FI63495B - STRAOLNINGSDETEKTOR - Google Patents

Description

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Suomi-Finland (FI) (72) Esko Riihimäki, Espoo, Finland-Finland (FI) (7 * 0 Ins.tsto Olli Heikinheimo Ky. (5 * 0 Radiation detector - Strälningsdetektor

The invention relates to a radiation detector which is intended for use in connection with, for example, X-ray tomography equipment. The detector comprises a sealed container in which a noble gas of high atomic weight is enclosed at a pressure higher than normal atmospheric pressure. Cathodes and anodes are placed inside the tank and are connected to the negative and positive voltages of the DC source, respectively.

Ionization chambers of the type described are used to detect X-rays. The purpose of such detectors is to detect both the amount and location of X-rays. An example of such a detector is the device disclosed in U.S. Patent 4,031,396. Ko. the device consists of an ionization chamber with a gas of high atomic weight at a pressure of 10 to 50 atmospheres. Inside the chamber are placed a number of parallel plate anodes separated by plate-like cathodes. The anode and cathode plates are placed perpendicular to the direction of radiation. With this device, the amount of radiation is measured by means of an electronic circuit in analog form, i.e. the amount of ionization current is measured. The disadvantage of such a device is the slow movement of positive ions in the chamber 2 63495 and the inaccuracy associated with measuring such very small currents and converting them to digital form. Crystal detectors are also known in which light is expressed in analog form by means of a photomultiplier tube.

With the device according to the invention, the disadvantages described above can be avoided, because it can measure each radiation quantum separately. This is achieved by the ionization chamber according to the invention, characterized in that the cathodes are metal plates spaced parallel to the beam of radiation to be detected and the anodes are metal wires spaced between the cathodes, which generate a voltage pulse from each absorbed X-ray. According to a preferred embodiment of the invention, the cathode plates are evenly spaced and frames are arranged between them. Anode wires are attached to these at regular intervals.

The device according to the invention has several advantages over known devices. And most importantly, now that each X-ray quantum can be measured individually, the susceptibility of the signal to interference is substantially reduced. Another and very significant advantage is that by allowing the pulses to be measured directly, the radiation dose can be significantly reduced. In certain cases, up to 1/5 or 1/10 of the amount of radiation that would be required to use known detectors may be recovered.

In the device according to the invention, the X-rays are detected in a noble gas having a high atomic weight. X-rays interact with gas atoms and form an ionization cloud consisting of electrons and positively charged noble gas atoms in an electric field. Electrons, whose velocities are very much higher than those of positive ions, move toward the nearest anode wire and, upon reaching the strong electric field around a thin wire, cause amplification, causing more electrons to detach from the atoms in collisions. This is how confirmation is obtained. Because the field is very strong in the vicinity of the wire, the electrons move very fast and cause a rapid, detectable voltage pulse in the wire. Because the detector has multiple wires for each location and the pressure is relatively low, similar electron pulses can be detected sequentially at different locations, even though the positive ions of the previous ionization cloud are still on their way to the cathode. The voltage pulse caused by positive ions is at least two orders of magnitude slower than 3 63495 than that of electrons, so that the pulse detection circuits are able to distinguish them; This also means that the X-rays themselves do not have to be pulsed during the measurement process, but continuous irradiation can be used.

Since it is important that the radiation is detected as completely as possible, the detector must be constructed long enough in the direction of the radiation to allow at least 70% of the X-ray quantum to be absorbed into the gas, even if the pressure in the chamber is not very high. This has two advantages: firstly, the detection of radiation takes place over a wide range, so that the positive ions have time to drift in the chamber to the cathodes, and secondly, the voltage required for the amplification of electrons does not rise very high.

In the following, the invention will be explained in detail with reference to the accompanying drawings. .

Figure 1 shows diagrammatically and partly in section an embodiment of the invention

Figure 2 shows the cathode plate of the detector of Figure 1. Figure 3 shows the anode arrangement of the detector of Figure 1.

X-rays interact with the detector gas to form an electron cloud and a cloud of positively charged gas ions. By adding a suitable molecular gas to the gas, the rate of electron drift can be increased. The probability of X-ray absorption depends on the atomic weight of the noble gas used and the number of radially located gas molecules. Thus, sufficient detection efficiency is obtained in the detector by making it long enough in the radial direction.

The gain of the electron pulse depends on the diameter of the wire, the voltage difference between the electrodes and the pressure of the filling gas. The ratio between these is chosen so that the wire has a gain of 100 to 1000 times. The measurements performed show that in this way voltage pulses of about 10 nanoseconds are achieved with the anode wire.

A chamber operating in this way can be called a proportionality counter, although the magnitude of the voltage pulse detected is irrelevant.

According to Fig. Ί, the detector according to the invention comprises a pressure vessel 10. The pressure vessel has the shape of a circular segment, whereby a point source is placed at the center of the circle.

4 63495

Inside the pressure vessel 10, the detector gas 12 is pressurized. One side of the pressure vessel 10 has a thin window 14 which is substantially transparent to X-rays. The detector gas 12 fills the pressure vessel and is substantially opaque to X-rays so that only a small portion of the radiation is absorbed by the window 14 and most is absorbed by the gas 12. The detector gas consists of a high atomic noble gas (such as Xenon) and an electron mobility molecule. The amount of carbon dioxide CO2 is preferably 5-10%. The cathodes, which are a high atomic weight metal, are located in the chamber in the direction of the radiation impinging on the detector and perpendicular to the longitudinal axis of the detector.

The anodes 16 are located between the cathodes 18 at the center and parallel to them. The detector consists of several, perhaps hundreds, cathodes and anodes. The cathodes 18 are electrically conductive connected to the negative terminal of the voltage source 28. The wires acting as anodes are connected to the pulse detector circuits by insulating bushings 22,

Figure 2 shows a cathode plate 18 consisting of a high atomic weight metal plate, typically 0.05 to 0.1 mm thick, and a length such that it extends almost from the front wall to the rear wall of the pressure vessel. The edges of the cathode are protected by an insulating material 40 to prevent electron discharge. The cathode plate can be manufactured e.g. tantalum, tungsten, molybdenum or gold.

Figure 3 shows an anode plate 16 which is approximately the same length as the cathode plate 18 and consists of a frame 54 and wires 50. The wires 50 are firmly attached to the frame 54. The number of anode wires is such that their spacing is approximately the same as the distance between the cathodes. The thickness of the frame is so small, preferably 0.01-0.05 mm, that it does not substantially cause the absorption of detectable radiation, but still provides a supportive frame for the anode wires. The thickness of the anode wires 50 is preferably 0.02 to 0.1 mm and they are made of, for example, tungsten, silver, steel, tantalum, gold or molybdenum.

The detection of pulses from wires on the same anode can be done either separately or partially or all in one detector. A typical distance between the electrodes is 2-10 mm. The advantage of this type of detector implementation is that each quantum can be detected in a sufficiently short time, because computed tomography imaging is characterized by such a large number of quantums per unit time that they have not been able to be detected individually by previous detectors.

In a detector such as the one described, 12 mm is suitable as the detector gas. Xenon, Argon and Krypton. In addition to these, it is advantageous to use a small amount, e.g. 5-10 l, of carbon dioxide. A suitable gas pressure is about 2-10 atmospheres. In this case, the suitable voltage is about 2-5 kV, respectively.

The invention has been described above with reference to only one preferred embodiment thereof. It is, of course, to be understood that the described embodiment is intended only as an example and that the invention is not intended to be limited in any way to that example only. On the contrary, many changes in the structure of the device according to the invention are possible while still remaining within the scope of the inventive idea set out in the following claims.

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Claims (5)

6 63495 A radiation detector for use, for example, in connection with X-ray tomography equipment, comprising a sealed container (10) enclosed by a gas having a high atomic weight at a pressure higher than normal atmospheric pressure and anodes (16) and cathodes (18) placed in said gas and a voltage source (28) for applying a positive DC voltage to the anodes and a negative DC voltage to the cathodes, and means (24) for detecting a voltage pulse detected at each anode (16) or anode array, the detector operating on a proportional counting principle; that the cathodes (18) are metal plates spaced parallel to the beam of radiation to be detected and the anodes are metal wires (50) spaced between the cathodes and forming a voltage pulse from each absorbed X-ray quantum. Detector according to Claim 1, characterized in that the cathodes (18) are arranged at regular distances from each other and the anodes (16) are located halfway between two parallel cathodes. A detector according to claim 1, characterized in that the anodes are thin metal wires (50) spaced at regular intervals on a frame (54) or the like. Detector according to one of the preceding claims, characterized in that the cathodes (18) are made of a high atomic weight metal such as tantalum, tungsten, molybdenum or gold. Detector according to one of the preceding claims, characterized in that the anode wires (50) are made of tungsten, silver, steel, tantalum, gold or molybdenum.