Compositions – Inorganic luminescent compositions – Zinc or cadmium containing
Reexamination Certificate
1999-05-14
2001-07-03
Koslow, C. Melissa (Department: 1755)
Compositions
Inorganic luminescent compositions
Zinc or cadmium containing
Reexamination Certificate
active
06254806
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to luminescent phosphors and, in particular, those phosphors used in x-ray imaging with CCD and other silicon-based detectors.
BACKGROUND OF THE INVENTION
Luminescent phosphor screens are used in conjunction with CCD detectors (or other silicon detectors) as high resolution, high dynamic range x-ray imagers in x-ray crystallography, medical and industrial imaging. The performance of these devices is heavily dependent on the characteristics of the phosphor material used. Desirable characteristics for a phosphor of this type include high photoluminescent efficiency, peak emission in the red or near-infrared wavelength range, and low afterglow. The desire for high photoluminescent efficiency is due to the obvious advantage of higher sensitivity. Peak emission in the red or near-infrared bands enables the phosphor output to match the maximum quantum efficiency of the silicon detectors. Low afterglow is important for high dynamic range imaging (high afterglow levels lead to ghost images and streaking).
Recently, phosphors based on ZnSe
1-x
Te
x
and ZnSe
1-x
Te
x
:Cu:Cl have been developed. These phosphors show high x-ray luminescence with a peak emission near 650-700 nm (depending on the exact doping composition used), and have an energy efficiency of about 20%. Furthermore, they show low afterglow levels as compared to previously available x-ray phosphors. These phosphors are described in detail in V. Valdna, et al., “ZnSe
1-x
Te
x
solid solutions,”
Joumal of Crystal Growth
, Vol. 161, 1996, pp. 177-180.
The phosphors disclosed by Valdna have many of the necessary qualities of a good imaging phosphor. However, it has been found that, in their basic formulation, phosphors of this type suffer from a nonlinear output in that, during the initial luminescence of the phosphor while being exposed to constant x-ray flux, the luminescence of the phosphors increases over a finite period of time before stabilizing. Moreover, the phosphors exhibit an undesirable amount of afterglow once the x-ray exposure has been discontinued.
SUMMARY OF THE INVENTION
In accordance with the present invention, a phosphor is provided that uses a zinc selenide host material that has a relatively high starting purity and appropriate grain size, typically 1-2 microns median. If necessary, the purity of the host material may be increased by driving off high vapor pressure contaminants with a vacuum purification step. Once a sufficiently pure host material is acquired, a dopant is added. A fluxing agent, such as zinc chloride (ZnCl
2
) is preferably used to facilitate diffusion of the dopant into the host. In the preferred embodiment, the dopant comprises a rare earth element and that rare earth element is combined with free oxygen. One method of providing such a combination is to dope the host with a material having the chemical structure XCl
3
, where Cl is chlorine and X is the desired rare earth material. In the preferred embodiment, the free oxygen is then added by diffusion of a chlorate or nitrate into the host. For example, materials such as potassium chlorate (KClO
3
), silver chlorate (AgClO
4
) or silver nitrate (AgNO
3
) may be used to provide the desired oxygen.
Some examples of rare earth components known to be effective in the aforementioned doping strategy include Europium (Eu), Samarium (Sm), Neodymium (Nd), Gadolinium (Gd), Holmium (Ho), Erbium (Er) and Yfterbium (Yb). Free oxygen is then added to the host. It is also possible to use combinations of rare earth dopants along with the co-dopants used to introduce free oxygen. Some examples of such combinations are Cerium-Terbium (Ce—Tb) and Cerium-Terbium-Erbium (Ce—Tb—Er). Naturally, different rare earth elements or combinations thereof may be selected to achieve a desired wavelength.
In another embodiment of the invention, a dopant is used that has the chemical structure XCl
2
, and which uses a secondary material that is not a rare earth element. These secondary materials may include copper, tellurium, cadmium, silver, potassium, manganese, magnesium, calcium, strontium, and barium. When using one of these secondary materials, it is not necessary to introduce free oxygen into the host, so no oxygen-contributing material (e.g., chlorate or nitrate) is added. Preferably, the dopants in this embodiment are added in an aqueous solution to a molar dopant concentration of approximately 0.0005, with an appropriate flux (e.g., ZnCl
2
) at 0.1-5.0% by weight. If the dopant is added in an aqueous solution, the water should be so-called “ultra-high purity distilled water,” (that is, greater than 18 M&OHgr;-cm resistivity).
In each of the embodiments discussed above, the addition of the dopant is followed by drying of the phosphor and segregation of the zinc selenide grains. The material is then annealed at a high temperature (e.g., 1000-1200° C.) to diffuse the dopants, to remove any fracture defects caused by the milling and, if a rare earth dopant is used, to form complex defects between the rare earth material and the subsequently added chlorate or nitrate. The phosphor is then washed again, dried and loaded into a two temperature zone container. The hofter zone (the zone containing the phosphor powder) is heated to approximately 1000-1200° C., while the container is slowly rotated. This causes the excess (that is, non-stoichiometric) metal components and high vapor pressure impurities to vaporize, and they thereafter condense out on a surface of the container located in the cooler zone. After this process, the material is cooled, and the cooled phosphor is sieved. If wet sieving or sedimentation is used, ultrapure water should be used, and the material should be subsequently heated again in a two-temperature zone container at lower temperatures so as to remove oxide layers that form on the surface of the power granules.
REFERENCES:
patent: 5751108 (1998-05-01), Kanemura et al.
patent: 2-306591 (1990-12-01), None
patent: 2-306585 (1990-12-01), None
V. Valdna et al., ZnSe1-xTexsolid solutions, Journal of Crystal Growth 161 (1996), pp. 177-180.
P. Schotanus et al., Detection of DcS(Te) and ZnSe(Te) scintillation light with silicon photodiodes, IEEE Transactions on Nuclear Science, vol. 39, No. 4, 1992, pp. 546-550.
V.D. Ryzhikov et al., A New ZnSe1-xTexScintillator Luminescence Mechanism, Nucl. Tracks Radlat. Meas., vol. 21, No. 1, 1993, ps. 53-54.
L.P. Gal'chinetskii et al., Determination of Scintillation Efficiency and Photometric Characteristics of X-Ray Phosphors from Results of Emission-Power Measurements, Scientific-Industrial Organization, No. 1, Jan.-Feb., 1991, pp. 88-90.
O.V. Vakulenko et al., Kinetic Characteristics of the X-Ray Luminescence of ZnSe:Te at High Levels of Excitation, Sov. Phys. Tech. Phys. 33(3), Mar. 1988, pp. 384-385.
Durst Roger D.
Valdna Vello
Bruker AX, Inc.
Koslow C. Melissa
Kudirka & Jobse LLP
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