HK1037405B - Photosensitive matrix electronic sensor - Google Patents

Description

Photosensitive array electronic sensor

Technical Field

The invention relates to a photosensitive array electronic sensor. More particularly, the present invention relates to X-ray radiation sensors. The object of the present invention is to improve the performance of such sensors.

Background

The radio frequency logic image enhanced screen is positioned relative to a detector and receives X-ray radiation on the other side, a configuration well known in the X-ray electronic sensor art. It is also known in the field of nuclear medicine to use scintillators for converting gamma rays (or X-rays) into visible radiation that can be detected by a detector. In the field of radiology, the most commonly used detector is a camera with a target or collection array of Charge Coupled Devices (CCDs). In the nuclear medicine field, devices are used which are also composed of a series of photomultiplier tubes and are connected to a center of gravity electronic circuit. All these sensors with detectors that are not capable of directly detecting X-rays are associated with scintillators whose task is to convert the X-rays into radiation in the approximate visible spectrum.

The material used for this conversion (the material of the scintillator) is typically gadolinium hydrosulfide. The latter is used in the form of a thin precipitate, typically 50 to 300 microns. The precipitate consists of particles of the material, which are bonded together by a binder. Throughout the entire thickness of this material, the emission of visible light in all directions leads to a loss of sensitivity of the detector (and thus of the sensor), and a loss of resolving power.

One proposal has been to deposit a gadolinium-containing film (plastic) on a luminescent imaging array detector, the latter consisting of a silicon integrated circuit.

The cesium iodide CsI doped with thallium in a needle form provides a good alternative for higher luminous efficiency in combination with a waveguide effect of the needle, and the section of the needle usually has a dimension of 3-6 micrometers. Traditionally, such materials have been used in radiographic intensifiers by coating the material on an input screen, which is typically comprised of an arched sheet of aluminum metal. Numerous implementations are also known in which a bundle of optical fibers is covered with such a material. The needles are oriented perpendicular to the surface of the support on which they are supported. The needles are only partially bonded to each other so that they provide a 20% to 25% porosity. These gas-filled pores, combined with the good refractive index of CsI (1.78), can tunnel the emitted visible photons in each needle and give higher sensitivity and resolving power.

However, CsI is still more difficult to use than gadolinium hydrogensulfide. In particular, the disadvantage that CsI will hydrate rapidly in ambient air and normal humidity is well known. The uptake of water has a destructive effect on the image obtained using the sensor, which causes an initial halation effect. This humidification effect then irreversibly damages the needles, with consequent loss of luminous efficiency and resolution of the sensor. It should be noted that this disadvantage is not encountered in a radiographic enhanced tube, since the CsI is in the vacuum tube.

However, thallium has high toxicity despite the presence of very small amounts of thallium in the needle. Thus, the weak mechanical fixation of CsI leads to the generation of dust and waste, the elimination of which must be carefully controlled. In some cases, passivation of the thallium-doped CsI can be achieved by evaporating a layer of aluminum on the surface of the scintillator.

Due to the weak mechanical fixation of CsI, it must be deposited on a solid support. In fact, the bending of the support causes visible defects in the image, and this support must usually be subjected to a (non-deforming) thermal treatment in order to diffuse the thallium at a temperature of the order of 300 °.

In the radiological image intensifier, the support is made of aluminum. Sometimes in combination with, or even replaced by, amorphous carbon, because such materials have a very high electrical resistance.

The deposition of cesium iodide on beryllium has been of interest outside the structure of picture-enhancing tubes, but this material has the disadvantage of being too expensive.

Disclosure of Invention

The object of the present invention is to solve these problems by growing the CsI layer on a substrate consisting of a machined graphite block. Preferably, the substrate comprised of the machined graphite block has a shallow roughened surface. In the present invention, it is proposed that the graphite used be a substrate whose surface has been subjected to a densification treatment step aimed at eliminating the natural porosity associated with graphite in the substrate. In addition, it is preferable to grind the layer requiring a dense primer so as to have a shallow roughness. Furthermore, it was found that when precipitating in the gas phase, CsI takes a completely beneficial increase: the needles are regularly spaced so that the surface of the resulting scintillator is almost flat, regardless of the presence of defects related to the roughness of the graphite.

Differences in sensitivity can occur in the resulting sensor if the substrate is not constructed through a densification process. One may try to correct this, for example if the surface of the graphite is striped (e.g. has parallel lines), then the appearance of these stripes can be identified in the image obtained after the sensor has been operated. By processing in software, it is possible to correct differences in sensitivity associated with various locations, particularly in the field of nuclear medicine. In a development according to the invention, one limits the magnitude of this correction by means of a densification process and/or a grinding process.

In all cases, the presence of the graphite substrate provides a solution to the problem of uneven spreading that occurs during thallium diffusion.

Graphite (as understood in the present invention) is a material different from amorphous carbon in the sense that it possesses a porous physical structure (unlike extremely dense amorphous carbon). Graphite can be machined using metal tools, while amorphous carbon can be machined almost exclusively using diamond-tipped tools.

This is why the use of a graphite block as support has proved to be particularly advantageous in applications of interest here, i.e. the precipitation of cesium iodide on a processed support intended to be placed in front of an array image detector.

Generally, the structure of graphite is not only porous but also layered. Thus, the processing thereof is further facilitated. Unlike amorphous carbon, its structure is essentially isotropic.

In principle, graphite is obtained by compressing carbon powder at high temperature, whereas amorphous carbon is produced by decomposition in the gaseous phase (cracking), i.e. decomposition when the growth of a thicker or thinner coating on a starting support reaches high tide. Thus, the use of graphite readily produces a workable mass, while the production of an amorphous carbon coating on a surface such as the dome-shaped surface of a radiographic-enhanced input screen appears to be somewhat easier.

One subject of the present invention is therefore a photosensitive array electronic sensor. This photosensitive array electronic sensor comprises an array image detector covered with a scintillator for converting high-frequency electromagnetic radiation (generally X-ray radiation) into low-frequency radiation (generally radiation in the visible range), characterized in that the scintillator comprises a cesium iodide plate (faceplate) supported by a graphite substrate arranged on the side receiving the high-frequency radiation.

Another subject of the invention is a process for constructing a sensor, characterized in that:

-constructing a graphite substrate which must be able to act as a support for the scintillator;

-grinding the graphite substrate;

-precipitating cesium iodide on a substrate in a gaseous phase;

-doping the cesium iodide precipitate with thallium;

-depositing a layer of synthetic resin on the cesium iodide precipitate under vacuum in a gaseous phase; -depositing a layer of liquid photo-coupling resin on the layer of synthetic resin;

-laying down a detector against the layer of liquid light coupling resin;

drawings

The invention will be better understood upon reading the following description and upon examination of the drawings. The drawings are given without limiting the description of the invention at all. The figures are described as follows:

FIG. 1: is a schematic description of a sensor structure consistent with the present invention;

-figure 2: is a schematic representation of an apparatus for implementing a process for passivating a cesium iodide layer;

Detailed Description

Fig. 1 depicts a light-sensitive electronic sensor 1 according to the invention. Preferably, an array sensor is used. The sensor 1 comprises a detector 2 covered on top with a scintillator 3. The purpose of this sensor is to convert X-ray radiation 4 or any other high frequency radiation (which may also be gamma radiation) into low frequency radiation 5. The low frequency radiation 5 can then be emitted in the visible light spectrum, so that the radiation 5 can be detected by the detector 2. The detector 2 may be a conventional detector. In a preferred example, the detector 2 is of the CCD type, as described above. Each CCD device array forms a row of detection points. The juxtaposed arrays are used to form the individual rows of an array image.

In essence, the sensor comprises a cesium iodide plate 6 supported by a graphite substrate 7. The substrate is arranged on the side on which the X-ray radiation is received. According to the invention, the graphite used is preferably graphite having a layered structure and obtained by hot pressing carbon powder. This type of graphite is relatively inexpensive to produce, and in particular is relatively inexpensive to machine, since it can be machined with metal tools, whereas structures based on amorphous carbon materials can only be machined with diamond-tipped tools.

The material used is therefore in the form of small, blocky sheets 10, stacked one on top of the other end-to-end. In one example, the graphite substrate 7 has a thickness on the order of 500 microns. In case the scintillator should be larger, an increase to 800 or 2000 microns is possible. If it is desired to have a smaller size, as small as 200 microns, is possible, graphite has the advantage of being lumpy, i.e. absorbing the emitted visible radiation in all its directions by the scintillator, in addition to its better X-ray permeability. These visible radiation contribute more to reducing the resolving power of the sensor than to increasing the sensor sensitivity. In a preferred example, the quality of the graphite substrate 7 would be such that: particle size-the length of the thin layer-will be less than 5 microns, preferably on the order of 1 or less than 1 micron. In fact, it was found that if the natural anisotropy of graphite is not controllable, it would result in particles having a size of 20 microns. In this case the quality of consistency of the thickness of the CsI plate 6 is poor, which would require more software corrections.

The graphitic substrate 7 is preferably coated with a layer of amorphous carbon 8 so that the graphitic substrate 7 has a densified surface. The amorphous carbon layer 8, whose thickness is of the order of 3-20 microns, makes it possible to fill the pores 9 present on the surface of the graphite substrate 7 (due to the pores it presents). The atoms of the carbon layer 8 are different from the atoms in the graphite substrate 7, wherein in the graphite substrate 7 the pores are larger and the carbon-graphite particles are oriented. Amorphous carbon layer 8 is an unstructured, denser layer, i.e. non-polycrystalline: the atoms gather disorderly one on top of the other, for example this layer of amorphous carbon is deposited on the graphite substrate 7 under vacuum in a gaseous phase.

As an additional requirement, or as an aid, at the location where the cesium iodide layer 6 is grown, the graphite substrate 7 should be densified beforehand by infiltration. Such permeation can be achieved, for example, by covering the surface of the graphite substrate 7 receiving cesium iodide with a thin film made of an organic resin. The device can then withstand extremely high temperatures (1000 deg.). The effect is to split the resin, separating the carbon atoms in the resin from the hydrogen atoms or other components bound to it. These impurities can be naturally eliminated by evaporation. The high temperature effect may also cause carbon atoms to migrate into the void spaces 11 of the graphite substrate 7 by diffusion. This infiltration operation may be repeated multiple times in order to further densify the active surface of the substrate, in order to increase densification. In one example, this infiltration operation is repeated four times.

As explained above, the prepared surface of the graphite need not be overly desirable, as in the case of a graphite surface that is not so desirable, the actual correction to the acquired image can be processed by the software after image acquisition. This is entirely acceptable. In the present invention, grinding of the graphite surface, particularly after densification, is accomplished using a grinding tool 12. Typically, the grinding action is omitted from the top layer of the graphite substrate 7, i.e. the corresponding layer 8, by a thickness of 5-10 microns. The precipitation of the layer 8 can take place either before or after grinding, which results in a roughness H of the order of 0.2 to 0.4 microns, whereas the natural roughness H without grinding, in particular without densification, can reach 130 microns, especially if the particle size of the graphite is of the order of 20 microns.

Thus, the CsI is grown by action in a conventional manner. The needle 12 is then obtained, the diameter of the cross section of which may be of the order of 3-6 microns. The cross-section of the needle 12 can be of various sizes, as is evident from fig. 1. In one example, the pins 12 are randomly spaced from one another by a spacing 13 of 1-3 microns, which makes it possible to construct a "varying media surface" 14 using the pins 12. The presence of this surface 14, in combination with the good refractive index of the CsI, results in a fiber-like behavior of the needle 12. In other words, the conversion of the radiation (scintillation phenomenon occurring in one needle 12) produces one radiation 5 to be directed. If this emission of radiation is directed towards the detector 2, it will normally exit the needle 12 through the reflection point 15. On the other hand, if this radiation 5 is tilted, it will not reflect off the surface 14 inside the needle 12 and eventually exit through the reflection point 15. The portion emitted towards the substrate is absorbed by the rear graphite substrate 7. In one example, the thickness of the layer 6 of CsI is 100 to 300 microns. Typically it measures 180 microns.

The layer 6 itself is then doped with thallium in a conventional manner.

Finally the CsI-doped layer 6 is covered with a passivation layer 16. In contrast to the prior art, in which the passivation layer 16 is a silicone gel layer, which relates to gadolinium hydrosulfide, the invention proposes to produce the passivation layer 16 in the form of a transparent polymeric synthetic resin. This polymeric resin has the advantage of being less permeable and preventing dust from volatilizing from CsI or from thallium, but also has the disadvantage of not producing a completely smooth outer surface. Thus, in the present invention, the passivation layer 16 is combined with a liquid resin layer 17 for optical coupling with the detector 2. In this manner, good thallium evaporation impermeability can be achieved without compromising sensor efficiency.

Fig. 2 depicts one apparatus that may be used to create passivation layer 16. This device comprises three parts connected together. In the first part 18, raw material for the generation of the tree finger is introduced. In one preferred example, this material is a xylene. In section 18, this material is vaporized at a pressure of 1 torr (pressure of 1 mm mercury) and a temperature of 175 °. The first part 18 is connected to the second part 19, wherein the vaporized material is vaporized and precipitated. For example, the combined xylene gas may be heated to 680 ℃ at a pressure of 0.5 torr. Under this pressure, the bi-p-xylylene starts to decompose and is converted into a single molecule of p-xylylene. The prepared paraxylene can then be introduced into the third section 20 at ambient temperature and at a very low pressure of 0.1 torr, where it diffuses onto the needles 12 of layer 6 as layer 16. Thus, paraxylene is recombined by condensation to form a poly-paraxylene polymer. This condensation results in the creation of a bridge over the pore space of the CsI layer without penetrating the gaps.

It is possible to use a synthetic resin other than the above-specified resin. However, the resins specified above have one advantage: on the one hand it adheres well to the CsI and on the other hand it allows bridges to be formed over the spaces 13 without filling these spaces. Preferably, the resin used has a refractive index of between 1.78 and 1.45. Therefore, a resin having a refractive index lower than that of CsI can form an antireflection layer by combining with CsI. In one example, layer 16 has a thickness of 1-25 microns.

The liquid resin layer 17 then spreads over the passivation layer 16 (and remains there) in order to ensure good optical coupling. Preferably, the resin has a refractive index of less than 1.45. For example of the type used in the construction of liquid crystal components. The thickness of layer 17 is of the same order of magnitude as the thickness of layer 16.

The detector 2 is then fixed to the graphite substrate 7 by conventional mechanical means.

Claims (15)

1. A photosensitive array electronic sensor comprising an array image detector (2) covered with a scintillator (3) for converting high frequency electromagnetic radiation (4) into low frequency radiation (5), characterized in that the scintillator comprises a cesium iodide plate (6) supported by a graphite substrate (7).

2. The sensor of claim 1, wherein: the particle size of the graphite is less than 5 microns.

3. The sensor of claim 2, wherein: the particle size of the graphite is less than or equal to 1 micron.

4. A sensor according to any one of claims 1 to 3, wherein: the substrate is made of graphite covered with a layer of amorphous carbon (8).

5. The sensor of claim 1, wherein: the substrate is composed of graphite impregnated with amorphous carbon.

6. The sensor of claim 1, wherein: the plates of the scintillator are insulated from the surrounding medium by a passivation layer (16) made of synthetic resin, which is covered by a liquid-optical coupling layer (17).

7. The sensor of claim 6, wherein: the passivation layer is made of a resin of the poly-p-tolylene type.

8. The sensor of claim 6, wherein: the refractive index of the resin of the passivation layer is 1.78-1.45, so as to form an anti-reflection layer.

9. The sensor of claim 6, wherein:

-the graphite substrate has a thickness of 200 to 2000 microns;

-the thickness of the cesium iodide slab is 100 to 300 μm; and

-the resin passivation layer has a thickness of 1-25 microns;

10. the sensor of claim 9, wherein: the thickness of the graphite substrate is 500-800 microns.

11. The sensor of claim 9, wherein: the thickness of the cesium iodide slab was 180 μm.

12. A process for manufacturing a sensor, characterized by:

-constructing a graphite substrate which must be able to act as a support for a scintillator;

-grinding the graphite substrate;

-precipitating cesium iodide on a graphite substrate in a gaseous phase;

-doping the cesium iodide precipitate with thallium;

-depositing a layer of synthetic resin on the cesium iodide precipitate under vacuum in a gaseous phase; -depositing a layer of liquid light coupling resin on the layer of synthetic resin;

-laying down one detector for the liquid light coupling resin layer.

13. The process of claim 12, wherein:

-treating the surface of the graphite by precipitating a layer of amorphous carbon before or after grinding.

14. The process of claim 12, wherein: the surface of the graphite is treated by infiltration with amorphous carbon prior to milling.

15. The process of claim 13, wherein: the surface of the graphite is treated by infiltration with amorphous carbon prior to milling.