APPARATUS FOR SLIT RADIOGRAPHY

Abstract: A dosimeter for ionizing radiation is of oblong shape. It comprises a gas-filled measuring chamber surrounded by a casing. The casing is formed of two oblong frames (7,8) of insulating material that are substantially similar in shape. The two frames are mounted on each other in a gas-tight manner. The anode wires (13) are situated in the plane of separation between the frames and are secured by the frames in contact with each other. At least one of the frames is provided with a cathode (14,15) over its whole length and parallel to the plane in which the anode wires are situated. Along the edge of the cathode extends a guard electrode which surrounds the cathode with a certain gap. The dosimeter is made of materials that are transparant to the ionizing radiation. It can be used particularly in slit radiography equipment in which the slitwidth can be controlled locally and independently along the length of the slit.

Description: The invention relates to a dosimeter for ionising radiation comprising a gas-filled measuring chamber sur­ rounded by a casing in which there extend a number of anode wires, which carry a fixed electrical voltage in operation, and at least one cathode element, which casing is provided with at least one entry window for the ionis­ ing radiation. Such dosimeters are already known from the Hand­ book on Synchrotron Radiation, vol. 1A, pages 323-328 by Ernst Eckhard Koch, published by North-Holland Publishing Company, Amsterdam, New York, Oxford, 1983. A drawback of said known dosimeters is that application thereof in slit radiography equipment, in which, during the produc­ tion of a radiograph, it has to be possible to measure and regulate the quantity of radiation in each diaphragm sec­ tion transmitted through a diaphragm slit at any instant, is not readily possible. An example of such slit radio­ graphy equipment, in which use is not made, however, of a dosimeter of the type described above, is described in Dutch Patent Application 8400845 . The known dosimeters are not designed to attenuate the radiation, the intensity of which has to be measured, as little as possible and to prevent the formation of a visible X-ray shadow image of the dosimeter itself as much as possible. This latter is, however, of great importance in slit radiography equipment because the radiation transmitted through the dosimeter has to produce the required radiograph. The shape and dimensions of the known dosimeters also make them unsuit­ able for application in slit radiography equipment. There is therefore a need for a dosimeter for ionising radiation, in particular X-ray radiation, which is suitable for application in slit radiography equipment. The object of the invention is to meet said need. For this purpose a dosimeter of the type described is characterized according to the invention in that the casing has an oblong, essentially rectangular shape and that the measuring chamber is an oblong, essentially rectangular cavity which is recessed in the casing and in which there extends a relatively large number of parallel anode wires, distributed over essentially the whole length of the measuring chamber, in a direction essentially transverse to the longitudinal direction of the measuring chamber. The invention will be described in more detail below by reference to the accompanying drawing of an exemplary embodiment. Figure 1 shows a diagrammatic cross-section of an embodiment of a dosimeter according to the invention; Figure 2 shows a section along the line II-II in Figure 1; Figure 3 shows diagrammatically how a dosimeter according to the invention can be applied in slit radio­ graphy equipment; Figure 4 shows an example of a cathode element for a dosimeter according to the invention; and Figure 5 shows diagrammatically an end view of a variation of figure 4. The dosimeter shown in figures 1 and 2 comprises an oblong measuring chamber 1 having parallel long sides 2, 2a, 4, 4a or 3, 3a, 5, 5a. When applied in slit radiography equipment, the measuring chamber is at least equally as long as the width of the flat X-ray beam, which is transmitted through the slit diaphragm and which spreads out fan-like, at the point of the intended dose measurement. If the dose measurement has to take place in the immediate vicinity of the slit diaphragm, the measu­ rement chamber may be equally as long as, or somewhat longer than, the diaphragm slit. The further away from the diaphragm slit the dose measurement is made, the longer the measuring chamber has to be. For measurement near the X-ray screen, the length of the measuring chamber may be, for example, 40 to 45 cm. The measuring chamber is formed in an oblong casing 6 made of insulating material such as, for example, glass or ceramic material. The casing consists of two, essentially mirror-image parts 7, 8 which are attached to each other in a gas-tight manner, for example by fritting , fusing or gluing. Each casing part con­ sists essentially of an oblong strip of an insulating material in which there is recessed an oblong groove approximately of the size of the measuring chamber to be formed so that an oblong frame is produced. As can be seen in figure 1, both frames are pro­ vided with a recess along the innermost long edges and on the sides facing each other, as a result of which both frames form shoulders 9, 10, 11, 12 which face each other and extend along the long edges. The purpose of said shoulders will be described in more detail below. Transversely to the longitudinal direction of the frames 7, 8 a large number of parallel thin anode wires 13, which carry a fixed electrical voltage in operation, extend through the measuring chamber 1. In a practical embodiment the anode wires may be, for example, gold-­ plated tungsten wires which, for example, have a mutual spacing of 2,5 mm and a thickness of 50 µm. The anode wires are mounted so as to be distri­ buted over essentially the whole length of the measuring chamber. The anode wires are situated in the plane of separation between the frames 7 and 8 and are secured by the surfaces of the frames which are in contact with each other. The anode wires are situated, moreover, in the centre plane between two parallel flat cathode elements 14, 15 which are mounted on the shoulders 10, 12 and on the shoulders 9, 11 of the two frames. The two cathode elements are electrically connected to each other. It is pointed out that in principle, however, it would be pos­ sible to make do with one cathode element. The cathode elements may consist of a wire grid, but according to a preferred embodiment of the invention, each is constructed of a thin insulating sheet-like carrier on which a conducting metal layer is provided on the side facing the anode wires. The carrier may be manufactured from a suitable plastic material or another suitable material such as glass. Figure 4 shows a preferred embodiment of the cathode elements 14, 15 which are here diagrammatically shown in their mutual relationship and in their relation­ ship to the anode wires 13, the rest of the dosimeter having been omitted for the sake of clarity. The cathode elements shown in figure 4 each comprise in turn a thin sheet-like carrier which is provided with a guard elec­ trode 40 or 41, which is connected to earth along the edges. Inside each guard electrode there is situated the actual cathode 44 or 45 which is insulated with respect to the guard electrode by a narrow gap 42 or 43. The guard electrodes serve to remove any leakage currents, as a result of which the signal/noise ratio of the output signal of the dosimeter is improved. In the embodiment shown in figure 4 the guard electrodes are discontinued at 46 in order to form a passage for a connecting section 47 of the cathodes. The operation of the guard electrodes can be optimized further, if desired, by constructing the guard electrodes without discontinuity. A connection bridging the guard electrodes then has to be provided for the cathodes. For this purpose, for example, a connection which is brought through an opening in the carrier may be provided, as is shown diagrammatically by way of example at 48 in figure 5 for the cathode element 14. The passage 48 is shown diagrammatically in figure 5 for the cathode element 14. The passage 48 or another electrical connection of the cathodes is situated preferably outside the region of the anode wires. In a practical embodiment the carriers may consist of glass film with a thickness of approx. 80 µm which is mounted on the shoulders 9, 11 or 10, 12 respec­ t1vely by fritting. A thin layer of gold or silver or another metal with good conductivity may be provided on the carriers by evaporation. At least one head end 1a of the measuring chamber 1 extends in the longitudinal direction beyond the flat cathodes and the region of the anode wires so that the cavity between the cathodes is in communication with the cavities on the sides of the cathodes facing away from the anode wires. The measuring chamber is, moreover, sealed in a gas-tight manner by windows 16, 17 which are manufactured from a material which does not attenuate, or scarcely attenuates, the X-ray radiation. In a practical embodi­ ment the windows may, for example, be manufactured from glass film having a thickness of 0.5 mm. The windows are preferably constructed in a curved manner because of strength considerations. In figure 1 the windows 16, 17 are constructed somewhat convexly. It is, however, also possible to construct the windows concavely as seen from the outside. Such a con­ struction can be applied with advantage if the internal pressure in the dosimeter is at least during some period greater than the ambient pressure, as may be the case, for example, during transportation by air. In the exemplary embodiment shown, both frames 7, 8 have an external flange 18 or 19 respectively (figure 1) which extends along the plane of separation between the frames and in the longitudinal direction of the frames. On one of the flanges, in the drawing flange 18, a narrow conducting track 20, for example a silver track, is provided in line with each anode wire 13. The end of an anode wire is mounted by soldering on each track. The flange 18 with the conducting tracks may advantageously be constructed as a connector so that the signals from the anode wires can be supplied in a simple manner to processing equipment for said signals. The flange 19 of the frame 8 shown in figure 1 could, if desired, be provided in a similar manner with connection tracks for the anode wires. This is not, how­ ever, necessary, and in the exemplary embodiment shown the flange 19 is constructed as a mounting flange, by means of which the dosimeter may be mounted on a carrier or the like. For this purpose, the flange 19 is provided with a number of fixing holes 21, one of which is shown. An advantage of the embodiment shown is that the two frames 7, 8 can have exactly the same shape, which simplifies the manufacture. As an alternative, however, the flange 19 may be omitted as shown in figure 2, and the flange 18 could be constructed somewhat longer as is indicated by the broken lines at 18′, and may be provided with fixing holes. In figure 2 a tubular stem 22 is also shown which is in communication with the measuring chamber and which can be connected to a vacuum pump to evacuate the measur­ ing chamber. The measuring chamber is then filled with a suitable gas such as, for example, a mixture of Ar and CH₄, which may have a pressure of 1 atm. The tubular stem can then be sealed by pinching. If desired, the other frame 8 may also be provided with such a tubular stem. The tubular stem may advantageously serve as a connecting conductor for the cathodes 14 and 15 or for the guard electrodes. In figure 2 an electrical connec­ tion is shown between the tubular stem 22 and the cathode 15 at 23. In relation to the gas situated in the measuring chamber it is further pointed out that the argon gas pro­ duces the output signal under the influence of ionising radiation at the electrodes, while methane gas is added as a quenching gas ("quenching agent") in order to counteract the so-called avalanche effect. Preferably, however, the anode/cathode voltage is chosen so that the avalanche effect does not normally occur. The methane gas could then be omitted. Since Ar is relatively transparent to ionising radiation, to obtain a sufficiently large output current (greater than approx. 10⁻¹⁰ A), it is necessary to work in the so-called gas-multiplication region, as a result of which a signal amplification by, for example, a factor of 1000 can be obtained, depending on the chosen anode/­ cathode voltage. If the dosimeter forms part of a re­ gulating circuit for controlling the amount of X-ray radiation transmitted per section of a slit diaphragm of slit radiography equipment, this also opens up at the same time the possibility of adjusting the gain for the regulating circuit belonging to each section of the slit diaphragm by means of the anode/cathode voltage of the dosimeter. Figure 3 illustrates some possibilities of appli­ cation of a dosimeter according to the invention in slit radiography equipment. It is pointed out that the dosi­ meter may also be applied in other situations and is in particular suitable, in general, for detecting the dis­ tribution and variation of the intensity of ionising radiation over an extensive region. If only the total dose of ionising radiation is of interest in the measurement region, the signals from the anode wires can be added together or the anode wires can be connected together. Figure 3 shows diagrammatically slit radiography equipment having an X-ray source 30 which can irradiate a body 33 to be investigated with a flat X-ray beam 32 hav­ ing a scanning movement indicated by an arrow 34 via a slit diaphragm 31 in order to form an X-ray image by means of an X-ray detector 35 placed behind the body. If it is only desired to determine the total X-­ ray dose to which the body 33 is exposed during one or more scanning movements, the dosimeter may be disposed in the vicinity of the slit diaphragm or even against the slit diaphragm as shown diagrammatically at 36. The output signals from the dosimeter cannot then be used, however, to control the quantity of radiation transmitted locally through the slit diaphragm in order to obtain an equaIized radiograph. For this purpose, the dosimeter has to be situated, as indicated at 37, between the body 33 and the X-ray detector 35 and obviously has to track the scanning movement of the X-ray beam 32. The dosimeter may be mounted, for example, on an arm 38 which moves synchronously with the slit diaphragm. The output signals from one anode wire or from a number of adjacent anode wires each time provide a measure of the irradiation intensity prevailing instantaneously in the associated sector of the X-ray beam and, there­ fore, also of the brightness of the part of the radio­ graph to be produced corresponding to said sector. Said output signals can therefore be used to control attenua­ ting elements 39 which interact with the corresponding section of the slit diaphragm in order to achieve image equalization. In order to prevent large differences between the output signals of (sets of) anode wires of the dosimeter which interact with adjacent sections of the slit dia­ phragm, the output signal from each set of anode wires belonging to a certain diaphragm section or, if one anode wire is present for each diaphragm section, from each anode wire may be combined, if desired, with the output signal from one or more anode wires belonging to adjacent sections of the slit diaphragm, in order to obtain the control signal for the section concerned. In a practical embodiment a dosimeter according to the invention may have a measuring chamber which is 40 cm long and contains 160 anode wires. If the slit dia­ phragm has, for example, 20 controllable sections, 8 wires are available per section. The signals from said 8 wires are then combined into a control signal for the associated diaphragm section. However, as described above, the output signals of one or more adjacent wires belonging to adjacent sections might also be additionally involved in the forma­ tion of the control signal. Depending on the type of X-ray detector used, it is possible, as an alternative, to control the attenua­ tion elements on the basis of the radiation transmitted by the X-ray detector 35. The dosimeter may then be sited behind the X-ray detector, as indicated at 40, and must therefore again move synchronously along with the scanning movement of the X-ray beam 32. In any case it is an advantage that a dosimeter according to the invention can be constructed with a very small thickness, in the order of 10 mm or less. Despite the fact that very thin anode wires may be used, there is the risk that the anode wires may give rise to artefacts in the form of thin stripes in the radiograph to be produced. If desired, this can be pre­ vented by ensuring that the wires extend somewhat ob­ liquely with respect to the scanning direction. This can be achieved in a simple manner by mounting the dosi­ meter itself somewhat obliquely with respect to the scan­ ning direction or by mounting the anode wires at a small angle with respect to the centre line of the dosimeter. It is pointed out that in addition to the above various modifications are obvious to those skilled in the art. Such modifications are considered to fall within the scope of the invention.

Claims: 1. Dosimeter for ionising radiation comprising a gas-filled measuring chamber surrounded by a casing in which there extend a number of anode wires, which carry a fixed electrical voltage in operation, and at least one cathode element, which casing is provided with at least one entry window for the ionising radiation, characteri­ zed in that the casing has an oblong, essentially rectan­ gular shape and that the measuring chamber is an oblong, essentially rectangular cavity which is recessed in the casing and in which there extends a relatively large number of parallel anode wires, distributed over essen­ tially the whole length of the measuring chamber, in a direction essentially transverse to the longitudinal direction of the measuring chamber. 2. Dosimeter according to claim 1, characterized in that the casing is formed from two oblong strips of insu­ lating material, essentially similar in shape, which are each formed into an oblong frame by the provision of an oblong recess, which frames are mounted on each other in a gas-tight manner, the anode wires being situated in the plane of separation between the frames and being secured by the frames in contact with each other. 3. Dosimeter according to claim 2, characterized in that a cathode element, which extends essentially over the whole length of the measuring chamber and is essen­ tially flat, is mounted in at least one of the frames parallel to the plane in which the anode wires are situa­ ted. 4. Dosimeter according to claim 3, characterized in that each frame is provided, along the innermost long edges thereof, with a shoulder extending parallel to the plane of the anode wires and at a short distance there­ from, and that the two shoulders of each frame carry a flat cathode element. 5. Dosimeter according to claim 3 or 4, characteri­ zed in that each cathode element consists of a thin insulating carrier on which a metal layer is provided. 6. Dosimeter according to claim 5, characterized in that each cathode element consists of glass film onto which a noble metal is deposited by evaporation. 7. Dosimeter according to claim 4, characterized in that each cathode element is mounted on the shoulders of the associated frame by fritting. 8. Dosimeter according to one of claims 3 to 7 incl., characterized in that each flat cathode element is pro­ vided with a guard electrode which extends along the edge of the cathode element, which is connected to earth dur­ ing operation and which surrounds the actual cathode with a certain gap. 9. Dosimeter according to claim 2, characterized in that the measuring chamber is sealed in a gas-tight man­ ner by means of thin glass plates mounted in a gas-tight manner on the surfaces, which face away from each other, of the frames. 10. Dosimeter according to claim 9, characterized in that the cavities between each glass plate and the asso­ ciated cathode element are in communication with each other and with the cavity situated between the cathode elements via at least one head end of the measuring cham­ ber extending beyond the cathode elements. 11. Dosimeter according to claim 2, characterized in that at least one of the frames has an oblong external flange which extends in the longitudinal direction of the frame and one of the surfaces of which coincides with the plane of separation, which flange is provided with conduc­ ting tracks extending parallel on the said surface trans­ versely to the longitudinal direction, each anode wire being mounted on one of the conducting tracks by solder­ ing. 12. Dosimeter according to claim 11, characterized in that the flange is constructed as a connector. 13. Dosimeter according to claim 2, characterized in that both frames have an oblong external flange extending in the longitudinal direction of the frames, one of the surfaces of at least one flange coinciding with the plane of separation and being provided with conducting tracks extending on the said surface transversely to the longi­ tudinal direction, each anode wire being mounted on one of the conducting tracks by soldering, and the other flange being provided with fixing holes. 14. Dosimeter according to claim 13, characterized in that the flange provided with conducting tracks is con­ structed as a connector. 15. Dosimeter according to claim 1, characterized in that a tubular stem is present which is in communication with the measuring chamber, which is sealed by pinching outside the casing, by means of which the cathode ele­ ments are electrically connected and which serves as an electrical connecting terminal for the cathode elements. 16. Dosimeter according to claim 1, characterized in that the anode wires extend somewhat obliquely with res­ pect to the longitudinal direction of the measuring chamber. 17. Dosimeter according to claim 1, characterized in that the measuring chamber is filled with argon gas and that the anode/cathode voltage during operation is such that gas multiplication occurs. 18. Dosimeter according to claim 17, characterized in that methane gas is added to the argon gas. 19. Method for applying a dosimeter according to one of the preceding claims, characterized in that the anode wires are grouped into a number of sets of anode wires and the signals from the anode wires belonging to each set are combined to provide an output signal belonging to the set concerned. 20. Method according to claim 19, characterized in that the signals from the anode wires belonging to each set of anode wires are combined with signals from one or more anode wires belonging to adjacent sets to obtain an output signal belonging to the set concerned. 21. Method according to claim 19 or 20, characterized in that the dosimeter is applied in slit radiography equipment with a slit diaphragm which is provided with controllable attenuation elements which are able to attenuate locally the radiation transmitted, or to be transmitted, through the slit diaphragm, the dosimeter being located at each instant in the radiation beam transmitted through the slit diaphragm in a manner such that each set of anode wires is located in a section of the radiation beam which corresponds to at least one specifically controllable attenuation element, each output signal belonging to a set of anode wires being used as a control signal for the corresponding attenua­ tion element.

Inventors: DUINKER, SIMON, VLASBLOEM, HUGO
Applicants: B.V. OPTISCHE INDUSTRIE "DE OUDE DELFT"
Application year: 1986
Main classification: G21K1
IPC codes: H01J47, G01T1