WO2013140310A2

WO2013140310A2 - Method for producing a scintillator for an x-ray detector

Info

Publication number: WO2013140310A2
Application number: PCT/IB2013/051992
Authority: WO
Inventors: Onno Jan WIMMERS
Original assignee: Koninklijke Philips N.V.
Priority date: 2012-03-19
Filing date: 2013-03-13
Publication date: 2013-09-26
Also published as: WO2013140310A3

Abstract

A method for producing a scintillator for an X-ray detector and a respective scintillator (3) and X-ray detector (1) is described. The method comprises providing a particles-in- binder scintillator substrate (11) comprising scintillating particles (13) incorporated into a binder matrix material (15) and subsequently abrading a surface (17) of the scintillator substrate (11). The abrading procedure may be performed using e.g. chemical- mechanical polishing. The lower surface (17) may be abraded such that scintillating particles (21) incorporated in the binder matrix material are flattened such that a surface of the scintillating particles (21) is planar and coincides with the surface (17) of the scintillator substrate (11). Due to reduced surface irregularities, on the one hand, and due to the fact that more scintillating material directly abuts a lower surface (17) of the scintillator substrate (11) compared to an unabraded conventional scintillator, imaging sharpness and light output of the X-ray detector using such polished scintillator may be significantly improved.

Description

METHOD FOR PRODUCING A SCINTILLATOR FOR AN X-RAY DETECTOR

FIELD OF THE INVENTION

The present invention relates to a method for producing a scintillator for an X- ray detector and to a method for producing an X-ray detector. Furthermore, the present invention relates to a scintillator and an X-ray detector as may be produced with the proposed method.

BACKGROUND OF THE INVENTION

X-ray detectors are used in various appliances for detecting X-rays. For example, in medical appliances, X-rays generated by an X-ray source, may be transmitted through a patient and information about internal X-ray absorbing structures within the patient may be determined based on detecting a two-dimensional distribution of the X-rays transmitted through the patient using an X-ray detector.

One type of X-ray detector uses scintillating material which emits light for example in the visual spectral range upon absorption of X-rays wherein this emitted light is detected with a photodetector.

A scintillating material may be provided in different forms. For example, a scintillator for an X-ray detector may use a single crystal of a scintillating material.

Alternatively, a scintillating material may be applied to a substrate using vapor deposition as described e.g. in US 6,936,304 B2, or DE 2 832 141. As a further alternative, scintillating material may be provided as a fine powder and may then be compacted and sintered, using e.g. hot pressing, thereby forming a polycrystalline ceramic-like scintillator as described e.g. in US 5,521,387.

One specific type of scintillator uses a so-called particles-in-binder (PIB) scintillator substrate. Therein, particles comprising a scintillating material are incorporated into a binder matrix material. The binder matrix material encloses the scintillating particles. While, during fabrication of the particles-in-binder scintillator substrate, the binder matrix material may be liquid, in the final product, the binder matrix material is in a solid state after for example a hardening processing, thereby providing for a sufficient mechanical integrity of the scintillator substrate. Particles-in-binder scintillator substrates may beneficially serve as a scintillator for an X-ray detector as they may be easily produced at low cost. However, it has been observed that X-ray detectors using such particles-in-binder scintillator substrates do not always reproducibly exhibit high imaging sharpness and light output.

SUMMARY OF THE INVENTION

There may be a need for a method for producing a scintillator for an X-ray detector, for producing an X-ray detector and for a scintillator or X-ray detector producible by such methods which allows high imaging sharpness and light output for a resulting X-ray detector.

Such a need may be met by the subject-matter of the independent claims. Advantageous embodiments are defined in the dependent claims.

According to a first aspect of the present invention, a method for producing a scintillator for an X-ray detector is proposed. The method comprises steps of providing a particles-in-binder scintillator substrate comprising scintillating particles incorporated into a binder matrix material and, subsequently, abrading a surface of the scintillator substrate.

According to a second aspect of the invention, a method for producing an X- ray detector is proposed to comprise providing of a scintillator being produced with a method according to the above first aspect of the invention and then attaching the scintillator in close contact to a photo- sensitive surface of a photodetector.

According to a third aspect of the invention, a scintillator for an X-ray detector is proposed to comprise a particles-in-binder scintillator substrate comprising scintillating particles incorporated into a binder matrix material. Therein, at a flattened surface, the scintillator substrate comprises scintillating particles being flattened with a surface of the scintillating particles being planar and coinciding with the surface of the scintillator substrate.

According to a fourth aspect of the invention, an X-ray detector comprises such a scintillator according to the above third aspect of the invention and furthermore comprises a photodetector wherein the flattened surface of the scintillator is attached to a photo-sensitive surface of the photodetector.

A gist of the invention may be seen as relating to the following ideas and observations:

In X-ray detectors using a scintillator, the scintillator is typically attached to a photo-sensitive surface of a photodetector. This can be done, for example, by clamping or gluing. An imaging quality of the X-ray detector may be deteriorated due to an unevenness of the surface of the scintillator lying on the photo-sensitive surface of the photodetector as light may be scattered in all directions before being absorbed in the photosensitive surface of the photodetector.

For specific types of scintillators, it has therefore been proposed to flatten a surface of the scintillator before attaching it to the photo- sensitive surface of the

photodetector. For example, the above-cited prior art references propose to polish a surface of a scintillator layer prepared by depositing a luminophore from a vapor phase on a substrate. The luminophore may be for example CsLTlI. Scintillators produced by such vapor deposition are known to have a needle-like uneven surface such that it was proposed to smoothen such extremely uneven surface using a polishing agent to abrasively erode the luminophore layer to a prescribed uniform layer thickness and to fill any fissures on the luminophore layer.

Furthermore, polishing of a surface of a scintillator being made from sintered powder particles has been proposed in the above-mentioned prior art references. Such polycrystalline ceramic-like scintillators are known to have uneven surfaces resulting from their powder-based nature and resulting from manufacturing processes used for compacting and sintering the scintillating powder particles.

However, for particles-in-binder scintillators, no additional processing for flattening a surface of the scintillator has been proposed yet. To the contrary, it has been assumed that such particles-in-binder scintillators exhibit a sufficiently smooth and flattened surface as the binder matrix material enclosing the scintillating particles is initially provided in a liquid form during manufacturing of such PIB scintillator substrates such that any excessively uneven surface should be prevented.

The inventors of the present application have found that, contrary to previous assumptions, abrading a surface of such PIB scintillator substrate may significantly enhance an imaging sharpness and light output of an X-ray detector using such scintillator attached to a photo-sensitive surface of a photodetector.

A process of abrading a surface of the PIB scintillator substrate may include various techniques for removing material from such surface including, inter alia, polishing, grinding, lapping, sanding, etching, etc. Typically, it may be sufficient to abrade only a thin layer from the surface of the PIB scintillator substrate, such thin layer having a thickness e.g. in a range of 0.1 to 10 μιη. Particularly, it may be advantageous to abrade a surface of the particles-in- binder scintillator substrate such that scintillating particles in the binder matrix material are flattened such that a surface of the scintillating particles is planar and coincides with the surface of the scintillator substrates.

In other words, the surface of the scintillator substrate may be abraded to a degree such that not only binder matrix material is removed but also part of the material of the scintillating particles is removed by abrasion. Accordingly, while with an unabraded surface of a PIB scintillator, a significant unevenness or roughness may result from both local protrusions and a general roughness due to the size of the scintillating particles, not only the protrusions may be flattened by abrading the surface of the scintillator substrate but also the scintillating particles itself may be partially abraded. Therein, an abrading procedure should be performed such that the scintillating particles are not broken out of the enclosing binder matrix material but remain within the binder matrix material and are abraded together with the binder matrix material.

As a result, the flattened surface of the scintillator substrate may be abraded to a degree such that maximum surface irregularity at the abraded surface are smaller than an average size of the scintillating particles comprised in the particles-in-binder scintillator substrate.

Such smoothened scintillator substrate surface having at most very small surface irregularities may be attached to a photo-sensitive surface of a photodetector in very close contact such that no significant light scattering occurs. Furthermore, due to the abrading processing, a significant portion of the exposed surface of the scintillator substrate comprises scintillating material instead of binder matrix material originally enclosing the scintillating particles such that the scintillating material may be in very close contact to the photosensitive surface of the photodetector. Both effects may provide for improved imaging sharpness and light output of the X-ray detector.

The abrasion processing may be performed using chemical-mechanical polishing as developed for semiconductor industry. Such chemical-mechanical polishing or planarization is a process of smoothing surfaces with a combination of chemical and mechanical forces, and may be thought of as a hybrid of chemical etching and free abrasive polishing. The process may use an abrasive for corrosive chemical slurry such as a colloid in conjunction with a polishing pad and retaining ring. The pad and the substrate may be pressed together by a dynamic polishing head and held in place by a retainer. By rotating a dynamic polishing head with different axes of rotation, material may be removed. Any irregular topography of the substrate surface may be evened out. In semiconductor industry appliances, such chemical-mechanical polishing may result in extremely planar surfaces with remaining irregularities being only at An gstrom level.

Alternatively, other abrading processes creating a smooth, planarized surface e.g. by rubbing and/or using chemical action, may be used.

At the abraded flattened surface, a surface roughness (Ra) to be achieved may be in a range of below 10 nm, preferably below 2 nm and more preferred below 0.5 nm.

The scintillating particles comprised in the particles-in-binder scintillator substrate may comprise a scintillating material including GdOS:Tb. Particles-in-binder scintillators comprising scintillating particles of such material have shown high scintillating efficiency and may be produced at low cost.

The binder matrix material for the particles-in-binder scintillator substrate may comprise an organic material. For example, epoxy in a solvent may be used as binder matrix material.

Possible features and advantages of embodiments of the invention are described herein with respect to different subject-matters. Particularly, some features and advantages are described with reference to the proposed method for producing a scintillator for an X-ray detector whereas other features and advantages are described for the produced scintillator or X-ray detector. One skilled in the art will understand that the various features may be combined or exchanged in order to result in additional embodiments and possibly enable synergy effects.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will be described with reference to the attached drawings. However, neither the drawings nor the description shall be interpreted as limiting the scope of the present invention.

Fig 1 schematically shows an X-ray detector with an unabraded scintillator attached to a photodetector.

Fig. 2 schematically shows an X-ray detector with a scintillator wherein a surface of a scintillator substrate has been abraded according to an embodiment of the present invention. Fig. 3 shows an MTF ratio between a polished and an unpolished PIB scintillator substrate used in an X-ray detector in dependence from a spatial frequency.

The figures are only schematically and not to scale.

DETAILED DESCRIPTION OF EMBODIMENTS

Fig. 1 schematically shows a cross-section of a conventional X-ray detector 1 ' . A scintillator 3' is glued via a gluing layer 5' to a photo-sensitive surface 7' of a photodetector 9'. The scintillator 3' comprises a particles-in-binder scintillator substrate 11 ' comprising roundish scintillating particles 13' incorporated into a binder matrix material 15'. The PIB scintillator substrate 11 ' may have been produced by mixing a powder of scintillating particles 13' into an originally liquid binder material 15' and subsequently hardening the binder matrix material 15'. Accordingly, the binder matrix material 15' completely encloses the scintillating particles 13' incorporated therein. As a result, a lower surface 17' of the PIB scintillator substrate 11 ' comprises a relatively smooth contour.

However, the lower surface 17' of the PIB scintillator substrate 1 Γ may show some macroscopic unevenness. Accordingly, a gap between the lower surface 17' of the PIB scintillator substrate 11 ' and an upper photo-sensitive surface 7' of the photodetector 9' may have varying dimensions. This gap is filled with the gluing layer 5'. Accordingly, due to differing refractive indices of the glue the gluing layer 5' and the materials comprised in the scintillator substrate 11 ', an imaging sharpness of the X-ray detector Γ may suffer from the varying gap dimensions.

Furthermore, as shown in Fig. 1, the roundish scintillating particles 13' are arranged adjacent to the lower surface 17'. Due to the roundish geometry of the scintillating particles 13' and their small dimensions, only a minor fraction of the entire lower surface 17' is created by the scintillating material of the scintillating pixels 13', whereas a major portion of the lower surface 17' is created by the binder matrix material 15'. Accordingly, light generated within the scintillating particles 13' upon impact of X-rays generally has to be transmitted through binder matrix material 15' first before entering the gluing layer 5' and then reaching the photo-sensitive surface 7' of the photodetector 9'. Accordingly, a significant portion of such scintillating light is scattered in various directions thereby reducing imaging sharpness and light output.

Fig. 2 shows an X-ray detector 1 produced using a method in accordance with an embodiment of the present invention. Therein, a lower surface 17 of a particles-in-binder scintillator substrate 11 has been polished thereby abrading partially the lower surface 17 of the scintillator substrate 11. The polished lower surface 17 is planar such that a gap between the photo-sensitive surface 7 of the photodetector 9 and the lower surface 17 of the scintillator substrate 11 has a constant width. Again, this gap is filled with a gluing layer 5 thereby fixing the scintillator substrate 11 to the photodetector 9.

Due to the constant width of the gap, imaging sharpness of the X-ray detector 1 may be improved and light output may be increased.

Furthermore, as shown in Fig. 1, abrading the lower surface 17 of the scintillator substrate 11 results in the scintillating particles 13 arranged at such lower surface 17 being partially abraded. Accordingly, such abraded lower-most scintillating particles 21 are flattened at their lower side such that a lower surface 19 of the abraded scintillating particles is planar and coincides with the surface 17 of the scintillator substrate 11.

Accordingly, a major portion of the lower surface 17 of the scintillator substrate 11 is created by the material of the scintillating particles 21 instead of being created by the material of the binder matrix as is the case for the conventional X-ray detector shown in Fig. 1. As a result, at least light generated in such lower- most scintillating particles 21 is not necessarily scattered within the binder matrix material 15 but may directly enter the intermediate gluing layer 5 and may then be transmitted directly to the photo-sensitive layer 7 of the photodetector 9. Consequently, light scattering is reduced and imaging sharpness and light output is improved.

Furthermore, with the planar abraded lower surface 17 of the scintillator 3, a thickness of the gluing layer 5 may be thinner than with an irregular lower surface 17' of an unabraded scintillator 3' as shown in Fig. l. Such thinner gluing layer 5 may further increase the sharpness of the resulting photodetector.

The lower surface 17 of the particles-in-binder scintillator substrate 11 may be abraded by chemical-mechanical polishing using for example a polisher as is conventionally used in semiconductor appliances. The polished scintillator substrate 3 may be clamped to a photo-sensitive surface 7 of a photodetector 9,

Fig. 3 shows a graph representing a ratio of an MTF (modulation transfer function) acquired before and after chemical-mechanical polishing a lower surface of a particles-in-binder scintillator substrate. It can be clearly seen that an improvement in MTF was measured. This illustrates that polishing the lower surface of a GdOS:Tb-based PID scintillator substrate improves the sharpness of an image of an X-ray detector. LIST OF REFERENCE SIGNS:

1 X-ray detector

3 scintillator

5 glue layer

7 photo-sensitive surface

9 photodetector

11 particles-in-binder scintillator substrate

13 scintillating particles

15 binder matrix material

17 lower surface of scintillator substrate

19 lower surface of abraded scintillating particle

21 abraded scintillating particle

Claims

CLAIMS:

1. A method for producing a scintillator (3) for an X-ray detector (1), the method comprising:

providing a particles-in-binder scintillator substrate (11) comprising scintillating particles (13) incorporated into a binder matrix material (15);

abrading a surface (17) of the scintillator substrate (11).

2. The method of claim I, wherein the surface (11) is abraded such that scintillating particles (21) incorporated in the binder matrix material (15) are flattened such that a surface (19) of the scintillating particles (21) is planar and coincides with the surface (17) of the scintillator substrate (11).

3. The method of claim 1 or 2, wherein the surface (17) of the scintillator substrate (11) is abraded to a degree such that maximum surface irregularities at the abraded surface are smaller than an average size of the scintillating particles (13) comprised in the particles-in-binder scintillator substrate (11).

4. The method of one of claims 1 to 3, wherein the surface (17) is abraded using chemical-mechanical polishing.

5. A method for producing an X-ray detector, the method comprising:

providing a scintillator (3) being manufactured with a method according to one of claims 1 to 4; and

attaching the scintillator (3) in close contact to a photosensitive surface (7) of a photodetector (9).

6. The method of claim 5, wherein the scintillator (3) is glued onto the photosensitive surface (7) of the photodetector (9).

7. A scintillator (3) for an X-ray detector (1), comprising:

a particles-in-binder scintillator substrate (11) comprising scintillating particles (13) incorporated into a binder matrix material (15);

wherein, at a flattened surface (17), the scintillator substrate (11) comprises scintillating particles (21) being flattened with a surface (19) of the scintillating particles (21) being planar and coinciding with the surface (17) of the scintillator substrate (11).

8. The scintillator of claim 7, wherein the flattened surface (17) of the scintillator substrate (11) comprises maximum surface irregularities being smaller than an average size of scintillating particles (13) comprised in the particles-in-binder scintillator substrate (11).

9. The scintillator of claim 7 or 8, wherein the scintillating particles (13) comprise a scintillating material comprising GdOS:Tb.

10. The scintillator of one of claims 7 to 9, wherein the binder matrix material comprises an organic material.

11. The scintillator of one of claims 7 to 10, wherein the binder matrix material comprises epoxy.

12. The scintillator of one of claims 7 to 11, wherein, at the flattened surface, a surface roughness (Ra) is in a range of below 10 nm.

13. An X-ray detector ( 1 ) comprising :

a scintillator (3) according to one of claims 7 to 12; and

a photodetector (9);

wherein the flattened surface (17) of the scintillator (3) is attached to a photosensitive surface (7) of the photodetector (9).

14. X-ray detector of claim 13, wherein the flattened surface (17) of the scintillator (3) is glued to the photosensitive surface (7) of the photodetector (9).