Compositions – Inorganic luminescent compositions – Compositions containing halogen; e.g. – halides and oxyhalides
Reexamination Certificate
2001-11-28
2004-05-04
Koslow, C. Melissa (Department: 1755)
Compositions
Inorganic luminescent compositions
Compositions containing halogen; e.g., halides and oxyhalides
C427S064000, C427S065000, C427S157000, C427S255230, C427S255280, C427S255390, C427S255395
Reexamination Certificate
active
06730243
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method for recording and reproducing images of objects made by high energy radiation. It relates especially to a method for manufacturing a cesium halide storage phosphor, an more particularly phosphors and a storage phosphor panel containing said phosphors.
BACKGROUND OF THE INVENTION
In computer radiography (CR), a photostimulable phosphor is used, which being incorporated in a panel, is exposed to incident pattern-wise modulated X-ray beam and, as a result thereof, temporarily stores energy contained in the X-ray radiation pattern. At some interval after the exposure, a beam of visible or infra-red light scans the panel in order to stimulate the release of stored energy as light that is detected and converted to sequential electrical signals which are can be processed in order to produce a visible image. For this purpose, the phosphor should store as much as possible incident X-ray energy and emit as little as possible stored energy until stimulated by the scanning beam.
The image quality that is produced by computer radiography largely depends on the construction of the phosphor screen. Generally, the thinner a phosphor screen at a given amount of absorption of X-rays, the better the image quality will be. This means that the lower the ratio of binder to phosphor of a phosphor screen, the better the image quality, attainable with that screen, will be. Optimized sharpness can thus be obtained when screens without any binder are used. Such screens can be produced, e.g., by physical vapour deposition, which may be thermal vapour deposition, sputtering, electron beam deposition or other of phosphor material on a substrate.
Use of alkali metal halide phosphors in storage screens or panels is well known in the art of storage phosphor radiology and the high crystal symmetry of these phosphors makes it possible to provide structured screens and binderless screens. In U.S. Pat. No. 5,055,681 e.g. a storage phosphor screen comprising an alkali metal phosphor in a pile-like structure is disclosed.
In U.S. Pat. No. 5,736,069 an alkali metal storage phosphor is disclosed corresponding to the formula:
M
1+
X·aM
2+
X′
2
·bM
3+
X″
3
:cZ
wherein
M
1+
is at least one member selected from the group consisting of Li, Na, K, Cs and Rb,
M
2+
is at least one member selected from the group consisting of Be, Mg, Ca, Sr, Ba, Zn, Cd, Cu, Pb and Ni,
M
3+
is at least one member selected from the group consisting of Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Al, Bi, In and Ga,
Z is at least one member selected from the group Ga
1+
, Ge
2+
, Sn
2+
, Sb
3+
and As
3+
,
X, X′ and X″ can be the same or different and each represents a halogen atom selected from the group consisting of F, Br, Cl, and I, and 0≦a≦1, 0≦b≦1 and 0<c≦0.2.
In EP-A-174 875 and EP-B-252 991 (and the corresponding U.S. Pat. No. 5,028,509), among other alkali metal stimulable phosphors a CsBr:Eu phosphor is disclosed, wherein Eu is incorporated in the CsBr by firing CsBr with europium oxide.
In U.S. Provisional Patent Application No. 60/142,276, filed Jul. 2, 1999 and U.S. Provisional Patent Application No. 60/159,004, filed Oct. 8, 1999, a novel CsX:Eu has been disclosed wherein the europium dopant is introduced in the CsX by firing CsX with a europium compound containing an halide atom. This phosphor shows high speed and can be used in order to prepare thin binderless screens with good speed.
Nevertheless, since in medical X-ray diagnosis the quest for systems making it possible to lower the patient dose and to keep the image quality still proceeds, there remains a need to have storage phosphors with enhanced speed.
OBJECTS AND SUMMARY OF THE INVENTION
It is a first object of the invention to provide a novel cesium halide phosphor exhibiting high speed.
It is a further object of the invention to provide a method for producing a novel cesium halide phosphor, where such phosphor exhibits a high speed.
It is a still further object of the invention to provide a panel, containing a cesium halide phosphor that exhibits a high speed.
Further objects and advantages of the invention will become clear from the detailed description hereinafter.
The objects of the invention are realized by providing a CsX:Eu phosphor showing upon excitation with light of 370 nm at a wavelength of &lgr;
max
a maximum emission intensity I
0
and at &lgr;
max
+30 nm an emission intensity I, such that I≦0.20 I
0
.
DETAILED DESCRIPTION OF THE INVENTION
It has been found upon experimentation that storage phosphor screens containing a CsX:Eu phosphor showing a narrow emission spectrum upon UV-excitation did show an enhanced speed when used as storage phosphor screen for radiography. It was found that it was especially important that the emission spectrum of the phosphor upon UV-excitation did have a low emission intensity at the higher wavelength region. Storage phosphor screens or panels incorporating such a phosphor did show a speed increase ranging from 50% to a factor 10 and even 20 when compared to screens incorporating phosphors having upon UV excitation a higher emission intensity at the higher wavelength region.
It has been found upon experimentation that the speed increase of phosphor screens, including a CsX:Eu phosphor with a narrow emission spectrum upon UV-excitation, was very pronounced when said CSX:Eu phosphor, was either a CsBr or CsCl phosphor wherein the Eu doping had proceeded by adding a europium compound containing a halogen atom.
It is clear that this higher speed is very beneficial while it gives more degrees of freedom in finding a trade-off between speed a image quality in computer radiographic system using those screens.
It seems that when a CsX:Eu phosphor with narrow emission spectrum upon UV excitation has absorbed X-ray energy, this energy is, upon stimulation, released as emitted light having also a narrow spectral distribution around a maximum emission, so that more useful light is emitted.
A phosphor showing a narrow emission spectrum upon UV excitation is defined as a phosphor showing upon excitation with light of 370 nm a maximum emission intensity I
0
at a wavelength of &lgr;
max
and at &lgr;
max
+30 nm an emission intensity I, such that I≦0.20 I
0
. It is even more preferred that a phosphor with a narrow emission spectrum upon UV excitation shows upon excitation with light of 370 nm a maximum emission intensity I
0
at a wavelength of &lgr;
max
and at &lgr;
max
+30 nm an emission intensity I, such that I≦0.15 I
0
.
The emission spectrum of a CsX:Eu phosphor upon UV excitation can be narrowed by incorporating in the production process of the phosphor a step of maintaining the phosphor for some time, between 10 minutes to about 15 hours, at a temperature between 80° C. and 220° C. Further on in this text the step of maintaining the phosphor for a given time at a given temperature will be called “annealing step”, although it is not sure that strictu sensu an “annealing of the crystal structure” takes place. Preferably during the “annealing step”, the temperature is kept between 100° C. and 180° C. and the time between 30 minutes and 10 hours. Most preferably the “annealing step”, is executed at a temperature between 130 and 170° C. for between 2 and 5 hours. It was found that to some extent, within the limits given above, time and temperature of the “annealing step”, are interchangeable.
Without being bound to any theory, it is believed that in a phosphor according to this invention the distribution of the europium dopant is changed so that the “impurity centre” is more active.
The “annealing step” can be introduced in the production process of the phosphor at different stages.
Thus the invention encompasses a method for preparing a CsX:Eu phosphor comprising the steps of :
mixing or combining (in another way) CsX with between 10
−3
mol % and 5 mol % of a europium compound,
heating (e.g., firing) said mixture at a temper
Leblans Paul
Struye Luc
Afga-Gevaert
Guy Joseph T.
Koslow C. Melissa
Nexsen Pruet , LLC
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