SE530171C2 - Gas-based detector - Google Patents

Abstract

A detector (9) for detection of ionizing radiation comprises a cathode (17, 41); an anode (19); an ionizable gas arranged between these electrodes; a radiation entrance (33) arranged such that ionizing radiation (1) can enter and ionize the ionizable gas; and a read-out arrangement. A voltage across the electrodes causes electrons created during ionization of the gas to drift towards the anode, where the read-out arrangement (19) detects them. To reduce the risk of occurrence of sparks, and/or to reduce the energy in occurring sparks, the cathode has at least the surface layer facing the anode made of a material having a resistivity of at least 5x10<-8> OMEGA m.

Description

25 530 171 2 events are particularly likely to occur for high gain detectors.

Such spark discharges block the detector for a period of time and can also be harmful to the detector and especially to the electronics in it.

Disclosure of the Invention A principal object of the invention is to provide a detector for detecting ionizing radiation, for which problems caused by spark discharges are eliminated or at least reduced.

It is in this respect a particular object of the invention to provide such a detector in which the energy in each spark appearing is low, so that relatively few charges are released in the gas.

A further object of the present invention is to provide such a detector which provides for rapid recovery after a spark discharge and which thus provides for faster detection and shorter periods of time during which an object to be examined is exposed to ionizing radiation. Yet another object of the invention is to provide such a detector which is efficient, accurate and of low cost.

A further object of the invention is to provide such a detector which is reliable and which has a long life.

It is in this respect a particular object of the invention to provide such a detector which protects the anode and read-out electronics such as, for example, preamplifiers from being damaged by high-energy sparks.

Such objects, among other things, are achieved by means of detectors in accordance with the appended claims. Additional features of the invention and advantages thereof will become apparent from the following detailed description of preferred embodiments of the invention, which are shown in the accompanying drawings.

Brief Description of the Drawings Fig. 1 schematically and concisely illustrates in a cross-sectional view a device for plane beam radiography according to a first embodiment of the present invention.

Fig. 2 is a schematic plan view of a cathode arrangement of a detector according to a second embodiment of the present invention.

Fig. 3 is an enlarged, schematic cross-sectional view of a cathode arrangement for a detector according to a third embodiment of the present invention.

Fig. 4 is a schematic plan view of a cathode arrangement for a detector according to a fourth embodiment of the present invention.

Detailed Description of Preferred Embodiments Fig. 1 is a sectional view in a plane perpendicular to the plane of a planar, fan-shaped X-ray 1 of a flat beam radiography apparatus according to a first embodiment of the present invention. The device comprises an X-ray source 3, which together with a first thin collimator window 5 creates the planar X-ray beam 1 for irradiating an object 7 to be imaged.

The beam transmitted through the object 7 enters a detector 9. Optionally, a thin slit-shaped or second collimator window II, which is aligned with the X-ray beam, forms the input of the X-ray beam 1 to the detector 9. The detector 9 9 is oriented so that X-ray photons can enter laterally between a cathode arrangement 17 and an anode arrangement 19, between which a space 13 capable of being filled with an ionizable gas or gas mixture is arranged. By means of a high voltage DC power supply unit 7, a voltage UI can be applied between the cathode 17 and the anode 19 for driving electrons and ions in the space 13. The cathode arrangement 17 and the anode arrangement 19 are preferably substantially parallel to each other.

The X-ray source 3, the thin collimator window 5, the optional collimator window 11 and the detector 9 are preferably connected and fixed with respect to each other by means of a suitable means, e.g. a frame or support (not shown in Fig. 1).

The ionizable gas or gas mixture comprises, for example, 90% krypton and 10% carbon dioxide or 80% xenon and 20% carbon dioxide. The gas may be under pressure, preferably in the range of 1-20 atm. The detector therefore comprises a gas-tight housing 31 with a slit-shaped entrance window 33, through which the X-ray beam 1 can enter the detector. In Fig. 1, the housing 31 encloses main parts of the detector 9. It should be obvious, however, that the housing 31 can be arranged in other ways as long as the space between the electrodes can be enclosed.

Furthermore, the detector 9 comprises a readout arrangement for separately detecting electrons which are driven towards the anode arrangement 19 and / or ions which are driven towards the cathode arrangement 17.

The readout arrangement may be the anode arrangement 19 itself as illustrated in Fig. 1, or a separate readout arrangement may be provided adjacent the anode arrangement 19, adjacent the cathode arrangement 17, or elsewhere.

The anode or readout arrangement 19 comprises a set of electrically conductive elements or strips 35 arranged side by side and electrically insulated from each other on a dielectric layer or substrate 37. The strips 35 can be produced by photolithographic methods. or electroforming, etc.

To provide an increased spatial resolution and to compensate for parallax errors in each detected image, the strips 35 are extended substantially in directions parallel to the direction of the incident X-ray photons of the beam 1 originating from the source 3 at each location. Thus, given a divergent beam 1, the read strips 35 are arranged in a fan-like configuration. The length and width of the strips 35 are adjusted for the particular detector to obtain the desired (optimal) room resolution and sensitivity.

Each of the strips 35 is preferably connected to read and signal processing electronics 14 by means of a respective separate signal conductor (of which only one is illustrated in Fig. 1), whereby the signals from each strip can be processed separately.

Since the readout strips 35 also constitute the anode, the signal conductors also connect the respective strip to the high voltage unit 7, with suitable connections for separation. In Fig. 1, such measures are indicated only by means of a separate earth connector.

The above-mentioned design of the readout arrangement provides possibilities for separate detection of electrons, which can be deduced mainly from ionization of transversely separated parts of the planar beam 1, by means of the strips 35.

In this way, one-dimensional imaging is made possible.

In the case where the readout arrangement is a separate arrangement, the anode strips 35 can be formed as a uniform electrode (without division into strips).

In an alternative configuration of anode / readout arrangement (not illustrated), the strips may be further divided into segments in the direction of the incident X-rays, where the segments are electrically isolated from each other. Preferably, there is a short distance extended perpendicular to the incident X-rays between each segment of the respective strip. Each segment is connected to the processing electronics by means of a separate signal conductor, the signals from each segment preferably being processed separately. Such a readout arrangement can be used when energy-resolved detection of radiation is required. In this respect, special reference is made to our pending Swedish patent application No. 0001167-6 entitled Spectral Dissolved Detection of Ionizing Radiation, filed March 31, 2000, which application is hereby incorporated by reference.

Furthermore, the detector 9 comprises an electron avalanche amplification device 21 for avalanche amplification of electrons, which are driven inside the space 13. For this purpose, the electron avalanche amplification device 21 is suitably connected to the high voltage unit 7. In one version, the electron avalanche amplification device 21 consists of a grid-like disk a plurality of holes through which electrons can pass during their path towards the anode arrangement 19.

Alternatively, other avalanche amplification arrangements or field concentration means may be provided so that the electrons released in the space 13 can be amplified prior to detection. A number of such avalanche amplification arrangements are described in our pending Swedish patent application No. 9901325-2 entitled Radiation Detector, device for use in planar radiography and method for detecting ionizing radiation, filed on April 14, 1999, which application is hereby incorporated by reference. .

In a particular version of the invention, avalanche amplification can be obtained simply by keeping the voltage U, (i.e. the electric fields created thereby) high enough in use to cause electron avalanche amplification inside the space 13. 10 15 20 25 30 53Ü 171 7 in use, the incident X-rays 1 enter the detector through the thin, slit-shaped collimator window 11, if present, and between the cathode 17 and the anode 19, preferably in a central plane between them as indicated in Fig. 1. The incident X-rays 1 travel then through the gas space in a direction preferably substantially parallel to the electrodes 17 and 19 and absorbed, thus ionizing the gas molecules in the space 13.

X-rays absorbed in the space 13 will cause electrons to be released, which are driven towards the anode arrangement 19 due to the voltage UI applied. If the voltage is kept high enough and / or if field concentration means are provided (as discussed above), the released electrons are amplified during their journey towards the anode. If an electron avalanche amplification device is provided, it is preferably maintained at an electric potential (electric potentials) so that a weak driving field is obtained between the cathode arrangement 17 and the amplifying device 21 and a strong avalanche amplifying field is obtained inside the amplifying device 21 (e.g. between an electrode thereof). 19).

The electrons induce charges in the strips 35 of the anode / readout arrangement 19, which are detected. If no avalanche amplification takes place, a major part of the signal is derived from the collection of the free charges.

Each incident X-ray generally causes an induced pulse in one (or more) of the anode strips. The pulses are processed in the read and signal processing electronics 14, which ultimately form the pulses and integrate or count the pulses from each strip representing a pixel. The pulses can also be processed to provide an energy measure for each pixel.

Due to the high electric field strengths that may be present in connection with the electrode plates, there is a risk that spark discharges will occur in the gas. Such spark discharges block the detector for a period of time and can also damage the anode arrangement 19 and the electronics connected thereto.

To reduce the risk of spark discharges occurring in the gas and to reduce the energy released by spark discharges which nevertheless occur, the present invention comprises providing the cathode arrangement 17 (and optionally the anode strips 35) with a material having a resistivity of at least 5x10

The cathode arrangement 17 is preferably of a material having a resistivity between 5x10 * Qm and 1x10 5 Qm, more preferably between 1x10 .

The material may be doped or undoped semiconducting material, preferably comprising a semiconductor material consisting of elements selected from Group IV of the Periodic Table (eg the substances silicon and germanium) or groups III-V (eg the substances GaAs, InP and InGaAsP ). Preferably, however, the cathode arrangement 17 is of doped or doped silicon. Alternatively, the material is an electrically conductive glass or an electrically conductive plastic. In fact, substantially any solid material having resistivity within the ranges mentioned above may be suitable for use in the cathode arrangement 17.

By such measures, a resistive cathode arrangement 17 is directed towards the space 13 and the avalanche reinforcement means 21, where strong high electric fields can be present. As a result, if a spark discharge is about to occur, electrons within a much smaller area will participate and be released from the cathode surface of the spark, and thus the energy of the spark discharge will be small. Thus, the influence from it can be controlled.

Nevertheless, such resistances limit the speed at which X-ray photons can be detected without significantly reducing the electric field strength of the detector. It is obvious that a suitable balance must be found between speed and the risk of spark discharges occurring (and their respective energies).

As an alternative to providing the cathode arrangement 17 made entirely of such a semiconducting material, only the surface layer 17a of the cathode arrangement 17 facing the space 13 can be made of a material having a resistivity of at least 5x10 In such a case, the surface layer may be formed on a conductive substrate or on a dielectric substrate provided with suitable electrical connections (not illustrated).

As yet another alternative, the surface layer 17a of the cathode arrangement 17 facing the space 13 may be partially covered by a plurality of electrically conductive elements which are electrically connected to each other only by means of said resistive material. Such a cathode arrangement is illustrated in Fig. 2, wherein one of said plurality of electrically conductive elements is designated by reference numeral 41. By means of such measures a faster detector can be provided, the surface area of high conductivity still being limited to local areas (i.e. the respective elements 41 ). Although the elements 41 in Fig. 2 are shown as elongated strips, they may have other shapes and be arranged in other patterns. For example, the electrically conductive patterns may be square or rectangular layers arranged in a two-dimensional matrix on the surface 17a of the resistive cathode 17.

The high voltage unit 7 is preferably connected to the back of the cathode 17 as indicated at 17b in Fig. 1 (i.e. at the surface opposite the surface 17a) so that the respective electrically conductive elements 41 are each connected to the high voltage supply unit 7 via the resistive cathode 17.

Furthermore, in the case where the cathode 17 in Fig. 2 is used together with extended anode / read-out strips as described above, the elements 41 should preferably be oriented with respect to the read-out strips 35 so that an electric field is obtained inside the detector, which reduces the presence of "pockets" inside the space 13 and the reinforcement device 21, i.e. areas where electrons and / or ions are not further driven and will thus accumulate. This is particularly important to avoid near the anode / readout arrangement 19. Thus, the plurality of electrically conductive elements 41 of the cathode 17 are oriented substantially in a first direction and the plurality of electrically conductive or semiconductor elements 35 of the anode / readout arrangement 19 are oriented substantially in a second direction. , the first and second directions being substantially non-parallel, and preferably substantially perpendicular.

In yet another embodiment of the cathode 17, as illustrated in Fig. 3, such a plurality of electrically conductive elements 41 are arranged on a surface 42a of a dielectric substrate 42.

The cathode arrangement is arranged so that the electrically conductive elements 41 and the surface 42a of the dielectric substrate 42 are directed towards the space 13 and the anode 19 of the detector 9. Each of said plurality of elements 43 is connected to an electrically conductive layer 45 arranged on the surface 42b h0S the dielectric substrate 42 which is opposite the surface 42a by means of a respective resistor 43.

Fig. 3 shows these resistances only symbolically and it should be understood that they can be implemented in a number of ways; for example as discrete components inside or adjacent to the substrate 42 or as integrated components inside the substrate 42. In the latter case, the entire cathode may be made in a semiconductor process with the resistors implemented as suitably composite layers between a layer of electrically conductive elements 41 and an electrically conductive conductive layer 45 for connection to the high voltage unit 7.

A further alternative of implementing the ideas of the Fig. 3 embodiment is illustrated in Fig. 4. Here, each of the electrically conductive elements and each of the plurality of resistors are provided on the surface 42a of the Fig. 3 embodiment. the dielectric substrate 42 in the form of a strip 41 having a narrow waist 41b in an end portion thereof, so that the strip has an elongate portion 41a constituting the electrically conductive element, a narrow waist portion 41b constituting the resistor and a wider connecting portion 41c for connection to the high voltage unit 7.

Preferably, the material composition of each of the strips 41 is inhomogeneous so that each elongate member 41a has a material composition having a resistivity which is lower, in particular considerably lower, than the resistivity of the material composition of each of the narrow waist portions 41lb. Such a design can be obtained by first depositing a bad conductor such as chromium to define the whole strips 4la-c, after which a good conductor such as gold is deposited on top of said chromium only at the extended parts 4la and possibly also on the connecting parts 4lc.

In the described embodiments, the gas spaces should be thin as this results in rapid removal of ions, which leads to little or no accumulation of space charges.

This makes use at high speed possible.

In the embodiments described, the interelectrode distances must be kept short as this leads to low operating voltages, which results in even, low energy of possible sparks. This also reduces the risk of damaging the electronics.

The focusing of field lines (which is typically performed in an electron avalanche amplifier) is also preferred to suppress the formation of so-called streamers. This leads to a reduced risk of sparks appearing.

Furthermore, while the described embodiments of the present invention are concentrated on the cathode arrangement, it should not be immediately understood that the anode arrangement may also be designed in a similar manner.

In general, the resistances included in the cathode arrangement should be kept low enough to accept high speed and still high enough to protect the electrodes from sparks. Although the invention has been described in connection with a number of preferred embodiments, it is to be understood that numerous modifications may still be made without departing from the spirit and scope of the invention as defined by the appended claims.

For example, although the invention has been described in connection with detectors where the radiation is incident from the side, the invention can be used for detectors where the radiation is incident in any direction. Thus, the invention can be particularly used in two-dimensional gas-based detectors for ionizing radiation, wherein the incident radiation enters the detector through the cathode arrangement.

A serious limitation in such an arrangement, however, is a parallax error which occurs due to divergent beams, elongated absorption paths and homogeneous, electric driving fields. Such a parallax error problem is solved in our present Swedish patent application no. 0003390-2 entitled Parallax-free detection of ionizing radiation, filed on 22 September 2000, which application is hereby incorporated by reference. The solution involves dividing the cathode and / or anode into segments electrically isolated from each other and keeping the different segments at different selected electrical potentials so that an electric field is obtained between the electrodes, whose field lines point to the radiation source of the divergent beam, for driving charge carriers ( eg electrons) created by ionization parallel to the field lines towards the electrodes (the anode in the case of electrons). Thus, it should be particularly apparent that such a solution may be advantageously combined with any of the embodiments of the present invention shown in Figures 2-4.

Claims (23)

10 15 20 25 530 171 14 PATENT CLAIMS A detector (9) for detecting ionizing radiation, comprising: - a cathode arrangement (17, 41) and an anode arrangement (19), between which a voltage (U1) can be applied, - a space (13) capable of being filled with an ionizable gas and arranged at least partially between said cathode and anode arrangement, - a radiation input (33) arranged so that ionizing radiation (1) can enter said space between said cathode and anode arrangement for ionizing the ionizable gas , and - a readout arrangement (19), wherein - said voltage is operable to drive electrons created by ionizing said ionizable gas against the anode arrangement and - said readout arrangement is arranged to detect the electrons driven towards the anode arrangement, or correspondingly created ions, characterized in that the cathode arrangement has at least a part of the surface layer which is directed towards the anode arrangement made of a material with a resistivity of at least 5x10 * Qm. A detector according to claim 1, wherein said surface of said cathode arrangement facing the anode arrangement is partially covered by a plurality of electrically conductive elements (41: 41-C) - The detector of claim 2, wherein said plurality of electrically conductive elements (41; 41a-c) are separated from each other. 10 15 20 25 53Ü 171 15 A detector according to any one of claims 2 or 3, wherein said plurality of electrically conductive elements are resistively connected to each other by means of said material having a resistivity of at least 5x10 A detector according to any one of claims 1 to 4, comprising a high voltage supply unit for applying said voltage between said cathode and anode arrangements, said high voltage supply unit being electrically connected to said material having a resistivity of at least 5x10 A detector according to any one of claims 1-5, wherein said material has a resistivity between 5x10 * Qm and lx105 Qm, preferably between lx10 * Qm and lx103 Qm, more preferably between lx10 * Qm and lQm and most preferably between lx10 * Qm and lxl0 * Qm. A detector according to any one of claims 1-6, wherein said material is a semiconducting material. A detector according to claim 7, wherein said semiconductor material comprises a semiconductor material composed of elements selected from group IV of the periodic table and / or from groups III and V of the periodic table. The detector of claim 8, wherein said semiconductor material is silicon. A detector according to claim 8 or 9, wherein said semiconductor material is doped. 11. ll. Detector according to any one of claims 1-10, wherein said cathode arrangement is made entirely of a semiconducting material. A detector according to claim 2 or 3, wherein said material having a resistivity of at least 5x10 * Qm is an electrical insulator. lO 15 20 25 30 530 171 16 A detector according to claim 12, wherein said plurality of electrically conductive elements are connected to each other via respective resistors (43; 4lb). A detector according to claim 12, comprising a high voltage supply unit (7) for applying said voltage between said cathode and anode arrangements, said plurality of electrically conductive elements being connected to said high voltage supply unit (7) via respective resistors (43). ; 4lb). A detector according to claim 14, wherein each of said plurality of electrically conductive elements and each of said plurality of resistors are arranged on said surface of said cathode arrangement directed towards the anode arrangement in the form of a strip ( 41), with a narrow waist (41b) at one end thereof, so that the strip has an elongate part (4la), which constitutes the electrically conductive element, a narrow waist part (41b) which constitutes the resistance and a wider connecting part (41c) for connection to said high voltage supply unit. The detector of claim 15, wherein the material composition of each of said strips is inhomogeneous such that each elongate portion thereof has a material composition of a first resistivity and said narrow waist portion thereof has a material composition of a second resistivity, said second resistivity is higher than said first resistivity. A detector according to any one of claims 1-16, wherein the anode arrangement comprises the readout arrangement. A detector according to any one of claims 2-4 or 12-16, wherein the surface of the anode arrangement facing the cathode arrangement comprises a plurality of electrically conductive or semiconducting elements (35). A detector according to claim 18, wherein said plurality of electrically conductive elements of said cathode arrangement are extending substantially in a first direction and said plurality of electrically conductive or semiconducting elements of the anode arrangement are extended substantially in a second direction, wherein said first and second directions are substantially non-parallel. The detector of claim 19, wherein said first and second directions are substantially perpendicular. A detector according to any one of claims 1-20, wherein said detector comprises an electron avalanche amplification device for avalanche amplification of electrons created upon ionization of said ionizable gas and wherein said readout arrangement is arranged to detect said avalanche amplified electrons, or correspondingly created ions. 22. A device for use in planar radiography, characterized in that the device comprises an X-ray source (3), means (5) for creating a substantially planar X-ray beam (1) located between said X-ray source and an object (7) to be imaged, and the detector (9) according to any one of claims 1-21 located and arranged to detect the planar X-ray beam after being transmitted through, or reflected from, said object. An apparatus according to claim 22, comprising a second and a further of detector (9) according to any one of claims 1-21, which detectors are stacked to form a detector unit, and means (5) for forming a substantially planar X-ray beam (1). ) for each detector, said means being located between the X-ray source (3) and said object (7), each detector being located and arranged to detect the respective plane X-ray beam after being transmitted through, or reflected from, said object.