DE2365189C2 - Image chamber for a radiographic facility - Google Patents

Description

where ρ is the position on the electrode gap area, V _{g is} the voltage drop at the gap in the gap center, D is the distance from an electrode gap area to the X-ray source, d is the distance between the electrode gap areas, D '= D + d and χ is the distance from ρ to the gap center in Is polar coordinates for plane electrodes and the axial distance of ρ from the gap center for cylindrical electrodes

3. Image chamber according to claim 1 or 2, characterized in that each of the first and second Electrodes (24 ', 21') a plurality of sections (40-45, 46-51) made of one material less Has electrical conductivity, each section being different and substantially uniform Has electrical conductivity

4. Image chamber according to one of claims 1 - 3, characterized in that the electrodes (21,24) flat parallel gap surfaces and a middle disk made of a material with high electrical conductivity own.

5. Image chamber according to one of claims 1 to 3, characterized in that the electrodes (21,24) concentric cylindrical gap surfaces and a central curved strip (30 ') from one Have material with high electrical conductivity.

6. Image chamber according to claim 1, characterized in that the electrodes (21, 24) are made of metal, that an e ^p ste dielectric plate (27; 60) on the first electrode (24) and a second dielectric plate (22; 61 ) define a gap between the plates on the second electrode (21), and that each of the plates has a dielectric constant which changes across the plate so that the electrostatic potential at the gap surfaces is equal to the electrostatic potential for concentric, spherical metal electrodes .

7. image chamber according to claim 6, characterized

• Draws that each plate has a first layer with a first dielectric constant and a second layer having a second dielectric constant wherein one of the layers has a maximum thickness and the thickness at a minimum value decreases at the edges, and the other of the layers being of complementary thickness

8. Image chamber according to one of claims 1-7, characterized in that the change in the Conductivity per surface area by changing the thickness of the electrode and / or by Change in the conductivity of the material of the electrode from the central zone to the edges is conditional

The invention relates to the generation of x-ray images without the use of conventional ones X-ray films, and particularly to an improvement in an electrode construction for an imaging chamber such as this For example, the subject of DE-OS 22 58 364 is In such a radiographic device is a X-ray impermeable gas between two spaced electrodes, which are connected to a common supply source, used in a picture camera to produce a photoelectric Generate current within this chamber, which is collected on a dielectric plate, which is on a of the electrodes, forming a latent electrostatic image. The latent image is then made visible by the methods of xerography. Collecting the primary photoelectrons that by which X-rays are generated, which enter the gas-filled gap between the electrodes are absorbed, and the secondary electrons, which are generated by the impact of the primary electrons on the gas atoms is created by applying an acceleration potential difference applied between the electrodes preferably on the order of a few to 10 kV

In conventional arrangements, the gas in the interelectrode gap fulfills two major requirements Tasks, namely firstly as the most effective absorber of X-rays to ensure high sensitivity in the figure, and secondly as a means of slowing down the primary electrons and to generate secondary electrons in the shortest possible distance from the generation point to maintain the resolution of the imaging system. For these reasons, the chosen gas is a gas higher Ordinal number, e.g. B. krypton or xenon, which is present in the inter-electrode gap at high pressure, preferably 20 atmospheres or more.

The maximum gas pressure in the gap is essentially

limited by the fact that it becomes more and more difficult with increasing pressure, the gas in a chamber with to include walls permeable to X-rays. For a given gas pressure, a increased absorption can be achieved by increasing the inter-electrode gap width. It this results in a slight increase in the lateral diffusion of electrons when this is up to move the collecting electrode. However, the spot size is due to diffusion for gap pressures of 20 Atmospheres or above, very small even for gap widths of 1 cm or more. The limiting factor in the

Image resolution is not the electron diffusion, but the geometric blurring that results from the oblique Incidence of X-rays in the imaging gap results

This is in FIG. 1, the sign of the growing reduction in the resolving power nut of deviation of the X-rays from the normal angle of incidence X-rays along the center beam line QAA 'occur to produce Photoe'sktronen along the line AA', which are multiplied in the gas (by generation of secondary electrons ) and which are accelerated by the electric field E , which runs perpendicular to the anode and cathode and thus parallel to the line AA 'The electrons are collected on the receiving plate in a spot around the point A , the width of which is small and through the area of the primary photoelectrons and is determined by diffusion of the secondary electrons. The effective spot diameter due to these effects is less than 0.1 mm. The charge distribution that is collected around the point A has a maximum at A and decreases with increasing distance from the point, as schematically in FIG. 2, on the other hand, X-rays incident along the line OBB ' pass through the gas at an angle θ to the perpendicular vector of the planes of the electrodes, and hence to the field E, and the electrons generated along the line BB' are not around the point B. around collected, but the line "around wherein Z?" ßß the projection of the point B 'on the receiving plate along the direction of e is a result of the attenuation of the x-ray beam is by the gas in passing through the gap along BB' more charges to B as collected to B "and the resulting charge image of the beam OBB 'is can be schematically shown in Figure 2 taken from the that a geometrically blurred picture, OÄB'ergibt from the absorption of the beam by the stronger B to and less strongly to ß'hin is valued.

For a beam incident at an angle θ and for an interelectrode gap width d , the extent of the blurred image is proportional to Qd for practical values of the gap width and the pressure (for which the mean free path - of the X-ray absorption is such that μ <ί <1, where μ is the X-ray attenuation coefficient. Thus, the geometric blurring can be reduced either by reducing θ, which includes a reduction in the field size since the maximum of Θ is equal to the ratio of the maximum image radius to the so-called film-focus distance, or by this that the gap width d is reduced, which results in a lower absorption of the incident X-rays and thus a loss of the quantum efficiency or the sensitivity of the arrangement

The fundamental cause of the geometric fuzziness in the case of the electrostatic latent image formed on the recording plate, the absence of the Coincidence between the line along which incident X-rays generate photoelectrons and the Field lines that accelerate these electrons towards the receiving plate. Since the electron generating orbits Are parts of straight lines or rays that all point to a common center, e.g. B. the X-ray source, indicate, to avoid a geometric blurring, the electric field lines in the Gas gap can also be parts of rays that point to the same center. To be more precise this means that the equipotential surfaces of the electrostatic potential in the gas gap parts must be concentric spheres that have their center in the X-ray source. This is then the case when the electrodes of the imaging chambers or the inner (facing the gap) surfaces these electrodes, which form the boundaries of the gas gap, are parts of concentric spherical shells, their midpoints at the assumed target position

ίο an X-ray source, so that the electric field between the electrodes runs radially with all field lines (or instead the extensions of the field lines beyond the electrodes) run through the common center point of the spherical shell electrodes. With such a configuration as shown in FIG. 3 and assuming that the X-ray source is arranged in the common center of the concentric electrodes, all of the lines AA ', BB', along whose primary electrons are generated, are field lines, and the electrons are thus accelerated along the same lines soft they were generated, with resulting "dot" images with dimensions defined only by the electron domain, which have no geometric fuzziness.

While the above solution to the problem of blurring is possible by using concentric, spherical cap electrodes, this solution has significant disadvantages, the most important of which is that it is difficult to produce planar images with spherical electrodes. If the dielectric recording plate which picks up the electrostatic latent image is flat, it is extremely difficult to stretch the image on a spherical surface which is topologically different from a flat or cylindrical surface, so that the surface becomes necessary to be elastic and uneven is stretched to conform to a spherical shape. The difficulty is increased by the fact that this has to be carried out within a pressure vessel with only a few millimeters between the walls. When a permanently attached dielectric is used as the latent image receiving plate on which the visible image is first formed, the difficulty arises in transferring the image obtained on this receiving plate to the end receiving plate, which must be flat. This, in turn, is very difficult to achieve in practice.
In summary, an imaging chamber with planar parallel electrodes has the advantage of simple attachment of the dielectric receiving plate and, more generally, of practical usefulness, but has the disadvantage of geometrical fuzziness. On the other hand, a chamber with spherical electrodes overcomes the problem of geometric fuzziness, but is difficult to implement in a practical arrangement. In certain applications it is desirable to form the electrodes of the imaging chamber cylindrical, because such an arrangement provides additional mechanical strength and the receiving plate can be more easily attached to the electrode surface. A cylindrically curved surface is topologically equivalent to a planar surface so that no stretching of a planar receiving plate is required, while the curvature allows a receiving plate advanced by a roller to contact the curved surface more easily than a planar surface

10

20th

25th

- is pulled. The electrode surfaces at the gap in the cylindrical configuration are common Parts of coaxial cylinders with their common axis going through the X-ray source. The equipotential surfaces in the gas gap of an imaging chamber with coaxial, cylindrical electrodes are also parts of coaxial cylinders, and therefore the problem of geometrical fuzziness similar to that occurs here as well Problem that exists with plane parallel electrodes, also in the case of cylindrical electrodes.

The object of the invention is to solve the problem of the geometric problem that occurs in image chambers of the known type Solve blurring by oblique X-ray incidence and at the same time a plane or cylindrical To maintain the surface of the dielectric support plate.

According to the invention, this object is achieved with the characterizing features of claim 1.

Further refinements of the invention are the subject of the subclaims.

The invention is described below in conjunction with the drawing on the basis of exemplary embodiments explained it shows

F i g. 1 is a schematic illustration showing the operation of an imaging chamber with planar parallel electrodes becomes visible,

F i g. FIG. 2 shows a schematic representation of the charge distribution for the configuration according to FIG. 1,

FIG. 3 is a schematic representation similar to that of FIG according to Fig. 1, in which concentric, spherical electrodes are shown,

F i g. 4 is a schematic representation of an X-ray arrangement with an imaging chamber in which the preferred embodiment of the invention is used,

F i g. 5 shows the imaging chamber according to FIG. 5 in an enlarged view. 4,

F i g. 6 schematically shows an equivalent circuit diagram of the imaging chamber according to FIG. 5,

F i g. 7 is a top perspective view of a cylindrical cathode which is a modified Embodiment of the invention represents

F i g. 8 is a view similar to that of FIG. 5 showing another embodiment

F i g. 9 is a view similar to that of FIG. 7, which the modified embodiment of Fig.8 in connection with a cylindrical electrode shows and

F i g. 10 is a view similar to that of FIGS. 5 and 8 for a further embodiment of the invention.

The arrangement according to FIG. 4 shows an X-ray tube 10 which directly irradiates an object 11 a table 12 can be arranged. An imaging chamber 13 that can accommodate the receiving plate 14 beneath the table, with x-rays from source 10 passing through object 11 go through and into the gas-filled gap 15 of the imaging chamber 13. The interpretation of the Imaging chamber itself is not the subject of the present invention, and various known ones can be included Imaging chambers including those indicated above in the prior art are used will.

The imaging chamber may have a housing 20 with a high resistance cathode 21, which rests on an insulating plate 22. The housing cover 23 can serve as electrical ground, wherein the means 23 of the anode 24 of high resistance to the cover via a fine, electrically conductive (e.g.

55

60 aluminum) wire or thin strip 26 connected! The anode is otherwise connected to the housing cover practicing a thin adhesive insulator 27 so that they only; is in electrical contact with the cover via the strip or wire 26. Conductive strips 28 and 29 are with the outer edges of the; Anode and cathode so that good electrical contact is obtained all around the edges of the electrodes. The outer edge of the anode is electrically across the strip 28 to the center

30 of the cathode via a variable resistor ^

31 connected. The outer edge of the cathode is connected to a supply source 32 via the strip 29. The | other terminal of the power source 32 is grounded or connected in ί equivalent manner with the housing cover 23jf. |

The electrical circuit formed by the above arrangement is schematically shown in FIG. 6 i shown. The chamber also has a device (not | shown) for introducing a gas under pressure have in the gap j. The imaging chamber according to FIG. 4j is used in the same way as in known arrangements i described in the above-mentioned patent and differs from known arrangements in the construction of the electrodes. '

As already mentioned, it is necessary to align the J electric field lines within the gas gap; Seal chamber with the incident X \ rays necessary that the equipotential surfaces; within the gap are parts of spheres, the center of which is the X-ray source. However, from the theory of the electrostatic field, it is known that in an arbitrary area which is free of wettable charge, the [ electrostatic potential and the gradient> £ = -νΦ, which by definition is the electric field vector completely within a range ) are determined by specifying the value of Φ at the limits of this range and that, of course, specifies the change in the dielectric constant E ! within this range The dielectric constant of the gas gap is about 1.

As applied to the problem of the imaging chamber with planar, parallel electrodes in electron radiography as shown in FIGS. 4 and 5, this means that when the potential at the surfaces S \ and; S _2, which define the gas gap (the gap is substantially! Smaller than the electrode dimensions, the edge effects due to the limited dimensions of the ^electrode! Surfaces are negligible) is specified, the] electrostatic potential and the electric field "within the gas gap clearly and are fully determined.

Therefore, if it is desired that the equipotential; surfaces within the plane gas gap are parts of j spheres with the center in the X-ray tube, it is | only necessary and sufficient that the electrical |, static potentials Φι and Φ ₂ on the surfaces Si | and S2 are chosen so that they have such equipotentia- | Ien correspond. In other words, this means that the g surfaces Sj and Si would not be equipotential surfaces (the equipotentials are spherical), and the values of Φ \ and Φ2 at given points on 5Ί and Sz would be functions of the position of these points. In reality, if the position of the points Pj and Pi on S \ and S _{2 were expressed} by the polar coordinates r \ and T ₂ and the equipotentials sought must be concentric spheres with their centers at a distance D and D + d from the surfaces Si and S ₂

the change of the potentials on S \ and Sj as functions of r \ and &

r σ

(D

(2)

(3)

where V ^ is the voltage drop across the gap in the middle of the gap and £>'= D + dist

The desired change in electrostatic potential along surfaces Si and S ₂ , as defined in equations (1) and (2), can be achieved by using variable resistance electrodes which pick up a potential difference between different points on their surfaces can. Of course, a current will flow along the electrodes due to the applied potential.

The following is a mathematical analysis of the problem: Let a current conductor in the form of a circular disk with variable thickness t (r) and current conductivity a (r) be used, where r is the radial polar coordinate measured from the center of the disk. The center of the disk is largely a perfect conductor up to a radius ro (e.g. made of aluminum). For a typical imaging chamber for a 30 by 45 cm platen, ro can be on the order of 1.25 to 2.5 cm. The annular region between r = ro and a maximum radius r _m is composed of a material having relatively small and varying in the radial direction Stromjeiifahigkeit from the partial equation of the current conductor y = oE and the law on the conservation ^ energy V · y = P, wherein j is the current density and E is the electric field, it can easily be shown that the conductivity and the thickness of the material approximate according to the equation

30th

35

40

are related to each other, where Φ '(ή is the derivative of the potential Φ (γ) with respect to r and / is a constant which is equal to the current flowing over a closed contour on the disc

Equation (3) shows that in order to achieve a certain change Φ (γ) in the potential at the electrode, either the conductivity (r), the thickness t (r) or both of each electrode can be changed in such a way that equation (3 ) is fulfilled It can be shown in a simple manner that the As an example for a typical set of parameters the following quantities are chosen: V ^ = IOOOOVoIt and / = 1 milliampere; shall be based on the case in which the current-conducting electrode is homogeneous, ie, a '(r) is a constant that applies ofr = ^ oo = 10- ^{5 ohm-1} cm- ^1; only the thickness changes, and r _o = 3cm, D = 100 cm and d- 1 cm are chosen. From equations (3) and (5) it follows that the thickness of the current-conducting electrode must change from ifro) = 2OO to i (r _m ) = 8 μ. The total power loss, which is calculated from equation (4), is about 5 watts, which means a negligible value for heating ^{over a typical exposure time of 1 Ao second.}

On the other hand, an electrically conductive electrode of uniform thickness and having a material of variable electrical conductivity can be used. Then, for the example given above, the uniform thickness is assumed to be 40 μ, the conductivity must range from 2 × 10 " ⁶ ohm-'cm-' at r ₀ ^{to 5} × 10 ~ 5 ohm - 'cm -' at r _m .

In the preferred range of current conductivities between 10- ⁶ ohm 'and 10- ⁴ ohm' cm- 'are many materials, including chalcogenide glasses (z. B. As45Se5s) and carbon impregnated plastics (for. Example, thermosetting epoxy resins with acetylene black) available , the latter is used for easier fitting and shaping and because of the low temperature dependence of the electrical conductivity. These materials can be cast in molds or machined to the desired thickness, and their conductivity can be changed from center to edge by changing their composition (e.g. by changing the conductivity and / or by filling the carbon black filler in the material). When conductors of non-uniform thickness are used, the insulating substrate to which they are connected is given an inverse curvature to ensure that the gap side of the electrode is flat, although slight deviations from the flat configuration are not critical.

The change in the thickness and / or the conductivity along the electrodes would be largely the same for the anode and cathode, since the potentials along the two electrodes have the same shape and are only different by replacing the length (D + d) for the length D between the Differentiate equations (1) and (2).

For the typical example given above, the change in potential between the center and the circumference is e.g. B. the anode about 5000 volts, and the total resistance R \ between the center of the anode and the periphery is about 5 megohms. The scope

Power P, which is lost in the conductor due to the current flowing through it, corresponds to the product

the center of the

(4)

For a potential of the form according to equation (1) or (2), namely

applies to the derivation or the ek! Irish field

Cathode must be connected via an additional resistor of 5 megohms (the variable resistor 31 from Fig. 6), and the applied voltage V, which is required between the cathode circumference and the center of the anode to generate a gap voltage of 10,000 volts at the gap center , is 15,000VoIt

Similar considerations apply to the case of the coaxial, cylindrically curved electrodes the X-ray source are centered. The radii for the cylindrical electrodes are large, and the figures of the The drawings also show this embodiment in which the cylindrical axis is perpendicular to the plane of the drawing A perspective view of a cylindri- (5) see cathode is in FIG. 7 on a reduced scale

' shown In the case of a coaxial cylindrical configuration

ίο

, structure can be shown in a simple manner that the "electrical potential at a point ρ on the cylindrical surface is according to the equation

♦ i (Pi)

Vf -ξ- Ο + Zl / Ö> T ^m

(6)

(7)

10

15th

must change, whereby

Vg is the gap voltage,

D is the cylinder radius,

d is the gap width,

D ' the sum "+ rfur.d

Z is the axial coordinate of the point ρ ,

d. i.e., the coordinate along the cylinder axis of the center of the cylindrical cap that forms the electrode is measured according to equation (1) and (2). There is thus an axial dependence of the potential in the case of cylindrical electrodes. the Equation according to equation (3) and the corresponding electrode current conductivity, thickness and derivative of the electrostatic potential in the case of the cylindrical electrodes

(8th)

30th

where S is the arc length of the cylindrical section that represents the electrodes and the thickness r and the conductivity σ of the electrode functions are only of the axial coordinate Z. The desired. Values for the electrode thicknesses, the material flow conductivities and the resulting values for current and power losses are of the same order of magnitude as in the case of flat electrodes.

The electrical connections with the cylindrical electrodes are basically the same as in the case of flat electrodes: The middle disk of the "perfect" conductor of each electrode is made up of a central, curved strip (30 'in Fig. 7) of "perfect" electrical conduction with a few centimeters Width, and the two curved edges (29 'in Fig. 7) of the electrode are electrically connected to one another. The curved edges of the anode are connected to the central, curved strip on the cathode 21 via the variable resistor 31. The curved edges of the cathode 21 are connected to the central strip of the anode 24 via the supply source 32. In a typical example, D may be on the order of 2 meters so that the edge of a 30 cm wide electrode is about 1 millimeter out of plane. The term "largely planar" is intended to encompass such an arrangement, both in planar and cylindrical embodiments.

The use of curved electrodes, referred to as virtual, also implies that the Field lines and the path of generation of primary photoelectrons in the gas gap are only coincident if the X-ray source is placed in the middle of the virtual concentric electrodes.

The middle zone with uniformly high electrical conductivity per unit area is preferred but is not critical.

This zone is the disk in the embodiment with planar electrode and the strip in the Embodiment with a cylindrical electrode. The problem with the oblique path of the X-rays is extremely little or nonexistent in the central zone. This zone higher Current conductivity results in a good surface for an electrical conductor connection and also simplifies the Process of making the electrode.

An approximation to the desired, ideal, concentric, spherical potential change can be achieved by Means can be achieved that are other than the continuous change in the conductivity and / or thickness of the electrodes. For example, a plurality of concentric rings, each of which Has constant current conductivity, used to form an electrode v / earth, the current conductivity of each ring from the center to the edge is selected to a value such that the The change in potential along the radial coordinate becomes the desired ideal change in potential in a step-like manner approaching.

A pair of electrodes 21 ', 24' having this construction are shown in FIG. 8, the elements corresponding to those according to FIG. 5 have the same reference numerals. In the electrode 24 'a plurality of concentric rings 40-45 are arranged around the conductive center 25 to the conductive edge 28. A similar construction is provided for the electrode 21' with concentric rings 46-51 . Each of the rings 40-51 is made of a low electrical conductivity material as used in the embodiment of Figure 5. Each ring has a uniform electrical conductivity, but the electrical conductivity varies from ring to ring and approaches the desired relationship discussed above. In the case of the in FIG. 8-8, only six rings are shown for clarity. However, a typical imaging chamber may use 15 rings, each 1.25 cm wide

The approximate configuration according to FIG. 8 is suitable for use with the cylindrical electrode configuration of FIG. 7, and a corresponding arrangement is shown in FIG. In the case of the electrode 21 'according to FIG. 9, strips 46'- 51 'are provided between the center 30' and the edges 29 '. Each of the strips 46 ' to 51' has a constant electrical conductivity, the electrical conductivity of adjacent strips being selected so that a stair-step approximation to the desired, ideal change in potential is achieved.

In a further embodiment of the invention the desired electric field is achieved by using metal electrodes, typically aluminum or beryllium, with dielectric plates of uniform thickness on each electrode surface, each plate containing at least two different dielectric materials. One such configuration shown in Fig. 10, the elements corresponding to those according to Fi g. 5 are provided with the same reference numerals. The dielectric embodiment is also applicable to the cylindrical configuration of FIG. 7 applicable. A dielectric plate 60 is received on electrode 24a and another dielectric plate 61 is received on electrode 21a.

The desired change in the electrostatic potential along the interfaces Si and S _{2 of} the gas gap can be achieved by using the composite dielectric plates 60, 61 between the

flat electrodes 24a, 21a and the surfaces Si, S _{2 of} the gas gap can be achieved. The dielectric plate consists of a pair of dielectric inserts 60a, 606 of variable thickness and uniform but unequal dielectric constants ei, ^ ₂ . The dielectric plate 61 is similarly composed of a pair of dielectric inserts 61a, % \ b of variable thickness and uniform but unequal dielectric constant β2, & Ϊ.

The dielectric constants ei, e \ and C2, ei are chosen so that the desired electrostatic potentials are realized _{at the surfaces S 1} and S _2. The total thickness of the dielectric t \ + t \ '= T \ between the surface S \ and the electrode 24a is constant, as is the thickness f ₂ + fe' = 7 ^ between the surface Si and the electrode 21a. The dielectric plate 60 has a thickness T \ with layers 6öa, 6öü of a thickness t \, t \. The dielectric plate has a thickness T ₂ with layers 61a, 616 of thickness t ₂ , t ₂ '. The individual thicknesses ti, t \ and f ₂ , ti are functions of Λ and r ₂ such that the desired potential is realized for Si and S _2. The boundaries bi and fc between the dielectric layers are not flat, but are determined by the desired potential distributions

The exact shape of the boundaries b \ and hi or, more simply, the exact functional dependencies fi (i \) and h (r ₂ ) of the thickness of the dielectric layers 60a, 61a as well as the total thicknesses T \ and T ₂ can be obtained by numerical calculation ( Resolution according to the limits that give the desired solution to the La Place equation). Qualitatively, however, each composite dielectric plate consists in one embodiment of an inner layer (i.e. adjacent to the electrode) of a relatively higher dielectric constant ei maximum thickness fi (0) in the middle, which decreases to a minimum thickness fi (pi max) at the edge of the viewing area, and an outer layer of complementary thickness ti ' (such that ti + ti - Ti) and lower dielectric constant ei'<ei. The structure between the electrode 21a and the surface S ₂ would be similar. Changes to this structure are possible, in particular by reversing the inner and outer dielectric layers.

On the other hand, the desired change in potential can be achieved with the help of a single layer in the dielectric Plate can be achieved with a variable dielectric constant. The operation of the embodiment with the dielectric plates is similar to that in the embodiment with the electrodes inferior Conductivity, with the exception that the effective surface charge flowing through the conductor in the conductor Electricity generated is replaced by static polarization charge. The dielectric embodiment has a disadvantage in that the thickness of the dielectric, which is the case in most applications is required, must be large, that a fairly high opposing field than the collected charge due to the Image build-up. Also, dielectric constant control is available in these days standing material considerably more difficult than controlling the electrical conductivity.

For this purpose 3 sheets of drawings

Claims (2)

Patent claims: 1. Image chamber for a radiographic device with a gas impermeable to X-rays between two electrodes which are arranged at a distance from one another and are connected to a common supply source, characterized in that the supply source (32) is connected to the center of the first electrode (24, 24 ') and is connected to opposite edges of the second electrode (21, 2V) , and that a resistor (31) is placed between opposite edges of the first electrode (24) and the center of the second electrode (21,21 '), such that the Current conductivity per unit area of each electrode changes from a central zone to the edges so that the electrostatic potential at the gap surfaces of the electrodes corresponds to the electrostatic potential for concentric spherical metal electrodes 2. Image chamber according to claim I _1, characterized in that the electrostatic potentials Φ \ and Φ ₂ at the gap surfaces of the electrodes