EP1015910A1

EP1015910A1 - Scintillation detector, refractor coating for scintillator and method for making such a coating

Info

Publication number: EP1015910A1
Application number: EP98944014A
Authority: EP
Inventors: Rémi CHIPAUX; Marie Geleoc; Philippe Belleville; Benoít Lambert
Original assignee: Commissariat a lEnergie Atomique CEA
Current assignee: Commissariat a lEnergie Atomique et aux Energies Alternatives CEA
Priority date: 1997-09-18
Filing date: 1998-09-17
Publication date: 2000-07-05
Also published as: WO1999014618A1; FR2768522B1; JP2001516888A; FR2768522A1

Abstract

The invention concerns a scintillation detector comprising: a block of scintillating material (110); a refractor coating (150), made of a solid material, with low index of refraction, formed on the block of scintillating material; and optionally a so-called overall layer (152) made of a light-diffusing or light-reflecting material, formed on the refractor coating (150). The invention is useful for making segmented detectors.

Description

SCINTILLATION DETECTOR, REFRACTOR COATING FOR SCINTILLATOR AND METHOD FOR MANUFACTURING SUCH

COATING

Technical area

The present invention relates to an optical coating comprising at least one light-refracting layer, and usable for trapping light in a material. The invention more particularly finds applications for trapping scintillation light in a scintillator crystal.

Scintillators are generally in the form of blocks of crystalline inorganic material, called scintillating. This material is capable of transforming all or part of the energy of an ionizing radiation passing through it into scintillation light. Scintillators are used in the manufacture of devices for detecting ionizing radiation in which they are associated with photodetectors sensitive to scintillation light.

Generally, in a scintillation detection device or the photodetectors cover only part of the walls of the block of scintillating material. Thus, part of the scintillation light emitted is not detected but leaves the scintillator or is absorbed in the scintillating material, before reaching a photodetector. The performance of a scintillation detection device depends on the amount of light received by the photodetector and therefore on the efficiency of light collection defined as the proportion of light detected in relation to the light emitted.

The collection yield depends essentially on the dimensions of the scintillator and the transparency of the scintillating material in the scintillation light.

The collection output can also be increased by trapping the scintillation light in the scintillator. This trapping, as indicated above, is one of the main applications of the coating object of the invention.

STATE OF THE PRIOR ART The appended figures 1 to 3, described below, show various known measures, envisaged to increase the collection yield of scintillator detectors. In these figures, identical references respectively indicate identical or similar parts.

In FIG. 1, the reference 10 designates a crystalline block of scintillating material forming a scintillator.

On the face 12 of the crystal 10 is fixed a photodetector 13 such as a photomultiplier, for example, to detect the scintillation light and deliver an electric detection signal.

In the example of FIG. 1, the walls 14 of the scintillator are polished in order to confine in the scintillator the light which undergoes a total reflection on the walls.

An arrow 18 indicates ionizing radiation such as γ radiation which penetrates into the material scintillating. At the point referenced 20 there takes place an interaction with emission of scintillation light. References 21, 22, 23 correspond respectively to three types of paths that a light ray produced during the interaction at point 20 can perform.

In the first case, a ray 21 performs a trajectory oriented directly towards the photodetector. Such a ray travels a short distance in the scintillating material. It is therefore not absorbed but is detected by the photodetector 13.

In a second case, a radius 22 reaches a wall 14 of the scintillator with an angle θ, measured relative to the normal to the wall 14, such as θ≥θc where θc = arcsin (1 / n) and where n is the refractive index of scintillating material.

In this case, the ray 22 is reflected by total reflection and does not leave the scintillator. It can thus possibly reach the photodetector. In a third case, a radius 23 reaches the wall 14 with an angle θ, measured relative to the normal to the surface of the wall 14, which is less than the angle θc defined above. The ray 23 is refracted outside the scintillator and can no longer be detected.

A solution envisaged to increase the collection yield is shown in FIG. 2. It consists in wrapping the scintillator 10 with a sheet 30 of material forming an opaque diffuser. This is, for example, a sheet of white paper.

The opaque sheet 30, which constitutes an envelope, is separated from the scintillator 10 by a thin layer of air 32. The very small thickness of this layer, of the order of 10 to 100 μm is greatly exaggerated in FIG. 2 for reasons of clarity. The thickness of the sheet 30 is also exaggerated. In FIG. 2, the references 21 and 22 respectively designate scintillation light rays directly reaching the photodetector or undergoing total reflection on a wall 14 of the scintillator. The reference 23 designates a ray which reaches the wall 14 with an angle such (θ <θc) that it is refracted out of the scintillator. However, the ray 23 is diffused by the opaque diffusing film 30 and returns to the scintillator where it can possibly reach the photodetector 13.

Thus, thanks to the envelope 30, the scintillation light is in a way confined in the scintillator and the collection yield is improved. A device according to FIG. 2 however suffers from a major drawback linked to the thickness of the opaque sheet 30 which is of the order of a few tenths of a millimeter. In fact, a large number of ionizing radiation detectors are segmented detectors comprising a plurality of juxtaposed scintillators each forming an elementary detector. However, for segmented detectors, the adjacent elementary detectors should be as contiguous as possible. In addition, the elementary detectors must be optically isolated and have a high collection efficiency.

Thus, although the opaque sheet 30 makes it possible to increase the collection yield, it constitutes, due to its thickness, an obstacle to the production of segmented detectors of great compactness.

In order not to harm the compactness of the detectors, another possibility consists in covering the scintillator of each detector with a thin opaque layer which is deposited directly on the walls of the block of scintillating material.

This solution is illustrated in FIG. 3. In this figure, a thin layer 40 of a light-diffusing or reflective material, for example a metallic layer or a layer of white titanium oxide paint is formed on the walls 14 of the scintillator 10. Such a layer, the thickness of which is very small, enables the elementary detectors to be joined and joined together.

In FIG. 3, the reference 21 always designates a ray reaching the photodetector directly and without reflection.

The rays referenced 22 and 23 are rays reaching a wall 14 of the scintillator. It is observed that these rays undergo a metallic reflection, or are diffused, independently of their angle of incidence on the wall 14.

Thus with the device of Figure 3, a light confinement of the scintillation light can be obtained.

However, in the case where the thin layer 40 is a metallic layer, the metallic reflection coefficient R on the thin layer is less than 1 and the multiplication of successive reflections on the walls leads to a loss of light. Furthermore, in the case where the layer 40 is made of a light-scattering material, the light scattering on the walls of the scintillator increases the length of the path of the light rays in the scintillator and therefore promotes the absorption of light by the material. scintillator.

Light losses by metallic reflection or by absorption have a negative impact on collection performance. Documents (1), (2) and (3), the references of which are given at the end of the description, describe scintillator devices and suggest associating with the scintillator spacing or coating layers with a low refractive index. . However, the dielectric materials proposed by these documents do not make it possible to obtain very high collection yields due essentially to their insufficiently low index.

Statement of the invention

An object of the present invention is to provide an optical coating which can be used, for example, for the optical confinement of scintillation light in a scintillator, and which does not have the drawbacks mentioned above.

An object is in particular to propose such a coating allowing effective optical isolation and having a small thickness, so that it can be used in detectors of segmented ionizing radiation, comprising contiguous elementary detectors.

An aim is also to propose a method of manufacturing such a coating which is suitable for a industrial implementation and which allows precise control of the thickness of the coating.

Another aim is to propose a scintillator which can be used in an ionizing radiation detector and which has an optical coating as mentioned above.

Another aim is to propose a coating having good resistance to ionizing radiation. Another aim is also to provide a coating with good mechanical resistance, thus allowing easy handling.

An object of the invention is finally to propose a scintillation detector exhibiting good light collection efficiency.

To achieve the goals mentioned above, the invention more specifically relates to a scintillation detector comprising: - a block of scintillating material, and - a refractive layer, made of a solid material having a refractive index of less than 1.3 , formed on the block of scintillator material.

The low refractive index of the refractive layer makes it possible to obtain a good light collection efficiency with a scintillator according to the invention.

This is particularly important when a scintillating material, such as Csl, itself has a relatively low index. Preferably, this index can be chosen less than or equal to 1.26.

The detector may also comprise a layer, called a covering layer, of a diffusing material or light reflector, formed on the refractory layer.

By scintillating material is meant a material capable of converting ionizing radiation, such as gamma radiation, for example, into light radiation generally located in the visible or ultraviolet spectrum. By way of example of scintillating material, mention may be made of lead tungstate Pb 0 ₄ or bismuth germanate (BGO). Furthermore, the terms scattering, reflection or refraction are understood to apply to the wavelengths of scintillation light.

The combination of the refractive layer of a solid material (that is to say non-gaseous) with a covering layer, capable of reflecting or diffusing light, makes it possible to obtain an excellent coating of optical confinement whose thickness is very weak. The thickness of the refractive layer is chosen to be sufficient to effectively allow light to refract. Preferably, this thickness can be chosen between 0.8 and 2 μm. Indeed, for too small thicknesses, the refractive layer could, depending on the material used, behave like an anti-reflective layer.

According to a particular embodiment of the coating, the refractory layer may comprise silica colloids coated with a binder. Such a material is particularly suitable for achieving a low refractive index. Such a layer can be easily formed by a sol-gel process. Such a method is particularly suitable for industrial implementation.

By way of illustration of the production of a layer by a sol-gel process, reference may be made to documents (4) and (5), the reference of which is given at the end of this description.

The covering layer can be a metal layer, for example a silver layer. Such a layer can be formed by vacuum evaporation according to conventional techniques known per se. Optionally, a protective layer, such as a polymer layer, can be provided to protect the metallic layer from oxidation. As a variant, the covering layer can also be a metal oxide layer, for example a titanium oxide layer. Such a layer can also be formed by a sol-gel process.

In this case, a metal oxide with a high refractive index is preferably used in the form of colloids, the particles of which have an average size sufficient to ensure diffusion of the wavelengths of the scintillation light.

It should be specified that, in an application to an ionizing radiation detector, the refractory layer and the covering layer can be formed on all or part of the surface of the scintillator, with the exception of the surface portion receiving a photodetector. In particular when the scintillator is a crystalline block having several faces, one or more faces can be lined with the coating formed by the layers mentioned above. The invention also relates to a method for forming a light trapping coating on a substrate, comprising the following steps: a) forming on the substrate a sol-gel layer comprising silica colloids coated in a siloxane binder , b) heat treatment to establish siloxane bonds in the binder.

In this process, operations a) and b) are repeated until a coating having a thickness of between 0.8 and 2 μm is obtained.

In particular, the substrate can be a block of scintillating material.

Thanks to this process, a uniform coating is obtained, free from cracks and / or internal cleavage planes. In addition, the layers formed are resistant to abrasion and to γ, X and neutron radiation.

Preferably, steps a) and b) can be preceded by the application to the substrate of an adhesion promoter layer. The sol-gel layer is then applied to the adhesion promoter layer.

The adhesion promoter is, for example, an epoxy-alkoxysilane. In addition, before applying a layer to the substrate, it is possible to carry out a cleaning step consisting in cleaning the substrate with an aqueous detergent solution and rinsing it with water and then with alcohol. A substrate is thus obtained, the surface of which is very hydrophilic.

In the process of the invention, the sol-gel layer can advantageously be deposited by soaking. According to another aspect of the invention, the method may further comprise, after steps a) and b), the formation of a covering layer made of a light-diffusing or reflecting material. The cover layer can be a light-reflecting metal layer or a light-diffusing metal oxide layer.

In the second case, the metal oxide layer can also be formed by dipping and according to a sol-gel type process.

Other features and benefits of

1 invention will emerge better from the description which follows, with reference to the figures of the accompanying drawings.

This description is given purely by way of non-limiting illustration.

Brief description of the figures

- Figures 1 to 3, already described, are simplified schematic sections of detectors of ionizing radiation with scintillator, of known type.

- Figure 4 is a simplified schematic section of an ionizing radiation detector with a scintillator according to the invention.

Description of methods of implementing the invention

In FIG. 4, parts identical or similar to parts of FIGS. 1 to 3 bear the same references to which 100 have been added. It is thus possible with regard to these parts, to refer to the description above.

The detector of FIG. 4 comprises a block 110 of scintillating material constituting a scintillator. It is, for example, a crystal of PbW0 ₄ .

On a face 112 of the crystal is fixed a photomultiplier 113 shown very schematically.

Finally, the outer walls 114 of the scintillator are covered with a two-layer coating 150, 152.

A first layer 150, called the refractory layer, is formed, either directly on the scintillating material, or on an adhesion layer 151 in an epoxyalkoxysilane, shown in broken lines. The refractive layer, with a thickness of between 0.8 μm and 2 μm has a refractive index of less than 1.3 and less than the refractive index of the scintillating material.

In the example described, the refractory layer is a colloidal layer. It includes silica colloids coated in a siloxane binder. A sol-gel manufacturing process guarantees this layer good resistance to abrasion.

A second layer 152, called the covering layer, is formed on the refractory layer 150. The layer 152 is a layer of silver deposited by evaporation under vacuum. This layer, with a thickness of the order of 0.1 μm, constitutes a reflective layer of the scintillation light. It can be protected by an outer layer 153 of polymer shown in broken lines in FIG. 4.

By analogy with the previous figures, the references 118 and 120 respectively denote ionizing radiation penetrating into the scintillator and a point of interaction of this radiation with the scintillating material.

The reference 122 indicates a light ray emitted by scintillation which is reflected on the wall 114 of the scintillator. The reference 123 indicates a light ray which is refracted in the refractive layer

150, out of the scintillator. This ray, when it reaches the covering layer 152 is reflected, and is returned in the scintillating material. Thus, the scintillation light is confined in the scintillating material and the collection efficiency is improved.

Alternatively, the silver layer forming the coating layer 152 can be replaced by a metal oxide layer such as a titanium oxide layer. The coating layer is then a light diffusing layer.

For this purpose, the layer preferably has a high refractive index, greater than 1.8. The coating layer comprises, for example, metal oxide colloids, the average particle size of which is sufficient to ensure the diffusion of the wavelengths of the scintillation light.

CITES DOCUMENT

(1)

EP-A-0 250 983 (2)

EP-A-0 534 683 (3)

DE-A-42 23 861 (4)

EP-A-0 600 022

(5)

US-A-4,397,666

Claims

1. Scintillation detector comprising: - a block of scintillating material (110), and - a refractive layer (150), made of a solid material having an internal refractive index of 1.3, formed on the block of scintillating material.

2. Scintillation detector according to claim 1, in which the refractive layer (150) has a thickness of between 0.8 and 2 μm

3. Scintillation detector according to claim 1, in which the refractory layer comprises colloids of silica coated with a binder. . Scintillation detector according to claim 1, further comprising a layer (152), called a covering layer, of a light-diffusing or reflective material, formed on the refractive layer (150).

5. Scintillation detector according to claim 4, in which the covering layer

(152) is a layer of metal, reflector of light.

6. Scintillation detector according to claim 4, in which the covering layer (152) is a layer of light-diffusing metal oxide.

7. Device for detecting ionizing radiation comprising a scintillation detector and at least one photodetector associated with the scintillation detector, characterized in that the scintillation detector conforms to claim 1.

8. Light trapping coating for a substrate comprising a refractory layer (150) of light comprising colloids of silica and having a refractive index less than 1.3.

9. Coating according to claim 8, in which the refractory layer has a thickness of between 0.8 and 2 μm.

10. Coating according to claim 8, characterized in that it further comprises a layer (152) of light diffusing or reflecting material formed on the light refracting layer. 11. Method for forming a light trapping coating on a substrate (110) comprising the following steps: a) forming on the substrate (110) a sol-gel layer (150) comprising coated silica colloids in a siloxane binder, b) heat treatment to establish siloxane bonds in the binder, in which operations a) and b) are repeated until a coating having a thickness of between 0.8 and 2 is obtained μm.

12. The method of claim 11, wherein step a) is preceded by the application to the substrate of a layer (151) of adhesion promoter, the sol-gel layer (150) being applied to the layer adhesion promoter (151).

13. The method of claim 12, wherein the adhesion promoter (151) is an epoxyalkoxysilane.

14. The method of claim 11, wherein the sol-gel layer (150) is deposited by dipping.

15. The method of claim 11, further comprising, after steps a) and b), the forming a covering layer (152) of a light diffusing or reflecting material.

16. The method of claim 15, wherein the covering layer (152) is a silver layer formed by vacuum evaporation.

17. The method of claim 15, wherein the covering layer (152) is a sol-gel layer using at least one metal oxide.