglow and that afterglow can reduced by adding another additive that essentially reduces the action of the shallow traps. The inventors found through their research that element M and Ce are elements capable of effectively reducing secondary afterglow in a rare-earth element oxysulfide using Eu as activator.
Either Tb or Pr can be used as the element M and part of Tb can be replaced by Pr. As no effect of reducing afterglow can be obtained with only one of element M and Ce, however, at least one mem

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