WO2007004099A1

WO2007004099A1 - Gd2o2s: pr for ct with a very short afterglow due to the use of yb as a scavenger for eu

Info

Publication number: WO2007004099A1
Application number: PCT/IB2006/052050
Authority: WO
Inventors: Cornelis Reinder Ronda; Günter ZEITLER; Herbert Schreinemacher
Original assignee: Philips Intellectual Property and Standards GmbH; Koninklijke Philips Electronics NV
Current assignee: Philips Intellectual Property and Standards GmbH; Koninklijke Philips NV
Priority date: 2005-07-05
Filing date: 2006-06-23
Publication date: 2007-01-11
Anticipated expiration: 2008-01-05

Abstract

The invention relates to a Gd2O2S: M fluorescent ceramic material, in which Yb serves as a scavenger to reduce undesired afterglow by Eu contamination in the Gd2O2S: M fluorescent ceramic material.

Description

Gd₂O₂S: Pr for CT with a very short afterglow due to the use of Yb as a scavenger for Eu.

The present invention is directed to a Gadolinium containing powder contaminated with Europium as well as a fluorescent ceramic contaminated with Europium.

The invention further relates to a method for manufacturing a fluorescent ceramic using single-axis hot pressing.

The invention still further relates to a detector for detecting ionizing radiation.

The invention still further relates to a use of said detector for detecting ionizing radiation.

Fluorescent members for detecting high energy radiation contain a phosphor that can absorb the radiation and convert it into visible light. The luminescent emission thereby generated is electronically acquired and evaluated with the assistance of light sensitive systems such as photodiodes or photomultipliers. Such fluorescent members can be manufactured of single-crystal materials, for example, doped alkali halides. Non-single-crystal materials can be employed as powdered phosphor or in the form of ceramic members manufactured there from.

A typical fluorescent ceramic material employed for detecting high energy radiation is doped Gd₂O₂S. However, the use OfGd₂O₂S is somewhat diminished in case the Gd₂O₂S is contaminated with Europium (Eu), which can result in an undesirable increased afterglow characteristics. Thus, there is a need to overcome this drawback.

This issue is also discussed in the EP 05103102.9 which is hereby incorporated by reference. A first object of the present invention is to provide a scintillating ceramic containing Europium with very short afterglow characteristics.

This object is solved by a ceramic material according to claim 1 of the present invention. Accordingly, a Gd₂O₂S: M fluorescent ceramic material with a very short afterglow is provided whereby M represents at least one element selected from the group Pr, Tb, Dy, Sm, Ce and/or Ho and the Gd₂O₂S: M fluorescent ceramic material comprises additionally:

Europium of < 1 wt. ppm based on Gd₂O₂S, and whereby the content of Cerium is < 0.1 wt. ppm whereby the Gd₂O₂S: M fluorescent ceramic material comprises

Ytterbium.

The inventors have found out that surprisingly the drawbacks of a contamination of Gd₂O₂S with Eu can be overcome or at least significantly reduced when the Gd₂O₂S comprises Ytterbium. It is believed that the role of the Yb is at least to a great deal as follows:

During the operation of the fluorescent ceramic material, part of the metal M in the Gd₂O₂S, which is usually present in the form of trivalent ions is oxidized as represented by the equation I:

M³⁺ -> M⁴⁺ + e ^" (I) In case that M is Praseodymium (which is a preferred embodiment of the present invention, as will be described below) this equation is specified as in the equation Ia:

Pr³⁺ -> Pr⁴⁺ + e ^" (Ia)

A part of the electrons, which are released by the equation I (or Ia) will then react with Europium as shown in equation II:

Eu³⁺ + e ^" -> Eu²⁺ (II)

These Eu²⁺ ions will then - by capture of a hole - be further oxidized back to Eu³⁺ ions, which are however in an excited state and subsequently emit light (several emission lines) in a wavelength range of about 400-750 nm, with its most prominent emission in the range 580 - 640 nm, which shows considerable overlap with the Pr³⁺ emission spectrum (equation III) h⁺ + Eu²⁺ -> Eu³⁺ + hv ( 400- 750 nm) (III) This results in an undesired afterglow of the ceramic material.

Without being determined to this mechanism, the inventors believe that the role of the Yb is that at least a part of the Yb reacts with the electrons released in equation I (or Ia) as shown in equation IV instead of the Europium: Yb³⁺ + e ^" -> Yb²⁺ (IV)

Therefore the afterglow resulting out of the reaction of the Europium will not occur or at least be strongly diminished. The Yb serves in a way as a "scavenger" for the Eu.

In some applications, the Yb will then subsequently react - correspondingly to the reaction of Eu as seen in equation III - as shown in equation V: h⁺ + Yb²⁺ -> Yb³⁺ + hv ( « 970 nm) (V)

However, since the emission lies in a different wavelength area, this can be filtered away without harming the performance behaviour of the ceramic material.

It should be noted that in some applications, no emittance of the Yb is seen, whereas in others there is a rather strong emission. The reason for this is yet unclear, however, due to the different wavelength area of the emission due to Yb³⁺ and Pr³⁺ this is no problem for the proposed embodiment making use of a dedicated filter.

According to a preferred embodiment of the present invention, the content of Yb is in a ratio of Yb to Eu (in wt:wt) of > 0.001: 1 to < 10000 : 1. When the content of the Yb is lower than this margin, the Yb will not serve as a scavenger any more, when the content is higher, undesired side reactions will take place (as will be described further below).

The object of the invention is also solved by a ceramic material according to claim 3 of the present invention. Accordingly, a Gd₂O₂S: M fluorescent ceramic material with a very short afterglow is provided whereby M represents at least one element selected from the group Pr, Tb, Dy, Sm, Ce and/or Ho and the Gd₂O₂S: M fluorescent ceramic material comprises additional:

Europium of < 1 wt. ppm based on Gd₂O₂S, and

Ytterbium wherein the content of Yb is in a ratio of Yb to Eu (in wt:wt) of > 0.001: 1 to < 10000 : 1 In the present invention, M represents at least one element selected from the group Pr, Tb, Dy, Sm, Ce and/or Ho. However, according to a preferred embodiment of the present invention, M represents Praseodymium (Pr).

The introduction of Yb ions can be carried out using aqueous solutions of corresponding salts: YbCl₃, YbBr₃, YbI₃, Yb(NO₃)₃, Yb₂(SO₄)₃ etc. Alternatively, the introduction of dopant ions can be carried out during a mechanical mixture of Gadolinium containing powders, such as Gd₂O₂S, with insoluble compositions comprising the dopant, like oxides, for example Yb₂O₃.

Still alternatively Gadolinium containing powders, such as Gd₂O₂S powder, may be mechanically mixed with water insoluble salts of Yb, like YbF₃, Yb₂S₃, Yb₂O₂S, Yb₂(CO₃)₃, Yb₂(C₂O₄)₃ and the like.

According to a preferred embodiment of the present invention, the doped pigment powder OfGd₂O₂S has a surface according to BET in the range of > 0.01 m²/g and < 1 m²/g, preferably of > 0.05 m²/g and < 0.5 m²/g and more preferably of > 0.1 m²/g and < 0.2 m²/g.

In general, Gadolinium containing powders, like Gd₂O₃, are used for the manufacture Of Gd₂O₂S: M fluorescent ceramic materials. The process for the preparation OfGd₂O₃ and OfGd₂O₂S: M fluorescent ceramic materials is complex and time consuming. However, the characteristics with respect to afterglow and other physical properties cannot be altered arbitrarily for the fluorescent ceramic material once prepared.

However, to obtain the desired very brief afterglow of the Gd₂O₂S: M fluorescent ceramic material according to the present invention it can be preferred that the amount of: - Europium is > 0.05 wt. ppm and < 1 wt. ppm, preferably of > 0.1 wt. ppm and < 0.5 wt. ppm, based on Gd₂O₂S, and/or

Ytterbium is > 0.05 wt. ppm and ≤IOOO wt. ppm, preferably > 0.05 wt. ppm and <500 wt. ppm

Europium can be contained as Eu³⁺, preferably as salt, for example EuCl₃, EuF₃, Eu₂O₂S, Eu₂(CO₃)₃, Eu₂(C₂O₄)₃, and the like. According to a preferred embodiment of the present invention, the content of Yb is in a ratio of Yb to Eu (in wt:wt) of > 0.01 : 1 to < 1000 : 1, preferably > 0. 1 : 1 to < 100 : 1 and most preferred > 1: 1 to < 10 : 1. These margins have been shown in practice to be the best suitable ratios of Yb to Eu (in wt:wt). According to a preferred embodiment of the present invention, the content of Yb is in a ratio of M to Yb (in wt:wt) of > 0.001 : 1 to < 10000 : 1, preferably >0.01 : 1 to < 1000: 1, more preferred > 0.1 : 1 to < 100 : 1 and most preferred > 1: l to ≤ lO : 1. with M being the sum of content(s) of the metals Pr, Tb, Dy, Sm, Ce and/or Ho in the Gd₂O₂S. In case M is Pr, which is a preferred embodiment of the present invention, the content of Yb (Pr in ratio to Yb) according to a preferred embodiment of the present invention is (in wt:wt) of > 0.001 : 1 to < 1000 : 1, preferably > 0.01: 1 to < 100 : 1 and most preferred > 0.1: 1 to < 100 : 1.

In case the content of the Yb (in ratio to M) is too low, the scavenger role of the Yb is somewhat diminished, in case the Yb content is too high, undesired side reaction take place.

Especially in case when M represents Pr, there is the possibility that if the Yb content is too high, a "direct" reaction between Pr and Yb may take place in the ceramic material as shown in equation VI: Pr³⁺ + Yb³⁺ -> Pr⁴⁺ + Yb²⁺ (VI)

It should be noted that it is believed that the above- given mechanism for the "scavenger role" of the Yb (as exemplified by equations IV and V) does not involve direct redox reactions between M and Yb, especially no reactions between Pr and Yb. In contrast it is believed, that the electrons, which are set free in equation I (or Ia) move around the ceramic material, until they react - either with the Eu (undesirable) or with the Yb (as it is the purpose of the present invention or several other reaction pathways which are of minor interest for the present invention.

Therefore it is a preferred embodiment of the present invention to keep the ratio of Yb to M as described above. According to a preferred embodiment of the present invention, the Pr³⁺ concentration in Gd₂O₂S is between > 10 and < 2000 wt. ppm, preferably > 100 to < 1000 wt. ppm and most preferred > 500 to < 1000 wt. ppm. These margins have shown in practice to be suitable in order with the present invention.

According to a preferred embodiment of the present invention, a Gd₂O₂S: M fluorescent ceramic material according to the present invention as described above exhibits an afterglow of > 0 ppm at 0.5s and < 80 ppm at 0.5s and preferably > 17 ppm at 0.5s and < 20 ppm at 0.5s.

However, it is preferred that a Gd₂O₂S: M fluorescent ceramic material according to the present invention, exhibits an afterglow of > 0 ppm at 0.5s and < 50 ppm at 0.5s, preferably > 5 ppm at 0.5s and < 40 ppm at 0.5s, further preferred > 10 ppm at 0.5s and < 30 ppm at 0.5s and more preferred > 15 ppm at 0.5s and < 25 ppm at 0.5s.

According to a preferred embodiment of the present invention, a Gd₂O₂S: M fluorescent ceramic material according to the present invention exhibits a relative light yield in the range of > 120% and preferably more than 230% of the light output OfCdWO₄.

Light output and afterglow was measured with a Hamamatsu PMT and a National Instruments ADC, whereby the photomultiplier is shielded against direct irradiation by lead shield. The afterglow was measured with 120 kV / 100 mA, 80 cm FDD (18-20 mGy/s), 2 s pulse, whereby all afterglow values are given in ppm of stationary signal. The signal values (light output) were measured on 4 x 4 mm² pixels, silicone glued to a photodiode. The afterglow is measured after the X-ray pulse has been switched off.

According to a preferred embodiment of the present invention, the Gd₂O₂S: M fluorescent ceramic material is transparent. It should be noted that the Gd₂O₂S: M fluorescent ceramic material can be yellow colored in case that Ce is present.

According to a preferred embodiment of the present invention, the transmission of the Gd₂O₂S: M fluorescent ceramic material at the wavelength of own emission at about 515 nm is 10% to 70%, preferably 20% to 60% and more preferred > 40% and most preferred > 50%, with respect to a layer thickness of 1.6 mm. The measurements of total transmission were carried out using a Perkin Elmer spectrometer. A further object of the present invention is directed to a Gadolinium containing pigment powder useful in the manufacture of a Gd₂O₂S: M fluorescent ceramic material according to the present invention.

The inventors have surprisingly found that a Gd₂O₂S: M fluorescent ceramic material with reduced afterglow can be obtained if a Gadolinium containing pigment powder is used which contains Yb as described above.

Preferably, the Gadolinium containing pigment powder can be selected from the group comprising Gd₂O₃, Gd₂O₂S and/or Gd₂O₂S : M, wherein M represents at least one element selected from the group Pr, Tb, Dy, Sm, Ce and/or Ho. In general the Gadolinium containing pigment powder is contaminated with Europium, such as Eu³⁺, of > 0.05 wt. ppm to < 1 wt. ppm based on Gd₂O₂S.

If the Gadolinium containing pigment powder comprises > 0.05 wt. ppm Eu³⁺ to < 1 wt. ppm Eu³⁺ the amount of Yb, such as Yb³⁺, added to said powder is according to a preferred embodiment of the present invention > 0.05 wt. ppm Yb to < 1000 wt. ppm Yb, preferably > 0.5 wt. ppm Yb to < 100 wt. ppm Yb, preferably > 1 wt. ppm Yb to < 10 wt. ppm Yb, based on said Gd₂O₂S.

According to a preferred embodiment of the present invention, in said Gadolinium containing pigment powder the ratio of Yb to Eu is adjusted to > 0.001 : 1 to < 10000 : 1, preferably > 0.01: 1 to < 1000 : 1, more preferably ≥ 0. 1: 1 to ≤ 100 : 1 and most preferred > 1 : 1 to < 10 : 1.

According to a preferred embodiment of the present invention the Gadolinium containing pigment powder has a powder grain size of 1 μm to 20 μm

A further object of the present invention is directed to a method for the manufacture of a Gadolinium containing pigment powder contaminated by Europium to be used in the manufacture of a Gd₂O₂S: M fluorescent ceramic material according to the present invention.

The method for the manufacture of a Gadolinium containing pigment powder contaminated by Europium to be used in the manufacture of a Gd₂O₂S: M fluorescent ceramic material with very brief afterglow, comprises the steps: a) detecting the amount of Europium in said Gadolinium containing pigment powder b) adding Yb, preferably in such a way that the ratio of Yb to Eu is adjusted to > 0.001: 1 to < 10000 : 1, preferably > 0.01: 1 to < 1000 : 1, more preferably > 0. 1: 1 to ≤ 100 : 1 and most preferred > 1 : 1 to ≤ 10 : 1.

It can be preferred in the case that the amount of Europium, for example Eu³⁺, is less then 0.05 wt. ppm or less then 0.01 wt. ppm based on Gd₂OiS, Yb may be added to the Gd₂O₂S only in minute amounts.

The method according to the present invention provides measures to avoid the manufacture OfGd₂O₂S: M (GOS) fluorescent ceramic materials having an undesired sustained afterglow. Suitable Europium, such as Eu³⁺, contaminated Gadolinium containing pigment powders can be selected from the group comprising Gd₂O₃, Gd₂O₂S and/or Gd₂O₂S : M

Since Gadolinium containing pigment powders as mentioned below are often contaminated with Europium, such as Eu³⁺, it is suggested by the inventors to detect the quantitative amount of Europium in said Gadolinium containing pigment powder.

Methods for a quantitative analysis of Europium, such as Eu³⁺, are general known in the art. According to the method of the present invention the amount of Europium, for example, the amount OfEu³⁺, can be measured by use of optical spectroscopy. It is most preferred for Eu³⁺ contaminated Gadolinium containing pigment powder to detect the amount of Eu³⁺ based on optical spectroscopy which measure the emission intensity of Eu³⁺ (excitation at about 254 nm UV radiation). An optical spectroscopy method which measures the emission intensity of Eu³⁺ allows an accurate determination of the Eu³⁺ content down to concentrations in the sub ppm range for an Eu³⁺ contaminated Gadolinium containing pigment powder.

The emission radiation of Europium, such as Eu³⁺, contaminated Gd₂O₃ precursor powder at excitation at 254 nm UV - obtained from a low pressure Hg- discharge - radiation provides a red colored visible radiation.

Emission spectra of Europium, such as Eu³⁺, contaminated Gd₂O₂S : M powders show an emission radiation amongst others in the range of 620 nm to 630 nm.

The concentration OfEu³⁺ is given in wt. ppm based on Gd₂O₂S. In a subsequent step Ytterbium, preferably as Yb³⁺, for example YbCl₃, is added, preferably in such a way that the ratio of Yb to Eu is adjusted to > 0.001 : 1 to < 10000 : 1, preferably > 0.01: 1 to < 1000 : 1, more preferably > 0. 1: 1 to < 100 : 1 and most preferred > 1: 1 to < 10 : 1. The content of Yb in the Gd₂O₂S material is preferably > 0.05 and

<10000 ppm, preferably >0.05 and <1000, more preferably > 0.1 and <100 ppm and most preferred >1 and <10 ppm.

The analysis of the Gadolinium containing powder with respect to the concentration Of Eu³⁺ has the advantage that amount of Yb, which is preferred according to the present invention, can easily be maintained simply by adding a suitable Yb source such as YbCl₃. Thus, the manufacture OfGd₂O₂S: M fluorescent ceramic materials (GOS) having an undesired sustained afterglow can be avoided. This can fasten the process of manufacture of the desired Gd₂O₂S: M fluorescent ceramic materials (GOS) having a very brief afterglow. A fourth object of the present invention is directed to a method for the manufacture of a fluorescent ceramic material according to the present invention using hot-pressing, said method comprising the steps: a) selecting a pigment powder of Gd₂O₂S : M as described above whereby the grain size of said powder used for hot-pressing is of 1 μm to 20 μm, and said hot- pressing is carried out at a temperature of 1000° C to 1400° C; and/or a pressure of 100 MPa to 300 MPa; b) air annealing at a temperature of 700° C to 1200° C for a time period of 0.5 hours to 30 hours, and optional between step a) and step b) an additional step c) is carried out, whereby step c) comprises annealing fluorescent ceramic under vacuum at a temperature of 1000° C to 1400° C for a period of time of 0.5 hours to 30 hours.

The pigment powder OfGd₂O₂S can comprise an amount of M from 0.1 ppm to 1000 ppm (weight fraction).

It has been found out that relatively coarse grained powders which are chemically stable in air can be successfully pressed to form a fluorescent crystal with improved characteristics. Thus, according to the present invention it can be preferred that the pressing mode is at a temperature of 1000° C to 1400° C, preferably of 1100° C to 1300° C, more preferably of 1150° C to 1250° C; and/or - a pressure of 100 MPa to 300 MPa, preferably of 180 MPa to 280 MPa and more preferably of 200 MPa to 250 MPa. Preferably, the vacuum during the step of uni-axial pressing according to the present invention is < 100 Pa and > 0.01 Pa.

According to the present invention the vacuum can be adjusted in the range of > 0.01 Pa and < 50 Pa, preferred in the range of > 0.01 Pa and < 10 Pa and most preferred the vacuum is adjusted to the range of > 0.01 Pa and < 1 Pa.

The fluorescent ceramic, after the step of hot-pressing under vacuum, can be further treated by air annealing at a temperature of 700° C to 1200° C, preferably of 800° C to 1100° C, more preferably of 900° C to 1000° C; whereby said time period for air annealing treatment is 0.5 hours to 30 hours, preferably 1 hours to 20 hours, more preferably 2 hours to 10 hours and most preferably 2 hours to 4 hours.

In an embodiment it is preferred that Gd₂O₂S pigment powder used according to the present invention has an average grain size in the range of 1 μm to 20 μm, more preferred of 2 μm to 10 μm and most preferred of 4 μm to 6 μm.

According to the present invention it is advantageous to introduce the vacuum annealing step for still further improving optical properties of resulting ceramics. During this step a further grain growth in the ceramics takes place which further improves transparency due to a decrease in porosity. Next to this, due to the grain growth an additional diffusion of dopant atoms in the lattice of oxysulfide enables still further improving scintillating properties of the ceramics. Therefore, according to one embodiment of the method according to the present invention between step a) and step b) an additional step c) can be carried out, whereby step c) comprises annealing fluorescent ceramic under vacuum at a temperature of 1000° C to 1400° C for a period of time of 0.5 hours to 30 hours.

Preferably, the annealing temperature is selected in the range of 1100° C to 1300° C, more preferably of 1200° C to 1250° C. The time period for vacuum annealing can be preferably set to 1 hour to 20 hours, more preferably to 2 hours to 10 hours and most preferably 3 hours to 5 hours.

The present invention further relates to a detector arranged for detecting ionizing radiation, said detector comprising a fluorescent ceramic as described above whereby the detector is preferably an X-ray detector, CT-detector or Electronic Portal Imaging detector .

The fluorescent ceramic according to the present invention can be used for example in - a scintillator or fluorescent member for detecting ionizing radiation, preferably x-rays, gamma rays and electron beams; and/or an apparatus or device used in the medical field, preferably for computed tomography (CT).

Most preferred at least one fluorescent ceramic according to the present invention can be used for a detector or apparatus adapted for medical imaging.

However, the fluorescent ceramic can be used for any detector known in the medical field. Such detectors are for example X-ray detector, CT-detector, Electronic Portal Imaging detector, and the like.

According to a preferred embodiment, the detector furthermore comprises a filter material which absorbs over the whole wavelength area of > 960 to < 980 nm.

The inventors have found out that in some applications, a fluorescence occurring from the Yb takes place, which is believed to be caused by the mechanism as shown in equations IV and V. This fluorescence is in a different wavelength area than the undesired afterglow occurring from the Eu. Therefore it can be filtered away by using a filter material which absorbs over the whole wavelength area of > 960 to < 980 nm. Preferably, the filter material absorbs over the whole wavelength area of > 950 to < 990 nm, most preferred > 940 to < 1000 nm. According to a preferred embodiment, the detector furthermore comprises a filter material which in addition absorbs over the whole wavelength area of >10 keV and < 200 keV to further shield the radiation sensitive electronics. When using such a filter, the electrical detection means in the detectors, such as a photo diode will not be harmed by the ionizing radiation, which increases the lifetime of the detector. Preferably, the filter material absorbs over the whole wavelength area of > 10 keV to <200 keV, most preferred > 40 keV to < 150 keV . According to a preferred embodiment, the filter material is based on interference, this implies the use of a fulter in which alternatingly materials with low and high refractive indices are used, an example being an interference filter with alternating thin layers of SiO₂ and Ta₂O₅ or SiO₂ and TiO₂. The optical thicknesses of these layers are usually a quarter of a given design wavelength or a multiple thereof. Normally the thickness of an interference filter is of the order of a few millimetres.

The invention further relates to a method of reducing the afterglow in a Gd₂O₂S: M fluorescent ceramic material comprising the following steps:

Adding Yb to the Gd₂O₂S: M fluorescent ceramic material in an amount suitable to serve as a scavenger for the reaction of Europium with electrons in the ceramic material, preferably in a ratio of Yb to Eu (in wt:wt) of > 0.001 : 1 to < 10000 : 1

Optionally filtering fluorescence of Yb by using a filter, which prevents radiation in a wavelength area of > 960 to < 980 nm to reach the detectors.

The aforementioned components, as well as the claimed components and the components to be used in accordance with the invention in the described embodiments, are not subject to any special exceptions with respect to their size, shape, material selection and technical concept such that the selection criteria known in the pertinent field can be applied without limitations.

Additional details, characteristics and advantages of the object of the invention are disclosed in the subclaims, the figures and the following description of the respective figures and examples, which —in an exemplary fashion- show a preferred embodiment of a detector according to the invention.

Fig. 1 shows a very schematic cross-sectional view of a detector according to a first embodiment of the present invention; and Fig. 2 shows a schematic cut-out top-view on the ceramic of Fig. 1 cut approximately at line I-I in Fig. 1.

Fig. 1 shows a schematic cross-sectional view of a detector 1 according to a first embodiment of the present invention. The detector comprises a ceramic material 10 as described above, which emits light upon emittance of ionizing radiation. As can be seen from Fig. 2, the ceramic material is provided in a brick-like matter to prevent optical cross-talk, which would reduce spatial resolution. It should be noted that all dimensions in Figs. 1 and 2 are highly schematic and the actual size and dimensions in the final applications may be greatly different.

Upon absorption of ionizing radiaton, the ceramic material will then itself emit light via fluorescence in the visible wavelength area.

This light will then pass the filter material 30, which is made as described above and enter an electric means, such as a photo diode 40, for further processing. In order to enhance this processing, ceramic material 10 is covered with a reflector 20 to guide all light towards the photo diode 40.

To provide a comprehensive disclosure without unduly lengthening the specification, the applicant hereby incorporates by reference each of the patents and patent applications referenced above.

The particular combinations of elements and features in the above detailed embodiments are exemplary only; the interchanging and substitution of these teachings with other teachings in this and the patents/applications incorporated by reference are also expressly contemplated. As those skilled in the art will recognize, variations, modifications, and other implementations of what is described herein can occur to those of ordinary skill in the art without departing from the spirit and the scope of the invention as claimed. Accordingly, the foregoing description is by way of example only and is not intended as limiting. The invention's scope is defined in the following claims and the equivalents thereto. Furthermore, reference signs used in the description and claims do not limit the scope of the invention as claimed.

Claims

CLAIMS:

1. A Gd₂O₂S: M fluorescent ceramic material with a very short afterglow whereby M represents at least one element selected from the group of Pr, Tb, Dy, Sm, Ce and/or Ho and the Gd₂O₂S: M fluorescent ceramic material comprises additional:

Europium of < 1 wt. ppm based on Gd₂O₂S, and whereby - the content of Cerium is < 0.1 wt. ppm whereby the Gd₂O₂S: M fluorescent ceramic material comprises Ytterbium.

2. The Gd₂O₂S: M fluorescent ceramic material according to claim 1, wherein the content of Yb is in a ratio of Yb to Eu (in wt:wt) of > 0.001 : 1 to

< 10000 : 1

3. A Gd₂O₂S: M fluorescent ceramic material with a very short afterglow whereby M represents at least one element selected from the group Pr, Tb, Dy, Sm, Ce and/or Ho and the Gd₂O₂S: M fluorescent ceramic material comprises additional: Europium of < 1 wt. ppm based on Gd₂O₂S, and Ytterbium wherein the content of Yb is in a ratio of Yb to Eu (in wt:wt) of > 0.001: 1 to < 10000 : 1

4. A Gd₂O₂S: M fluorescent ceramic material according to any of the claims 1 to 3, whereby the content of Yb is in a ratio of Yb to Eu (in wt:wt) of > 0.01 : 1 to < 1000 : 1, preferably > 0. 1 : 1 to < 100 : 1 and most preferred > 1 : 1 to < 10 : 1.

5. A Gd₂O₂S: M fluorescent ceramic material according to any of the claims 1 to 4, whereby the content of Yb is in a ratio of M to Yb of > 0.001 : 1 to < 10000 : 1, preferably > 0.01: 1 to < 1000 : 1, more preferred ≥ 0.1: 1 to ≤ 100 : 1 and most preferred > 1: 1 to < 10 : 1 with M being the sum of content(s) of the metals Pr, Tb, Dy, Sm, Ce and/or Ho in the Gd₂O₂S

6. A detector arranged for detecting ionizing radiation, said detector comprising a fluorescent ceramics according to any one of the preceding claims 1 to 5, whereby the detector is preferably a X-ray detector, CT-detector or Electronic Portal Imaging detector .

7. A detector according to claim 6, whereby the detector furthermore comprises a filter material which absorbs in the whole wavelength area of > 960 to < 980 nm.

8. A detector according to claim 6 or 7, whereby the detector furthermore comprises a filter material which absorbs in the wavelength area of > 10 keV to < 200 keV, most preferred > 40 keV to < 150 keV .

9. Use of a detector according to any of the claims 6 to 8 in an apparatus adapted for medical imaging, wherein the detector is preferably an X-ray detector, CT- detector or Electronic Portal Imaging detector.

10. Method of reducing the afterglow in a Gd₂O₂S: M fluorescent ceramic material comprising the following steps:

Optionally filtering fluorescence of Yb by using a filter which absorbs in a wavelength area of > 960 to < 980 nm.