DOSIMETER FOR IONIZING RADIATION

Abstract: A dosimeter for ionizing radiation is of oblong shape. It comprises a gas-filled measuring chamber surrounded by a casing. Two opposite side walls (9,10) are manufactured of material transparent to the ionizing radiation. One of the transparent side walls (9) is provided with a transparent plate-like electrode (12). The other transparent side wall (10) is provided with a number of strip-like electrodes (11) extending transversely to the longitudinal direction of the measuring chamber. A guard electrode (13) surrounds the plate-like electrode. The dosimeter being transparent to X-rays 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 ionizing radiation comprising a gas-filled measuring chamber sur­ rounded by a casing in which there extends a number of electrode elements between which an electrical voltage exists during operation, the casing being provided with at least one entry window for the ionizing radiation. Such dosimeters are already known from the Hand­ book on Synchrotron Radiation, Volume 1A, pages 323-328 by Ernst Eckhard Koch, published by North Holland Pub­ lishing Company, Amsterdam, New York, Oxford, 1983. A drawback of said known dosimeters is that application thereof is not readily possible in slit radiography equip­ ment, where it has to be possible to measure and regulate the quantity of radiation per diaphragm section trans­ mitted through a diaphragm slit at any instance during the production of a radiograph. An example of such slit radiography equipment, which does not, however, employ a dosimeter of the type described above, is described in the Dutch Patent Application 8,400,845. The known dosi­ meters are not designed to attenuate the radiation, the strength of which has to be measured, as little as pos­ sible and to prevent the formation of a visible X-ray sha­ dow image of the dosimeter itself as far as possible. This latter is, however, of great importance in slit ra­ diography equipment because the radiation transmitted through the dosimeter serves to produce the required ra­ diograph. The shape and dimensions of the known dosi­ meters also make them unsuitable for application in slit radiography equipment. The object of the invention is to meet this need. For this purpose, a dosimeter of the type described is characterized according to the invention in that the cas­ ing has an oblong shape and in that the measuring chamber is an oblong cavity recessed in the casing, at least two side walls of the casing, which are situated opposite each other, being manufactured from material transparent to ionizing radiation and there being disposed on the inner surface of the one side wall transparent to ioniz­ ing radiation a plate-like first electrode which largely covers said inner surface, while there is disposed on the inner surface of the second side wall a large number of strip-like second electrodes extending essentially trans­ versely to the longitudinal direction of the measuring chamber. The invention will be described below in more de­ tail with reference to the accompanying drawing of an exem­ plary embodiment. Figure 1 shows in perspective a part of an embodi­ ment of a dosimeter according to the invention; Figure 2 shows a cross-section of the dosimeter of Figure 1; Figure 3 shows a frame for a dosimeter according to the invention; Figure 4 shows a first cover plate for the frame of a dosimeter according to the invention; Figure 5 shows a second cover plate for the frame of a dosimeter according to the invention; Figure 6 shows the electrical circuit of a dosi­ meter according to the invention; Figure 7 shows how a dosimeter according to the invention can be applied in slit radiography equipment; and Figure 8 shows a variation of Figure 5 diagram­ matically. Figure 1 shows in perspective an exemplary embodi­ ment of a dosimeter according to the invention. The dosimeter comprises an oblong, in this example substantially a rectangular, frame 1 which surrounds an oblong, in this example substantially rectangular, cavity 2 (Figure 3). The frame has two short limbs 3, 4 and two long limbs 5, 6 and may be manufactured, for example from a flat plate of a suitable insulating material such as glass or perspex so that the side surfaces of the limbs jointly define two parallel side faces 7, 8. Cover plates 9, 10 made of a suitable insulating material such as glass or perspex are mounted in a vacuum-­ tight manner, for example by glueing, against the side faces 7, 8. With the cover plates the frame therefore forms a sealed casing which contains an oblong measuring chamber 2. On the surfaces of the cover plates which face each other there are disposed electrodes between which an electrical field exists during operation. On the inner surface of the one plate 9 there is disposed, uniformly distributed over the length of the measuring chamber 2, a number of strip-like electrodes 11 of a conducting material which extend substantially transversely to the longitudinal direction of the measuring chamber. This is again shown in Figure 4, which figure shows the inner surface of the plate 9. On the inner surface of the plate 10 there is disposed a flat electrode 12 which essentially occupies the whole of the inner surface of the plate 10 not occupied by the frame. In the preferred embodiment shown in Figure 5 the flat electrode is surrounded all round by a guard elec­ trode 13 which extends along the edges of the plate 10, which guard electrode is also disposed on the surface of the plate 10. The flat electrode and the guard electrode are separated from each other by a small gap 14. In the example shown the guard electrode is interrupted at at least one position to allow a connecting section for the flat electrode through which extends to the edge of the plate 10. In the example shown two of said connecting sections 15, 16 are provided and the two connecting sec­ tions are situated on the same edge 17 of the plate 10. It is pointed out that the operation of the guard electrode may be further optimized, if desired, by omit­ ting the brake(s). The flat electrode may then be pro­ vided with an electrical connection via a vacuum-tight leadthrough through the plate 10 as shown diagrammatically in Figure 8. The leadthrough 80 is preferably situated outside the region situated opposite the electrodes 11 and may be connected with a wire or, as shown, via a conducting strip 81 disposed on the outside of the plate 10. The measuring chamber is filled with a suitable gas which can be ionized by the radiation to be measured. Such a suitable gas is, for example, xenon. In order to be able to fill the measuring chamber with the gas and to be able to evacuate it beforehand, there are disposed, at two positions in the example shown, holes 18, 19 in the short limbs of the frame, in which holes small tubes of, for example, copper are placed. Such a small tube is indicated in Figure 1 by 20. After the measuring chamber has been evacuated via the small tubes and then filled with the gas, the small tubes are sealed in a vacuum-tight manner, for example by pinch sealing and soldering. The elctrodes may be formed, for example, by de­ position of a suitable conducting material by evaporation, the areas which are not to be covered with electrode ma­ terial being temporarily masked. In a practical embodi­ ment, with a casing manufactured from perspex, the elec­ trodes are formed by depositing a thin layer of nickel having a thickness of approximately 1 µm at the required positions by means of a sputtering technique. Such elec­ trodes do not attenuate, or virtually do not attenuate, X-ray radiation. In said practical embodiment the measur­ ing chamber had a length of approximately 42 cm and a height of approximately 3.5 cm, and 160 strip-like elec­ trodes were used having a pitch of approximately 2.54 mm and a gap between them of approximately 1 mm. The total thickness of the dosimeter was approximately 10 mm. The strip-like electrodes 11 may serve as anode strips, in which case the flat electrode 12 is connected as cathode strip. However, it is also possible to con­ nect the strip-like electrodes 11 as cathode strips, while the flat electrode 12 is then connected as anode. Such a circuit is shown diagrammatically in Figure 6. In the example shown in Figure 6 a positive volt­ age V is applied to the flat electrode, which is in this case the anode. The guard electrode 13 is earthed and serves to discharge any leakage currents. Depending on the specific application of the dosimeter, the cathode strips 11 are connected jointly or per group or separately to an associated amplifier 21 which provides, at an out­ put terminal S, the amplified measurement signal which is produced by ionization of the gas in the measuring chamber under the influence of, for example, X-ray radiation. If xenon is used as the gas filling of the measur­ ing chamber, the anode-cathode voltage may be chosen in the flat region of the current-voltage characteristic which is valid for gases. Such a characteristic gives the relationship between the anode-cathode voltage for a certain constant dose of radiation and the signal cur­ rent which appears as a result of the ionizing radiation. In said flat region the signal current is virtually inde­ pendent of the anode-cathode voltage so that the signal current depends exclusively on the number of quanta of ionizing radiation received. If xenon is used, it is pos­ sible to work in this region because xenon has a rela­ tively high absorption factor (large photon cross-section) for ionizing radiation and provides an adequately high signal current even in said flat region of the charac­ teristic. It is therefore not necessary to employ a higher anode-cathode voltage in the so-called gas multi­ plication region. An advantage of this is that the setting of the anode-cathode voltage is not very critical. The anode-cathode voltage V may be, for example, 600 V. Another advantage of the dosimeter described is that, as a result of the chosen configuration, the field lines of the electrical field between the anode and cath­ ode electrode(s) extend essentially perpendicularly bet­ ween the plates 9 and 10. As a result of this the output signals of the dosimeter are virtually independent of the distance between the two plates. As a result of this the dosimeter described is insensitive to variations in the atmospheric pressure. The electrodes may be connected electrically in a simple manner by making the plates 9 and 10 somewhat larger than the frame so that one of the long edges, over which the electrodes then have to continue, of the plates 9 and 10 extend outside the frame. The electrical con­ nections may then be produced, for example, by means of a suitable connector which can be pushed over the project­ ing edge of a plate. Although the plates 9 and 10 in the exemplary embodiment shown are equally as large as the frame, two recesses 22 and 23 respectively are formed along two outermost longitudinal edges of the frame which are situ­ ated diagonally opposite each other, which recesses ex­ tend over the whole length of the frame, so that the same effect is achieved. Figure 7 shows some possibilities of application of a dosimeter according to the invention in slit radio­ graphy 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 and is in particular suitable for performing said detection without substantially affecting the radiation to be detected. If only the total dose of ionizing radiation is of interest in the measurement region, the signals from the strip-like electrodes can be added together or the strip-like electrodes can be connected together. Figure 7 shows diagrammatically slit radiography equipment having 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 equalized radiograph as described in Dutch Patent Application 8,400,845. For this purpose, the dosi­ meter 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 strip-like electrode at a time or from a number of strip-like electrodes situated next to each other provide a measure of the radiation intensity pre­ vailing 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) strip-like electrodes of the dosimeter which interact with adjacent sections of the slit diaphragm, the output signal from each set of strip-­ like electrodes belonging to a certain diaphragm section or, if one strip-like electrode is present for each dia­ phragm section, from each strip-like electrtode may be combined, if desired, with the output signal from one or more strip-like electrodes 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 contain for example 160 anode wires. If the slit diaphragm has, for example, twenty controll­ able sections, eight strip-like electrodes are available per section. The signals from said eight electrodes are then combined into a control signal for the associated diaphragm section. However, as described above, the out­ put signals of one or more adjacent electrodes 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 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 strip-like elec­ trodes may be used, there is the risk that said electrodes may give rise to artefacts in the form of thin strips in the radiograph to be produced depending on the electrode material used. If desired, this can be prevented by en­ suring that the strip-like electrodes extend somewhat obliquely 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 strip-like electrodes at a small angle with respect to the centre line of the dosimeter. It is pointed out that if nickel electrodes as described above are used, no troublesome artefacts occur. 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 ionizing radiation comprising a gas-filled measuring chamber surrounded by a casing in which there extends a number of electrode elements bet­ ween which an electrical voltage exists during operation, the casing being provided with at least one entry window for the ionizing radiation, characterized in that the casing has an oblong shape and in that the measuring chamber is an oblong cavity recessed in the casing, at least two side walls of the casing, which are situated opposite each other, being manufactured from material transparent to ionizing radiation and there being dis­ posed on the inner surface of the one side wall trans­ parent to ionizing radiation a plate-like first elec­ trode which largely covers said inner surface, while there is disposed on the inner surface of the second side wall a number of strip-like second electrodes extending essentially transversely to the longtitudinal direction of the measuring chamber. 2. Dosimeter according to Claim 1, characterized in that the casing is formed from an oblong frame which is mounted in a gas-tight manner between the side walls. 3. Dosimeter according to Claim 2, characterized in that the casing is manufactured from glass. 4. Dosimeter according to Claim 2, characterized in that the casing is manufactured from perspex. 5. Dosimeter according to Claim 1, characterized in that a guard electrode which surrounds the plate-like electrode is disposed on the one side wall. 6. Dosimeter according to one of the preceding claims, characterized in that the electrodes are formed on the side walls by depositing a layer of conducting material in the required pattern by evaporation. 7. Dosimeter according to one of Claims 1 to 5 incl., characterized in that the electrodes are formed by depositing a layer of metal in the desired pattern on the side walls by means of a sputtering technique. 8. Dosimeter according to Claim 6 or 7, character­ ized in that the electrodes consist of nickel. 9. Dosimeter according to Claim 1, characterized in that at least one strip, extending along one of the long edges, of each side wall reaches outside the casing and in that the electrodes are provided with connecting sec­ tions which extend over said strip. 10. Dosimeter according to Claim 9, characterized in that the strips, reaching outside the casing, of the side walls are constructed as a connecting connector. 11. Dosimeter according to Claim 9 or 10, character­ ized in that the strips, reaching outside of the casing, of the side walls are obtained by providing a recess, which extends over essentially the entire length of the frame, along two outermost long edges of the frame situ­ ated diagonally opposite each other. 12. Dosimeter according to Claim 1, characterized in that there is provided in at least one of the limbs of the frame a hole in which a small tube is placed for eva­ cuating the measuring chamber and then filling it with a suitable gas, which small tube is sealed after the gas is introduced into the measuring chamber. 13. Dosimeter according to Claim 1, characterized in that the measuring chamber is filled with xenon. 14. Dosimeter according to Claim 13, characterized in that the potential difference between the plate-like electrode on the one hand and the strip-like electrodes on the other hand during operation is such that no gas multiplication occurs. 15. Dosimeter according to Claim 1, characterized in that the strip-like electrodes extend somewhat obliquely with respect to the longitudinal direction of the measur­ ing chamber. 16. Method for applying a dosimeter according to one of the preceding claims, characterized in that the strip-­ like electrodes are divided into a number of groups and in that the signals from the electrodes belonging to each group are combined to provide an output signal belonging to the group concerned. 17. Method according to Claim 16, characterized in that the signals from the electrodes belonging to each group are combined with signals from one or more elec­ trodes belonging to adjacent groups to provide an output signal belonging to the group concerned. 18. Method according to Claim 16 or 17, 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 trans­ mitted through the slit diaphragm in a manner such that each group of strip-like electrodes is located in a sec­ tion of the radiation beam which corresponds to at least one specifically controllable attenuation element, each output signal belonging to a group of strip-like elec­ trodes being used as a control signal for the correspond­ ing attenuation element. 19. Method according to Claim 18, in which the strip-­ like electrodes extend perpendicularly to the longitudi­ nal direction of the dosimeter, characterized in that the dosimeter is moved synchronously with the scanning move­ ment of the X-ray beam of the split radiography equipment, the longitudinal direction of the dosimeter running some­ what obliquely with respect to the scanning movement.

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