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 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 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 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:
BACKGROUND OF THE INVENTION 
     The invention relates to a dosimeter for ionizing 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 ionizing radiation. 
     Such dosimeters are already known from the Handbook 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 production of a radiograph, it has to be possible to measure and regulate the quantity of radiation in each diaphragm section transmitted through a diaphragm slit at any instant, is not readily possible. An example of such slit radiography equipment, in which use is not made, however, of a dosimeter of the type described above, is described in Dutch patent application No. 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 unsuitable for application in slit radiography equipment. 
     SUMMARY OF THE INVENTION 
     There is therefore a need for a dosimeter for ionizing 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. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be described in more detail below by reference to the accompanying drawing of an exemplary embodiment. 
     FIG. 1 shows a diagrammatic cross-section of an embodiment of a dosimeter according to the invention; 
     FIG. 2 shows a section along the line II--II in FIG. 1; 
     FIG. 3 shows diagrammatically how a dosimeter according to the invention can be applied in slit radiography equipment; 
     FIG. 4 shows an example of a cathode element for a dosimeter according to the invention; and 
     FIG. 5 shows diagrammatically an end view of a variation of FIG. 4. 
     The dosimeter shown in FIGS. 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 measurement 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 consists 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 FIG. 1, both frames are provided 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; anode wires 13&#39; (two shown) in lieu of anode wires 13 are illustrated as being disposed obliquely to the longitudinal direction of the frames 7,8. In a practical embodiment the anode wires may be, for example, gold-plated tungsten wires which, for example, have a spacing of 2.5 mm and a thickness of 50 μm. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The anode wires are mounted so as to be distributed 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 center 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 possible 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. 
     FIG. 4 shows a preferred embodiment of the cathode elements 14, 15 which are here diagrammatically shown in their mutual relationship and in their relationship to the anode wires 13, the rest of the dosimeter having been omitted for the sake of clarity. The cathode elements shown in FIG. 4 each comprise in turn a thin sheet-like carrier which is provided with a guard electrode 40 or 41, which is connected to ground 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 FIG. 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 FIG. 5 for the cathode element 14. The passage 48 is shown diagrammatically in FIG. 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 respectively 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 embodiment 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 FIG. 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 construction 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 (FIG. 1) which extends along the plane of separation between the frames and in the longitudinal direction of the frames. On one of the flanges, e.g., the flange 18, referring to FIG. 2 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 FIG. 1 could, if desired, be provided in a similar manner with connection tracks for the anode wires. This is not, however, 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 FIG. 2, and the flange 18 could be constructed somewhat longer as is indicated by the broken lines at 18&#39;, and may be provided with fixing holes. 
     In FIG. 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 measuring chamber. The measuring chamber is then filled with a suitable gas such as, for example, a mixture of Ar and CH 4 , 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 FIG. 2 an electrical connection 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 produces the output signal under the influence of ionizing radiation at the electrodes, while methane gas is added as a quenching gas (&#34;quenching agent&#34;) in order to counteract the so-called avalanche effect. Preferably, however, the anode/cathode voltage is chosen so that the avalance effect does not normally occur. The methane gas could then be omitted. 
     Since Ar is relatively transparent to ionizing 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 regulating 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. 
     FIG. 3 illustrates some possibilities of application of a dosimeter according to the invention in slit radiography equipment. It is pointed out that the dosimeter may also be applied in other situations and is in particular suitable, in general, for detecting the distribution and variation of the intensity of ionizing radiation over an extensive region. 
     If only the total dose of ionizing 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. 
     FIG. 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 having 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 equalized 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, therefore, also of the brightness of the part of the radiograph to be produced corresponding to said sector. Said output signals can therefore be used to control attenuating 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 diaphragm, 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 diaphragm 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 formation of the control signal. 
     Depending on the type of X-ray detector used, it is possible, as an alternative, to control the attenuation 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 prevented by ensuring that the wires extend somewhat obliquely with respect to the scanning direction. This can be achieved in a simple manner by mounting the dosimeter itself somewhat obliquely with respect to the scanning direction or by mounting the anode wires at a small angle with respect to the centre line of the dosimeter. 
     While the invention has been described in connection with several exemplary embodiments thereof, it will be understood that many modifications will be apparent to those of ordinary skill in the art; and that this application is intended to cover any adaptations or variations thereof. Therefore, it is manifestly intended that this invention be only limited by the claims and the equilvalents thereof.