RADIATION DETECTOR, AN APPARATUS FOR USE IN RADIOGRAPHY AND A METHOD FOR DETECTING IONIZING RADIATION

Abstract: A detector (64) for detection of ionizing radiation, an apparatus for use in planar beam radiography, comprising such a detector, and a method for detecting ionizing radiation. The detector comprises a chamber filled with an ionizable gas; first and second electrode arrangements (2, 1, 18, 19) provided in said chamber with a space between them, said space including a conversion volume (13); means for electron avalanche amplification (17) arranged in said chamber; and at least one arrangement of read-out elements (15) for detection of electron avalanches. A radiation entrance is provided so that radiation enters the conversion volume between the first and second electrode arrangements. In order to achieve well-defined avalanches the means for electron avalanche amplification includes a plurality of avalanche regions. The detector, apparatus and method is advantageous since it prevents the formation of harmful sparks.

Description: RADIATION DETECTOR, AN APPARATUS FOR USE IN RADIOGRAPHY AND A METHOD FOR DETECTING IONIZING RADIATION FIELD OF THE INVENTION The invention relates to a detector for detection of ionizing radiation, to an apparatus for use m radiography and to a method for detecting ionizing radiation. BACKGROUND OF THE INVENTION AND RELATED ART A detector, an apparatus and a method of the kind mentioned above are described m the copendmg Swedish application SE- 9901325-2, which is incorporated herein by reference. The detector described therein includes a gaseous parallel plate avalanche chamber. The detector provides good position resolution, high X-ray detection efficiency, and the ability to count every photon absorbed m the detector. This enables numerous possibilities for processing the detection signals, such as energy detection, discriminating detection signals from photons m certain energy ranges or from photons incident at certain distance ranges from the anode or the cathode. US-5 614 722 discloses a radiation detector based on charge amplification m a gaseous medium. Said detector includes a substrate having a cavity defined therein, an anode surface positioned m the bottom of the cavity and a cathode positioned adjacent the cavity opening. A drift electrode is juxtaposed over the substrate opposite the cavity and defines a region containing a gaseous medium. Said document also discloses that the cathode terminates m a cathode sidewall, which preferably terminates short of the cavity opening, and the cathode sidewall may terminate at greater distances from the cavity opening. Moreover, in said document it is mentioned that the gaseous medium of the detector can be replaced with a liquid such as, for example, liquid argon or liquid xenon. A drawback with conventional gaseous parallel plate avalanche chambers is that the electrical field between the electrodes, i.e. the voltage applied between the electrodes, needed for efficient operation of the instrument is close to the limit where sparks are produced which could harm the detector. SUMMARY OF THE INVENTION A main object of the invention is to provide an improved detector for detection of ionizing radiation, which employs avalanche amplification, and ensures stronger focusing effect than in conventional designs, and which can be manufactured in a simple and cost effective way. The stronger focusing effect will render the streamer formation more difficult compared to prior art designs . This and other objects are attained by a detector for detection of ionizing radiation, comprising a chamber filled with an ionizable medium, at least one first electrode arrangement provided m said chamber and including an electron avalanche amplification unit, which includes at least one avalanche cathode and at least one avalanche anode between which a voltage is to be applied for creation of an electric field for avalanche amplification, at least one arrangement of read-out elements for detection of electron avalanches, said at least one avalanche cathode having at least one hole or opening, wherein the area of said at least one hole or opening in the avalanche cathode is lager than the area of at least one associated anode, as seen from above, and wherein said at least one anode is centered with respect to said at least one hole or opening m the avalanche cathode, characterized in that at least one focusing electrode is arranged between said at least one avalanche cathode and said at least one avalanche anode and has an opening or hole the area of which is smaller than the area of the at least one hole or opening in said avalanche cathode but larger than the area of said at least one avalanche anode. The detector also can operate at high X-ray fluxes without performance degradation and has a long lifetime. The detector of claim 1 also achieves effective detection of any kind of radiation, including electromagnetic radiation as well as incident particles, including elementary particles. It is also an object of the invention to provide an apparatus for use m radiography comprising said detector for detection of ionizing radiation, which employs avalanche amplification, provides well-defined avalanches, and can be manufactured in a simple and cost effective way. This and other objects are attained by an apparatus for use in radiography, comprising an X-ray source, and a detector for detection of ionizing radiation, comprising a chamber filled with an ionizable medium, at least one first electrode arrangement provided m said chamber and including an electron avalanche amplification unit, which includes at least one avalanche cathode and at least one avalanche anode between which a voltage is to be applied for creation of an electric field for avalanche amplification, at least one arrangement of read-out elements for detection of electron avalanches, said at least one avalanche cathode having at least one hole or opening, wherein the area of said at least one hole or opening m the avalanche cathode is lager than the area of at least one associated anode, as seen from above, wherein said at least one anode is centered with respect to said at least one hole or opening m the avalanche cathode and wherein at least one focusing electrode is arranged between said at least one avalanche cathode and said at least one avalanche anode and has an opening or hole the area of which is smaller than the area of the at least one hole or opening m said avalanche cathode but larger than the area of said at least one avalanche anode. The apparatus for use m radiography may also include a simple and inexpensive detector that can operate with high X-ray detection efficiency and with good energy resolution for X-rays. The apparatus for use m radiography may also include a detector which can operate at high X-ray fluxes without a perform- ance degradation and has a long lifetime. It is also an object of the invention to provide a method for detection of ionizing radiation, which employs avalanche amplification, provides well-defined avalanches and focusing of the electrons by means of at least one focusing electrode, and can be implemented m a simple and cost effective way. This and other objects are attained by a method for detecting ionizing radiation, wherein the radiation interacts with atoms m a volume filled with an ionizable medium, for creation of released electrons, said method comprising the steps of: forming m each of at least one region, including at least one avalanche cathode and at least one avalanche anode, a concentrated electric field for causing electron avalanches, detecting the electron avalanches with read-out elements, focusing the electrons with at least one anode having a smaller area than associated holes or openings m the at least one avalanche cathode, wherein the area of the at least one anode is centered with respect to the associated holes or openings m the at least one cathode, subjecting the electrons to a first electric field m a conversion and drift volume, said first electric field forces the electrons to enter the at least one region with a concentrated electric field, c h a r a c t e r i z e d b y the further step of focusing the electrons with at least one focusing electrode arranged between the at least one avalanche cathode and the at least one avalanche anode, said at least one focusing electrode having a hole or opening, the area of which is smaller than the area of the hole or opening m the at least one avalanche cathode . The method can be used at high X-ray fluxes. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 illustrates schematically, m an overall view, a general embodiment of a detector for radiography. Figure 2 is a schematic, partly enlarged, cross sectional view, taken at II-II m Figure 1, of a detector according to a first specific embodiment of the invention. Figure 3 is a schematic, partly enlarged, cross sectional view, taken at II-II m Figure 1, of a detector according to a second specific embodiment of the invention. Figure 4 is a schematic cross sectional view of an alternate embodiment of an anode. Figure 5 is a schematic, partly enlarged, cross sectional view, taken at II-II m Figure 1, of a detector according to a third specific embodiment of the invention. Figure 6 is a schematic, partly enlarged, cross sectional view, taken at II-II m Figure 1, of a detector according to a fourth specific embodiment of the invention. DESCRIPTION OF PREFERRED EMBODIMENTS Fig. 1 is a sectional view m a plane orthogonal to the plane of a planar X-ray beam 9 of a detector 64 for radiography. A major fraction of the incident X-ray photons are detected m the detector 64, which includes a conversion and drift volume 13 and an electron avalanche amplification unit 17. The X-ray photons may enter sideways between two electrode arrangements 1, 2, between which an electric field for the drift of electrons and ions m the conversion and drift volume 13 is created or alternatively enter the detector from any direction m the range of 0-180° where 0° is defined as coinciding with the above-mentioned sideways entry direction. The detector 64 and its operation according to the invention will be further described below. The detector 64 includes a first drift electrode arrangement being a cathode plate 2 and a second drift electrode arrangement being an anode plate 1. The plates 1,2 may be mutually parallel and a gap or region 13 between said plates includes an ionizable medium, such as a gaseous medium, liquid medium or a solid medium, and the plates enclose the conversion and drift volume 13 and the electron avalanche amplification unit 17. Alternatively, the plates 1,2 are non- parallel . A voltage is applied between the anode plate 1 and the cathode plate 2, and one or several voltages is (are) applied on the electron avalanche amplification unit 17. This results m a drift field causing a drift of electrons and ions m the gap 13, and electron avalanche amplification fields m the electron avalanche amplification unit 17. In connection with the anode plate 1 is an arrangement 15 of read-out elements for detection of electron avalanches. Preferably the arrangement of read-out elements 15 also constitutes the anode electrode. Alternatively, the arrangement of read-out elements 15 can be formed m connection with the cathode plate 2 or the electron avalanche amplification unit 17. The arrangement of read-out elements 15 can also be formed on the anode or cathode plate separated from the anode or cathode electrode by a dielectric layer or substrate. In this case, the anode or cathode electrode should be semi-transparent to induced pulses, e.g. the anode or cathode electrode should be formed as strips or pads . In case of X-rays incident sideways on the detector 64 between the cathode plate 2 and the anode plate 1, the detector 64 can easily be made with an interaction path long enough to allow a major fraction of the incident X-ray photons to interact and be detected. A collimator may be used for this purpose and should preferably be arranged so that the thin planar beam enters the detector 64 close to the electron avalanche amplification unit 17 and preferably parallel therewith. The gaseous ionizable medium can be a mixture of for example 90% krypton and 10% carbon dioxide or a mixture of for example 80% xenon and 20% carbon dioxide. The gas can be under pressure, preferably a range 1 - 20 atm. Therefore, the detec- tor 64 includes a gas tight housing 91 with a slit entrance window 92, through which the X-ray beam 9 enters the detector 64. The window 92 may be made of a material, which is transparent for the radiation, e.g. Mylar®, or a thin aluminum foil . Detecting sideways incident beams m a gaseous ava- lanche chamber 64 is a particularly advantageous additional effect of the invention. The slit entrance window 92 is arranged at another place on the housing 91 than indicated figure 1 when X-rays enter the detector from a direction which is not sideways, i.e. ≠ 0 or 180°, alternatively the housing 91 may be made of a material transparent to the X-rays. The liquid ionizable medium may for instance be TME (Tπ Methyl Ethane) or TMP (Tπ Methyl Pentane) or any liquid ionizable media with similar properties. The solid ionizable medium may for instance be a semi conducting material, for instance, silicon. When the ionizable medium is solid the housing 91 can be excluded. In operation, the incident X-rays 9 enter the detector 64 and travel through the gas, liquid or solid ionizable medium m a direction m the range of 0-180° as defined above. Each X-ray photon produces a primary lonization electron- ion pair withm the gas, liquid or solid ionizable medium as a result of interaction with an atom the medium. This production may be caused by photoeffect, Compton-effect or Auger-effect. Each primary electron 11 produced loses its kinetic energy through interactions with new atoms, causing further production of electron-ion pairs (secondary lonization electron-ion pairs) . Typically between a few hundred and a few thousand secondary lonization electron-ion pairs are produced from a 20 keV X-ray photon m this process. The secondary lonization electrons 16 (together with the primary lonization electron 11) will drift towards the electron avalanche amplification unit 17 due to the electric field m the conversion and drift volume 13. When the electrons 11, 16 enter regions of focused field lines of the electron avalanche amplification unit 17, they will un- dergo avalanche amplification, which will be described further below. The movements of the avalanche electrons and ions induce signals the arrangement 15 of read-out elements for the detec- tion of electron avalanches. The signals are picked up m connection with the electron avalanche amplification unit 17, the cathode plate 2 or the anode plate 1, or a combination of two or more of said locations. The signals are further amplified and processed by readout circuitry 14 to obtain accurate meas- urements of the X-ray photon interaction points, and optionally the X-ray photon energies. Figure 2 shows a schematic, partly enlarged, cross sectional view, taken at II-II Figure 1, of a detector 64 according to a first specific embodiment of the invention. As seen, the cathode plate 2 includes a dielectric substrate 6 and a conductive layer 5 acting as a cathode electrode. The anode 1 includes a dielectric substrate 3 and a conductive layer 4 act- mg as an anode electrode. Between the gap 13 and the anode 1, the electron avalanche amplification unit 17 is arranged. The electron avalanche amplification unit 17 includes an avalanche amplification cathode 18 and a focusing electrode 19, sepa- rated by a dielectric 24. The dielectric 24 could be a gas, liquid or a solid substrate 24 carrying the cathode 18 and the focusing electrode 19, as shown Figure 2. Between the cathode 18 and the focusing electrode 19 a voltage is applied by a DC power supply 7 for creation of a very strong focused electric field an avalanche amplification region 25. The avalanche amplification region 25 is formed a region between and around the edges of the avalanche cathode 18 which are facing each other, where a concentrated electric field occurs due to the applied voltages. The DC power supply 7 is also connected with the cathode electrode 5 and the anode electrode 4 (19) . The voltages applied are selected so that a weaker electric field, drift field, is created over the gap 13. Electrons 11, 16 (primary and secondary electrons) re- leased by interaction m the conversion and drift volume 13 will drift, due to the drift field, towards the electron avalanche amplification unit 17. The electrons 11, 16 enter the very strong avalanche amplification fields and are accelerated. The accelerated electrons 11, 16 interact with other at- oms m the region 25 causing further electron-ion pairs to be produced. Those electron-ion pairs will also be accelerated the field, and will interact with new atoms, causing further electron-ion pairs to be produced. This process continues during the travel of the electrons m the avalanche region 25 to- wards the anode 4 and an electron avalanche is formed. The avalanche region 25 is formed by an opening or channel the cathode 18 and m the dielectric substrate 24, if present. The opening or channel can be of any shape for instance circular, square, triangular, rectangular, elliptical etc., when viewed from above, or continuous, longitudinal extending between two edges of the substrate 24, if present, and the cat- hode 18. In the case the openings or channels are circular when viewed from above, they may be arranged rows, each row of openings or channels including a plurality of circular openings or channels. A plurality of longitudinal openings or channels or rows of circular channels may be formed beside each other, parallel with each other or with the incident X- rays . Alternatively, the circular openings or channels can be arranged other patterns. As an example, the longitudinal channels m the cathode 18 can have a width m the range 0.01-1 mm. The longitudinal openings or holes m the focusing electrode 19 are smaller than the associated longitudinal openings or holes the cathode 18. The dielectric 24 may have an inclined surface 26 with respect to a normal to a surface of the avalanche cathode 18, the m- clined surface 26 forming a limiting surface of a region for avalanche amplification. The size of the area of the anode 4, as seen from above, is smaller than the associated area of the focusing electrode 19. The difference m size of the holes or openings m the cathode 18, the focusing electrode 19 and the anode 4 ensure stronger focusing effect compared to holes or openings of equal sizes. If a streamer appears at high ga s they will be self- quenched. The reason for the self -quenching is the focused field lines and the radial dependency of the field strength seen from the avalanche region 25. Alternatively the conductive layers 5, 4 can be replaced by a resistive carrier of e.g. silicon monoxide, conductive glass or diamond, with the dielectric substrates 3, 6 being replaced by a conductive layer. An electric field is created between the anode electrodes 4 and the focusing electrode 19. This field could be a drift field, i.e. a weaker field, or an avalanche amplification field, i.e. a very strong electric field. As an alternative, the electric field the conversion and drift gap 13 (volume) can be kept high enough to cause electron avalanches, hence to be used m a pre-amplification mode . The distance between the cathode plate 2 and the electron avalanche amplification unit 17 may be m the range of 0-10 mm, but preferably withm the range of 100-5000 μm. The distance between the avalanche amplification cathode 18 and the focusing electrode 19 may be m the range of 0-10 mm, but preferably withm the range of 50-500 μm. The distance between the focusing electrode 19 and the anode substrate 3 may be m the range of 0-1 mm, but preferably withm the range of 50-500 μm. Figure 3 shows a schematic, partly enlarged, cross sectional view, taken at II-II Figure 1, of a detector 64 according to a second specific embodiment of the invention. As seen, the cathode plate 2 includes a dielectric substrate 6 and a con- ductive layer 5 acting as a cathode electrode. The anode 1 includes a dielectric substrate 3 and an extended conductive layer 4 acting as an anode electrode. The avalanche cathode 18 acts m this embodiment, as the avalanche amplification unit 17, i.e. the distance m Figure 2 between the avalanche cathode 18 and focusing electrode 19 is reduced to 0 m this embodiment . Due to the shape of extended anode 4 more field lines per unit area may be incident at the top of said anode compared to the inclined surface, thereby increasing the ga of the detector 64. The area of the extended anode 4, as seen from above, is smaller than the associated area of the hole or opening m the avalanche cathode 18. It is noted that the top portion of the anode 4 may be any shape (for example, flat rounded or pointed) as long as the shape allows more field lines per unit area to be incident at the top of the anode 4 as compared with the inclined surface. In figure 4, another embodiment of the extended anode 4 ar- ranged on the substrate 3 is shown. In this embodiment the extended anode 4 has a smooth shape, e.g. like a half ellipse. The height of the extended anode could be as high as the distance between the substrate 3 and the focusing electrode 19 if present or the avalanche cathode 18 m absence of said focus- mg electrode 19. Figure 5 shows a schematic, partly enlarged, cross sectional view, taken at II-II m Figure 1, of a detector 64 according to a third specific embodiment of the invention. The anode 1 includes a dielectric substrate 3 and an extended conductive layer 4 acting as an anode electrode. The avalanche cathode 18 and the focusing electrode 19 act, m this embodiment, as the avalanche amplification unit 17. The distance between the cathode 2, including a dielectric substrate 6 and a conductive layer 5, and the avalanche cathode 18 is reduced to 0 m the embodiment as shown m figure 5, thereby eliminating the volume 13, i.e. the drift and conversion volume. In this embodiment the incident X-rays will enter the detector 64 directly the avalanche region 25 where the electrical field is very high. Due to the shape of extended anode 4 more field lines per unit area will be incident at the top of said anode compared to the inclined surface, thereby increasing the position resolution of the anode 4. The area of the extended anode 4, as seen from above, is smaller than the associated area of the hole or opening the avalanche cathode 18 and the area of the hole or opening m the focusing detector. Figure 6 shows a schematic, partly enlarged, cross sectional view, taken at II-II m Figure 1, of a detector according to a fourth specific embodiment of the invention. The anode 1 includes a dielectric substrate 3 and an extended conductive layer 4 acting as an anode electrode. As seen, the cathode plate 2 includes a dielectric substrate 6 and a conductive layer 5 acting as a cathode electrode. Between the gap 13 and the anode 1 an electron avalanche amplification unit 17 is arranged. The avalanche cathode 18 and the focusing electrode 19 represent, m this embodiment, the avalanche amplification unit 17. The voltages applied are selected so that a weaker electric field, drift field, is created over the gap 13. Electrons 11, 16 (primary and secondary electrons) released by interaction the conversion and drift volume 13 will drift, due to the drift field, towards the avalanche amplification unit 17. The electrons 11, 16 enter the very strong avalanche amplification fields and are accelerated. The accelerated electrons 11, 16 interact with other atoms m the region 25 causing further electron-ion pairs to be produced. Those produced electron-ion pairs will also be accelerated m the field, and may interact with new atoms, causing further electron-ion pairs to be produced. This process continues during the travel of the electrons m the avalanche region towards the anode 4 and an electron avalanche is formed. In compaπ- son to figure 2, the distance between the focusing electrode and the dielectric substrate 3 is reduced to 0 in the embodiment as shown in figure 6. Due to the shape of extended anode 4, more field lines per unit area will be incident at the top of said anode compared to the inclined surface, thereby increasing the ga of the apparatus. The area of the extended anode 4, as seen from above, is smaller than the associated area of the hole or opening the avalanche cathode 18 and the area of the hole or opening in the focusing detector. An apparatus comprising the inventive detector, as described above, may also include an X-ray source, for irradiation of an object to be imaged. The X-rays from the X-ray source are transmitted through the object and enter the inventive detector 64. In general, for all embodiments, the volume of gas, liquid or solid may be very small, which results in a fast removal of ions, which leads to low or no accumulation of space charges. This makes operation at high rate possible. In general, for all embodiments, smaller distances lead to low operating voltages, which results in low energy and a reduced possibility for sparks, which is favorable for the electro- nics. A streamer is a form of channel of plasma in which a spark can form. The focusing of the field lines m the embodiments is also favorable for suppressing streamer formations. This also leads to a reduced risk of sparks. Although the invention has been described in conjunction with a number of preferred embodiments, it is to be understood that various modifications may still be made without departing from the spirit and scope of the invention, as defined by the appended claims. For example the voltages can be applied m other ways as long as the described electrical fields are created.

Claims: CLAIMS 1. A detector (64) for detection of ionizing radiation, comprising : a chamber filled with an ionizable medium, - at least one first electrode arrangement provided said chamber and including an electron avalanche amplification unit (17) , which includes at least one avalanche cathode (18) and at least one avalanche anode (4) between which a voltage is to be applied for creation of an electric field for avalanche amplification, at least one arrangement (15) of read-out elements for detection of electron avalanches, said at least one avalanche cathode (18) having at least one hole or opening, wherein the area of said at least one hole or opening m the avalanche cathode (18) is larger than the area of the at least one associated anode (4), as seen from above, and wherein said at least one anode (4) is centered with respect to said at least one hole or opening m the avalanche cathode (18), c h a r a c t e r- l z e d l n that at least one focusing electrode (19) is arranged between said at least one avalanche cathode (18) and said at least one avalanche anode (4) and has an opening or hole the area of which is smaller than the area of the at least one hole or opening m said avalanche cathode (18) but larger than the area of said at least one avalanche anode (4) . 2. The detector according to claim 1, wherein the ionizable medium is a solid medium. 3. The detector according to claim 1 or 2, wherein - said at least one avalanche cathode (18) , said at least one focusing electrode (19) and said at least one avalanche anode (4) are arranged as stacked layers with interposed dielectric material (24). 4. The detector according to claim 3, wherein a surface of said dielectric material (24) forms at least one limiting surface of a region (25) for avalanche amplification said surface being inclined with respect to a surface of said at least one avalanche cathode (18) . 5. An apparatus for use m radiography, comprising: a chamber filled with an ionizable medium, - at least one first electrode arrangement provided m said chamber and including an electron avalanche amplification unit (17) , which includes at least one avalanche cathode (18) and at least one avalanche anode (4) between which a voltage is to be applied for creation of an electric field for avalanche amplification, at least one arrangement (15) of read-out elements for detection of electron avalanches, said at least one avalanche cathode (18) having at least one hole or opening, wherein the area of said at least one hole or opening the avalanche cathode (18) is larger than the area of the at least one associated anode (4) , as seen from above, wherein said at least one anode (4) is centered with respect to said at least one hole or opening m the avalanche cathode (18) , and wherein at least one focusing electrode (19) is arranged between said at least one avalanche cathode (18) and said at least one avalanche anode (4) and has an opening or hole the area of which is smaller than the area of the at least one hole or opening m said avalanche cathode (18) but larger than the area of said at least one avalanche anode (4) . 6. A method for detecting ionizing radiation, wherein the radiation interacts with atoms in a volume filled with an ionizable medium, for creation of released electrons, said method comprising the steps of: - forming in each of at least one region, including at least one avalanche cathode and at least one avalanche anode, a concentrated electric field for causing electron avalanches , detecting the electron avalanches with read-out elements, - focusing the electrons with at least one anode having a smaller area than associated holes or openings m the at least one avalanche cathode, wherein the area of the at least one anode is centered with respect to the associated holes or openings in the at least one cathode, - subjecting the electrons to a first electric field in a conversion and drift volume, said first electric field forces the electrons to enter the at least one region with a concentrated electric field, c h a r a c t e r i z e d b y the further step of - focusing the electrons with at least one focusing electrode arranged between the at least one avalanche cathode and the at least one avalanche anode, said at least one focusing electrode having a hole or opening, the area of which is smaller than the area of the hole or opening in the at least one avalanche cathode.

Inventors: FRANCKE TOM, PESKOV VLADIMIR, FRANCKE, TOM, PESKOV, VLADIMIR
Applicants: FRANCKE TOM, PESKOV VLADIMIR, XCOUNTER AB, FRANCKE, TOM, PESKOV, VLADIMIR
Application year: 2001
Main classification: G01T1
IPC codes: G01T1