Scintillator compositions of alkali and rare-earth tungstates

Details Scintillator compositions of alkali and rare-earth tungstates Scintillator compositions of alkali and rare-earth tungstates

2001-07-30

2003-07-01

Koslow, C. Melissa (Department: 1755)

Compositions

Inorganic luminescent compositions

Tungsten containing

C252S30140R, C252S301170, C423S263000

Reexamination Certificate

active

06585913

ABSTRACT:

BACKGROUND OF THE INVENTION
This invention relates to scintillator compositions and, more particularly, to solid-state X-ray scintillator compositions containing alkali and rare-earth tungstate. The present invention also relates to methods of producing such compositions.
Solid-state scintillator materials are in common use as component of radiation detectors in apparatus such as counters, image intensifiers and computed tomography (“CT”) scanners. Scintillator materials especially find widespread use in X-ray detectors. One embodiment of the present generation of solid-state ceramic scintillators comprises oxide mixtures in which a rare-earth oxide is present as an activator, along with various combined matrix elements, which are also usually rare-earth oxides. Other metallic compounds may also be present as additives for specific purposes. These scintillators have been characterized by the advantageous properties of high efficiency, moderate decay time, low afterglow and little or no radiation damage upon exposure to high X-ray doses.
One important property of CT systems is scan time which is the time required for a CT system to scan and acquire an image of a slice of the object under observation. Scan times of CT systems are related to primary decay time (sometimes simply “decay time” hereinafter) of the scintillator roughly by a factor of 1000. Thus, a scintillator having a decay time of 1 millisecond will typically produce a scan time of about 1 second. The scanning units containing the present generation of scintillators have scan times on the order of 1 second, and in any event no lower than about 0.7 second.
In future generations of CT scanners and the like, shorter scan times are desired. This is true because decreasing scan time makes possible an increase in patient volume covered in a given time, usually a single breath hold. Also, it reduces image blurring due to motion of internal organs and of non-cooperating patients, including pediatric patients.
Scan times of this magnitude may be achievable if the primary decay time of the scanner is shortened. In general, scan time in seconds is associated with a primary decay time of an equal number of milliseconds. As the speed of data processing in CT scanners increases due to advances in electronic circuit designs, it is desired to have faster scintillators, i.e., shorter time between receipts of stimulating radiation pulses so to fully take advantage of the capability of the scanner. Therefore, any measurable percentage decrease in decay time from that exhibited by the present generation of ceramic scintillators would be a distinct improvement, particularly when accompanied by the other advantageous properties described above.
Among the preferred scintillator compositions in the present generation of CT scanners are the ceramic scintillators employing at least one of the oxides of lutetium, yttrium, and gadolinium as matrix materials. These are described in detail, for example, in U.S. Pat. Nos. 4,421,671, 4,473,513, 4,525,628 and 4,783,596. They typically comprise a major proportion of yttria (Y
2
O
3
), up to about 50 mole percent gadolinia (Gd
2
O
3
) and a minor activating proportion (typically about 0.02-12, preferably about 1-6 and most preferably about 3 mole percent) of a rare earth activator oxide. Suitable activator oxides, as described in the aforementioned patents, include the oxides of europium, neodymium, ytterbium, dysprosium, terbium, and praseodymium. Europium-activated scintillators are often preferred in commercial X-ray detectors by reason of their high luminescent efficiency, low afterglow level, and other favorable characteristics. Europium is typically present therein in amounts up to 30 and most often up to about 12, preferably in the range of 1-6 and most preferably about 3 mole percent. Decay times of such scintillators are on the order of 0.9-1.0 millisecond.
The search thus continues for ceramic scintillator compositions having shorter decay times in combination with the aforementioned other advantageous properties.
SUMMARY OF THE INVENTION
The present invention provides improved scintillator compositions comprising alkali and rare-earth tungstates useful in the detection of high-energy radiation, such as X, &bgr;, or &ggr; radiation. Particularly, the scintillators of the present invention have higher light output, reduced afterglow, short decay time, and high X-ray stopping power in X-ray detection applications.
The scintillator compositions of the present invention are alkali and rare-earth tungstates and have a general formula of AD(WO
4
)
n
; wherein A is at least one element selected from the group consisting of Na, K, Rb, and Cs; D is at least one rare-earth element selected from the group consisting of Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu; and n is greater than or equal to 2. Such a tungstate is also commonly referred to as a pyrotungstate because each unit cell contains multiple WO
4
2−
ions. When n is equal to 2, it is commonly referred to as a double tungstate.
According to one aspect of the present invention, the scintillator composition has a formula of CsY
1−x
Gd
x
(WO
4
)
2
or CsLa
1−y
Lu
y
(WO
4
)
2
, wherein 0≦x≦1 and 0≦y≦1, and is useful as an X-ray scintillator. Such a scintillator efficiently absorbs X radiation and emits electromagnetic radiation in the visible region.
According to another aspect of the present invention, a method for producing a scintillator composition comprises the steps of: (1) providing amounts of (a) oxygen-containing compounds of at least one alkali metal selected from the group consisting of Na, K, Rb, and Cs, (b) oxygen-containing compounds of at least one rare-earth element selected from the group consisting of Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu, and (c) at least one oxygen-containing compound of tungsten; (2) mixing together the compounds to form a mixture; (3) optional adding at least one fluxing compound selected from the group consisting of halides of Na, K, Rb, Cs, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, and W in the mixture in a quantity sufficient to act as a flux; and (4) firing the mixture at a temperature and for a time sufficient to convert the mixture to a solid solution of alkali and rare-earth tungstate.
In another aspect of the present invention, a solution of (a) compounds of at least one alkali metal selected from the group consisting of Na, K, Rb, and Cs, (b) compounds of at least one rare-earth element selected from the group consisting of Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu, and (c) at least one compound of tungsten is precipitated in a basic solution to obtain a mixture of oxygen-containing compounds of at least one alkali metal, at least one rare-earth metal, and tungsten. The precipitate is calcined in an oxidizing atmosphere and then fired at a temperature for a time sufficient to convert the calcined material to a solid solution of alkali and rare-earth tungstate.
In still another aspect of the present invention, an alkali and rare-earth tungstate having a formula of AD(WO
4
)
2
; wherein A is at least one element selected from the group consisting of Na, K, Rb, and Cs; and D is at least one rare-earth element selected from the group consisting of Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu; is incorporated in an X-ray detector of a CT system.
Other features and advantages of the present invention will be apparent from a perusal of the following detailed description of the invention and the accompanying drawings in which the same numerals refer to like elements.


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Trunov et al, “Double Tungstates of Caesium

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