DE2715965A1 - ION CHAMBER ARRANGEMENT WITH REDUCED DEAD SPACE - Google Patents

Description

Ion chamber arrangement with reduced dead space

The invention relates to ion chamber type X-ray detectors. The invention particularly relates to arrangements for increasing the likelihood of the detection of x-ray photons interacting with a detection gas in an area between ion chamber electrodes and a grounded entrance window.

In the German patent application P 26 42 7 1.8 ^ X-ray detectors have been proposed from the ion chamber type for use in the determination of the spatial distribution of X-ray photons in computers equipped with imaging systems. The order

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contains a plurality of substantially parallel and planar anodes separated by parallel planar cathodes and in a gas of high atomic weight at a pressure in the range of about 10 atmospheres to about 50 atmospheres. X-ray photons interact with the gas and create photoelectron / ion pairs. In the presence of an electric field the electrons generated in this way are collected on the anodes and the ions on the cathodes and generate electrical ones Currents proportional to the X-ray intensity in the vicinity of these electrodes.

The anodes, cathodes and the detection gas of such ion chamber arrangements are typically surrounded by a metal pressure vessel that contains at least one thinned wall or window, that is relatively transparent to x-ray photons is. In a preferred embodiment of the ion chamber arrangement the window contains a thin layer of aluminum that is perpendicular to the collecting electrodes (i.e. cathode and anode) and in a distance of about 0.5 mm to about 1.5 now from the collecting electrodes is arranged.

The electric fields in such known ion chamber arrangements are such that the electron / ion pairs generated in the space between the collecting electrodes and the window do not reach the detector plates and therefore make no contribution to the electrical output signal. For an assembly filled with xenon gas at a pressure in the range of about 20 atmospheres to about 30 atmospheres, about 5 or 10 % of 60 KeV X-rays passing through the window will be absorbed in this "dead space"; this is a factor that significantly reduces the quantum cognitive efficiency and the noise equivalent absorption of the rays. The x-ray dose that must be administered to a patient undergoing treatment in a system incorporating this detector is therefore substantially increased by the presence of the "dead space".

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According to the invention a layer of dielectric material on the inner surface of the window of an ion chamber assembly appropriate. The electric field in the area between the window and the collecting electrodes is changed so that the probability of collecting the ions / electron pairs generated in this area is increased. In a preferred Embodiment of the invention is a thin conductive layer arranged on the surface of the dielectric opposite the window and is held at anode potential.

In accordance with the present invention it is possible, essentially collect all electrons and ions generated by those x-rays that are in the "dead space" between the window and absorbed by the collecting plates. The active absorption length of the ion chamber arrangement is thereby correspondingly extended.

In the following, exemplary embodiments of the invention are explained in more detail with reference to the drawing.

In the figures show:

Figure 1 shows an ion chamber x-ray detector assembly known type;

Fig. 2 an ion chamber arrangement according to the invention, the one contains a dielectric layer on the inner surface of an containment window;

3 shows a further embodiment of the detector arrangement according to FIG. 2, in which the collecting electrodes are in contact disposed with the dielectric layer; and

Fig. H, a preferred embodiment form of the detector of Figure 2, which includes a conductive electrode on the inner surface de_r dielectric layer. 709842/0 3 8 4

X-ray photons interact with atoms of a heavy gas to create electron / ion pairs. The X-ray photons are generally absorbed by a gas atom or undergo Compton scattering at the gas atom, whereby the gas atom emits a photoelectron from one of its electron shells. The photoelectrons move through the gas and interact with or ionize other gas atoms and generate an abundance of electrons and positive ions generated by suitable electrodes are collected and generate an electrical current flow. If these electron / ion pairs are in a range between two Electrodes of opposite polarity are generated, they drift along electrical field lines to the electrodes and call an actual flow of electrical current between the electrodes. The electrical current flow between the electrodes is therefore a function of the total number of x-ray photons interacting near these electrodes.

Figure 1 shows part of an embodiment of a multi-cell X-ray detector, which is disclosed in the above-mentioned German patent application P 26 * <2 7 * 11.8, to which hereby Payments are made.

An array of parallel planar anodes

10 and cathodes 12 is in a sub-high atmosphere Xenon gas 18 under pressure is provided within a receptacle (not shown). The cathodes 12 are of a Voltage source 16 held with a grounded positive terminal at a negative electrical potential. The anodes 10 are kept close to the earth potential and are via current sensing or measuring circuits 14 connected, the electrical Output signals in proportion to the current flow from the anodes.

Although the collecting electrodes in the illustrated embodiments of the detector arrangements are designated as cathodes and anodes for the sake of simplicity of description, the polarity reverse the voltage source 16, then by perception

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and measuring the flow of current from the cathode elements of the same type Signals are obtained.

An x-ray beam 22, the intensity of which typically varies along the length of the detector array, strikes the detection gas 18 in a direction substantially parallel to the anode plates 10 and cathode plates 12. This beam enters the area filled by the detection gas 18 through a thin window 20 which is relatively transparent with respect to X-rays. The term "relatively transparent", as used herein, means that the probability of the X-ray absorption in the window assembly than the probability is substantially smaller X-ray interaction in the detection gas 18th In a preferred embodiment of the "inventive detector, the window typically contains a layer of aluminum with a thickness in the range between approximately 3 mm to approximately 6 mm, which is part of an electrically grounded pressure vessel and is electrically connected to this vessel.

The dashed lines in FIG. 1 indicate equipotential lines in the area between the anodes 10, the cathodes 12 and the grounded window 20. The field distribution in the area between collecting plates 10 and 12 and window 20 is generally directed towards the window so that electrons generated in this area by X-ray interactions tend to the window rather than to the anodes 10 and to that of the perceptions - and measuring circuits 14 make no contribution.

FIG. 2 shows a detector according to the invention which, in addition to the elements shown in FIG. 1, contains a thin dielectric layer 24 on the inner surface of the window 20 in the vicinity of the collecting plate em 10 and 12. This dielectric layer 24 can be any material which is normally for this purpose in the framework of X-ray detectors

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is used, e.g. a Mylar polycarbonate layer with a thickness of about 0.12 mm. The in the area between the window 20 and the collecting plates 10 and 12 by interactions Generated electrons drift preferentially along the electric field lines to the dielectric layer 24 on the surface the grounded window 20 where they collect and charge the layer and create a negative electrical potential, which changes the electric field and drives more electrons back to the anode plates 10.

FIG. 3 shows a further embodiment of the detector according to FIG. 2. A thin dielectric layer 24 is, as in the embodiment according to FIG. 2, on the inner surface of the conductive window 20. The anode plates 10 and the cathode plates 12 are located in FIG of this embodiment in contact with the dielectric layer 24, eliminating the dead space between the plates and the window.

FIG. 4 shows a further preferred embodiment of a detector according to the invention. In this embodiment, the anode plates 10 and the cathode plates 12, as already described in connection with the description of the prior art according to FIG. 1, are arranged in a detection gas which is under high pressure. The anodes 10 are grounded via current measuring circuits 14, while the cathodes 12 are held at a negative voltage by a supply source 16 containing a grounded positive terminal. X-rays 22 enter the detector through a thin conductive window 20 in a direction that is substantially parallel to the anodes 10 and cathodes 12. The window 20 forms part of the pressure vessel and is kept at ground potential. A thin dielectric layer 24, which may include, for example, 0.12 mm thick Mylar® polycarbonate plastic material, is on the inner surface of the window 20. A thin conductive electrode 26, which may include, for example, a 0.05 mm thick layer of aluminum on the surface of the dielectric 24 facing away from the window 20

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arranged. Alternatively, the electrode 26 can be an aluminum layer or another metallized layer on the upper surface of the dielectric layer contained by vacuum evaporation or other known methods is applied.

The electrode 26 is connected to the cathode potential and is held by the supply source 16 at this potential. That in this way in the "dead space" between the window electrode 26 and the electric field generated by the collecting electrodes 10 and 12 is illustrated by the equipotential lines shown in FIG. The electric field in this area is directed to the anodes 10, so that electrons generated in this area on the anodes where they are collected and passed through Current measuring circuits 14 are measured.

Although the embodiment according to FIG. 4 is necessarily more complex than the embodiment according to Figures 2 and 3, since it contains an additional electrode 26 and connections to the cathode supply source, this embodiment provides a much more stable electrical field formation and is less prone to errors caused by the discharge of charge (Charge draining) can originate from the dielectric layer 24, which occurs in the embodiment according to FIG electrical leakage current to ground can make it necessary to readjust the potentials before each measurement or restore some x-ray pulses to be delivered to the collector. Leakage currents through the dielectric Layer 24 according to FIG. 3 can also represent undesired leakage currents which are related to the measuring circuits Add 14 measured currents.

The detectors according to the invention have a significantly increased Quantum efficiency and reduced noise levels as detectors of known type. It has been calculated that the increased active detector area, which results from the structures according to the invention, the x-ray dose of an examination

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Subjected patients do 9.1 to 13.2 % of the X-rays reduced in the 120 kVp to 80 kVp range.

The arrangement according to the invention also enables the construction of detectors with increased spacing between the collecting electrodes and the window arrangement. This increase in the allowable distance allows a reduction in manufacturing tolerances and results in lower detector costs.

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Claims (10)

Expectations i. X-ray detector with several essentially flat, parallel anode and cathode plates in one ionizable detection medium, with means to keep the anodes at or close to earth potential, with means for applying an electric field between the cathodes and the anodes, with devices for measuring the current flow from the anodes to ground, and with a container to hold the anodes, cathodes and the Detection medium, the container containing a thin window which is substantially transparent to X-rays, which is arranged perpendicular to the anodes and the cathodes and in the vicinity of the anodes and cathodes and at ground potential is held characterized in that a thin layer (24) of dielectric material is disposed on an inner surface of the window (20) in the vicinity of the anodes (10) and cathodes (12). 709842 / 098- ORIGINAL INSPECTED 2. X-ray detector according to claim 1, characterized in that a thin conductive electrode (26) is arranged on the surface of the dielectric layer (24) facing away from the window (20). 3. X-ray detector according to claim 2, characterized, that devices are provided that the thin conductive electrode (20) at the potential of the cathode (12) keep. 4. X-ray detector according to claim 3, characterized, that the dielectric layer (24) Polycabcmat-Kimststot'fmaterial contains. 5. X-ray detector according to claim 4, characterized, that the dielectric layer (24) is approximately 0.12 mm thick. 6. X-ray detector me claims 2 to 5, characterized, that the thin conductive electrode (26) is an aluminum layer contains. 7. X-ray detector according to claim 6, characterized, that the aluminum layer is now about 0.05 thick. 8. X-ray detector according to claims 2 to 5, characterized in that the conductive layer (26) is a metallized layer deposited on the surface of the dielectric layer. 709842/0984 9. X-ray detector according to one or more of claims 1 to 8, characterized in that the anodes (10) and the cathodes are angeord net in contact with the surface of the dielectric layer (24) (12). 10. Window for an X-ray detector assembly of the ionization chamber type,
characterized by a first layer (20) of electrically conductive material, a thin layer (2A) of dielectric material disposed on an inner surface of the first layer (20), a second layer (26) made of conductive material, which is arranged on that surface of the dielectric layer (24) which is facing away from the first layer (20) of conductive material, Means for maintaining the second layer (26) of conductive material at or near the potential of one or more cathode assemblies, causing electrons generated by x-ray interactions in an area between the window and the cathode assemblies to become one or more Neigh disclosed anode assemblies to flow for collection and measurement purposes. 709842/0984