FR2623940A1 - RADIATION DETECTOR AND METHOD FOR MANUFACTURING ITS INPUT WINDOW - Google Patents

Abstract

The structure and manufacture of the entrance window of this detector is simplified. The radiation detector comprises: - a sealed enclosure 2 containing a pressurized fluid, and in which is arranged a detection assembly, - an inlet window of the sealed enclosure 2 through which the radiation to be detected penetrates into the 'sealed enclosure 2, and - means for fixing the inlet window to the sealed enclosure 2. According to the invention, the inlet window is formed by a preformed sheet 10, by a force field such as deformation of the sheet 10 is at least equal to that obtained by a non-preformed sheet subjected to the operating pressure of the detector. Application to dosimetry, medical tomography, radiology and for non-destructive testing.

Description

i COu2623940

  RADIATION DETECTOR AND METHOD OF MANUFACTURING SAME

INPUT WINDOW

DESCRIPTION

  The invention relates to a radiation detector such as an ionization detector, that is to say an ionization chamber, and more particularly to the input window of a

  such detector, as well as the manufacturing method thereof.

  This type of detector is used in particular in dosimetry, medical tomography, radiology and for nondestructive tests, where the detection of radiation for imaging such as X-rays is used. An ionization detector comprises an entrance window, through which the incident rays penetrate, to be detected inside a sealed chamber of the detector containing a pressurized ionizable fluid. This window must be as thin as possible to have good radiation transparency, given nevertheless that it is to close the pressure tight enclosure. The window must also be very homogeneous, for

  ensure a good uniformity of the detector response.

  With reference to FIG. 1, a first type of detector according to the prior art comprises a sealed enclosure 2, inside which is placed a detection assembly consisting of conductive plates 4, delimiting one, or as in FIG. , several sensor cells. The technique used here to obtain an input window is to cut it into the mass from an ingot 6. The main disadvantage of this type of window is due to the lack of homogeneity of the base material. Indeed, it is risky to tailor, for example by milling a window of small thickness for example less than 3 mm for aluminum and its alloys, under penalty of having to reject the part because of leakage or crack. A second disadvantage is that after a machining of this type, the internal stresses of the ingot can be released, which affects the accuracy of a window for a gas-tight assembly. U.S. Patent No. 4,622,466 discloses another type of X-ray detector having a pressure sealed enclosure. This detector uses an entrance window with a multilayer structure, combining a sheet of metal with a carbon fiber sheet, and an elastic isolation sheet which may be polyaramid. This technique is more complex than the previous one, and is more expensive because of the diversity of the materials involved and their assembly. On the other hand, since the detector must be used in very variable conditions, differences in temperatures cause stress in the multilayer structure. These constraints

  affect the homogeneity and efficiency of the entrance window.

  The object of the invention is to avoid the disadvantages mentioned above, by proposing a detector with a window

  simple entry and lower realization cost.

  For this purpose, one of the main objects of the invention is a radiation detector comprising, a sealed chamber containing a pressurized fluid and in which is disposed a detection assembly, an inlet window of the sealed enclosure through which radiation enters the sealed enclosure, and means for fixing the inlet window on the sealed enclosure. This detector is characterized in that the

  entrance window is formed by a preformed sheet.

  The preformed sheet is constituted by a ductile material chosen as a function of the energy and the dose of the radiation to be detected and the operating pressure of the detector, such as a metal (aluminum and its alloys, stainless steel, etc.). ) or a plastic and / or composite material (glass

epoxy, polycarbonate, ...).

  In the case of detectors intended to be exposed to high doses of irradiation, the entrance window is preferably made of a metal foil, the metal having a better resistance to radiation than plastics and / or composites. The fact that the sheet is preformed allows it to undergo, under the pressure operation of the detector, less stress than if it were not preformed. In addition, the use of a single material avoids creating constraints during variations. of

  temperatures that the detector experiences.

  An important characteristic of the sheet is that this preforming is such that the deformation of the sheet is greater than or equal to that obtained by a non-preformed sheet and subjected to the operating pressure of the detector. In this way, the stress field exerted by the pressure of the detector on the preformed sheet remains in the elastic range, which guarantees the geometric stability of this

leaf.

  The fixing means of such a sheet may comprise a flange and clamping elements of the flange on the sealed enclosure, the sheet being clamped between the flange and

the waterproof enclosure.

  These fixing means may also consist of a weld seam or glue of the sheet on

the waterproof enclosure.

  According to the invention, it is preferable to use a seal placed between the sealed enclosure and the sheet to seal the detector. This seal is for example

of toric form.

  Another main object of the invention is a method of manufacturing a detector, as described in the preceding paragraphs, and forming the input window from a sheet by the following steps: fastening the non-preformed sheet, using specialized fastening tools and reproducing the means for fixing the sheet on the sealed enclosure, - application to the sheet to deform a force field of the same type than that applied to the sheet during operation of the detector and amplitude at least equal thereto. Such a manufacturing method is simpler than a mass machining and a multilayer window embodiment and allows, for a given material, to have a thinner window thickness than with these methods and therefore a better transparency. radiation. In addition, its cost is reduced compared to the two prior art techniques mentioned above. According to the invention, such a force field can be applied to the sheet hydraulically, the sheet being

  directly in contact with a liquid under pressure.

  The force field can also be applied mechanically to the sheet by means of a matrix whose shape

  corresponds to the desired deformation of the sheet.

  Depending on the ductility of the material constituting the sheet, it may be advantageous to bring the material constituting it at a temperature sufficient to allow deformation of the sheet both mechanically

than hydraulic.

  According to a further characteristic of the invention, provision is made for the hardest material constituting the sheet or the enclosure to be sandblasted on a peripheral zone,

  to promote the fixation of the feulle on the sealed enclosure.

  In some applications, to avoid electrical disturbances, it is advantageous to electrically isolate the inner face of the window relative to the detection assembly. For this we deposit, at least on the

  part of the window next to this set, an insulating film.

  The invention and its characteristics will be better

  The description that follows, and which is

  accompanied by the following figures: FIG. 1, already described above, represents a detector of a type according to the prior art, FIG. 2 schematically represents a detector

2 23940

  according to the invention, - Figure 3 shows schematically an input window of a detector according to the invention, - Figure 4 shows the method of manufacturing the detector according to the invention, according to a first version, - Figure 5 represents the manufacturing process of a

  detector according to the invention, according to a second version.

  Referring to Figure 2, the detector according to the invention comprises a sealed chamber 2 comprising metal sheets 4 placed inside thereof and forming a detection assembly. They delimit between them one, or as in the case of the figure, several sensor cells. The entrance window is constituted, according to the invention, a sheet 10 such as a laminated metal sheet. This is preformed using a specialized tool reproducing the fixing systems of the sheet 10 on the sealed enclosure 2. Such preforming aims to establish in the deformed material a stress field, during the operation, of lesser importance compared to what would exist in a flat plate. To achieve this result, the sheet 10 must be preformed, so that its deformation is greater than or equal to that obtained by a non-preformed sheet and subjected to the operating pressure of the detector. At the moment of pressurization of the detector, the deformations of a window made with such a sheet 10 remain in the elastic domain. Positioning stability

  The window relative to the internal parts is provided.

  The fastening means of the metal sheet 10 may comprise a flange 12 and clamping elements 14 of the flange on the sealed enclosure 2, the sheet 10 being clamped between the -brid and the sealed enclosure. These fixing means may be screws, the sealed chamber 2 being provided with tapped holes. The film fixing means may also consist of a simple weld seam or ribbon (not shown) thereof on the sealed enclosure. It also provides the use of a seal 8, for example ring, placed on the sealed enclosure, relatively close to the cavity having the detection assembly 4. It seals the enclosure during operation. The detection assembly may therefore consist of several conductive plates 4 delimiting sensor cells. The detector shown in this figure is of the type

multicellular and ionization.

  In FIG. 3, the preformed sheet 10, intended to serve as an input window for the detector, is shown seen from above. A peripheral zone 22 of this sheet may be intended to receive a sanding treatment to promote the attachment of this sheet 10 to the sealed chamber 2 of the detector. Between this peripheral zone 22 and the preformed zone 26 located in the center, there is shown in phantom lines 24 the

  line of support of a seal.

  To obtain the preforming of the sheet, it is possible to use according to the invention, to a first method, shown in FIG. 4. The non-preformed sheet is fixed, using a specialized fastening tool and reproducing the conditions clamping means 12 and 14 of Figure 2. It can be used as fixing means, clamping jaws 20, clamping the sheet at its periphery. Once the sheet is firmly fixed, a field of forces is applied to one of its faces using a pressurized fluid 16. This force field must be at least equal to that of the detector and may be equal to 1.5 times this one. The sheet is then deformed by the pressure exerted by the fluid 16. This deformation is a function of the material of the sheet, its proportions and the pressure exerted. It is therefore done freely under the sole effect

  uniformly distributed pressure.

  With reference to FIG. 5, the metal sheet is positioned in the same manner, the force field being obtained mechanically, by means of a die 18 actuated by a press (not shown). The shape of the matrix corresponds to the desired deformation of the sheet, this form corresponding to the natural deformation obtained by the means

described with reference to Figure 4.

  The choice of dimensions and the nature of the material used are dictated by the technical specifications of the detector to be manufactured, and in particular the operating pressure, the minimum transparency, and the radiation. the dose of radiation. The thickness is adapted to these conditions, as well as the chosen material, to remain within the usual safety limits. For example, an aluminum sheet to withstand a pressure of 14 bar (about 1,400,000 Pa) operating pressure, should have a thickness of 1 mm and a width of 40 mm. For a pressure of 40 bar (about 4,000,000 Pa), an aluminum plate should have a thickness of

  1.5 mm for a width reduced to 18 mm.

  The sanding of the sheet is done on the plate, when the material thereof is harder than that of the sealed chamber of the detector. Otherwise, sanding is

  performed on the detector enclosure.

  The sealed enclosure may be for example of forged aluminum. The detector material has therefore undergone a heat treatment to release the internal stresses of the material. As has been seen previously, the material used to make the entrance window must be an easily deformable material, that is to say ductile and malleable, while being transparent to the radiation to be detected. The sheet may therefore be laminated aluminum, for example from the duralumin family. If the metal foil is made of stainless steel, its thickness may be much less than aluminum foil. Of course, as we have seen above, it is also possible to use plastic and / or composite materials. Moreover, an adequate heat treatment can allow

  to improve the ductility of certain materials. -

  The reduction in the thickness of the sheet allows a very significant improvement in the transparency of the radiation input window. Problems due to temperature variations are eliminated because only one material

constitutes this input window.

  This manufacturing method makes it possible to lower the manufacturing cost of a window compared to the methods according to the prior art.

Claims (11)

  A radiation detector comprising: a sealed enclosure (2) containing a fluid under pressure, and in which a detection assembly is arranged; an inlet window of the sealed enclosure (2) through which the radii to be detected penetrate into the sealed enclosure (2), and - means for fixing the inlet window on the sealed enclosure (2), characterized in that the inlet window is formed by a preformed sheet (10).   2. Detector according to claim 1, characterized in that the preformed sheet (10) is constituted by a ductile material chosen as a function of the energy and the dose of the radiation to be detected and the operating pressure of the   detector, such as a metal or a plastic material.   3. Detector according to claim 1, characterized in that the preforming of the sheet (10) is such that the deformation of the sheet is at least equal to that obtained by a non-preformed sheet and subjected to the pressure of operation of the detector.   4. Detector according to claim 1 or 2, characterized in that the fastening means of the sheet (10) comprise a flange (12), and clamping elements (14) of the flange on the sealed enclosure (2). , the sheet (10) being sandwiched between the   flange (12) and the sealed enclosure (2).   5. Detector according to claim I or 3, characterized in that the means for fixing the sheet (10) on   The sealed enclosure (2) consists of a weld bead.   6. Detector according to claim I or 3, characterized in that the means for fixing the sheet (10) on   the sealed enclosure (2) consists of a bead of glue.   7. Detector according to any one of the claims   preceding, characterized in that a seal (8) is   placed between the sealed enclosure (2) and the sheet (10).   8. Method of manufacturing a detector according to one   any of the preceding claims, characterized in that   it consists in forming the entrance window from a sheet by the following steps: - fixing the non-preformed sheet with a specialized fastening tool (20) and reproducing the tightening conditions of the sheets; fastening means (12, 14) of the sheet (10) on the sealed enclosure (2), and - applying to the sheet, to deform, a field of forces of the same type as that applied to the sheet ( 10) during the operation of the detector and amplitude at least equal to this one.   9. Method according to claim 7, characterized in that the force field is applied to the sheet (10) hydraulically, the sheet (10) being directly in contact with a liquid (16) under pressure.   Method according to claim 8, characterized in that the force field is applied to the sheet (10) mechanically by means of a matrix (18), the shape of which corresponds to   the desired deformation of the sheet (10).   11. Method according to any one of claims 8   at 10, characterized in that the hardest material constituting the sheet (10) or the enclosure (2) is sandblasted on a peripheral zone (22), so as to facilitate the anchoring of the sheet (10) on the sealed enclosure (2).