Classifications

• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
  • H01J47/00—Tubes for determining the presence, intensity, density or energy of radiation or particles
  • H01J47/02—Ionisation chambers
  • H01J47/026—Gas flow ionisation chambers
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01T—MEASUREMENT OF NUCLEAR OR X-RADIATION
  • G01T1/00—Measuring X-radiation, gamma radiation, corpuscular radiation, or cosmic radiation
  • G01T1/16—Measuring radiation intensity
  • G01T1/185—Measuring radiation intensity with ionisation chamber arrangements
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
  • H01J47/00—Tubes for determining the presence, intensity, density or energy of radiation or particles
  • H01J47/02—Ionisation chambers

Definitions

• the invention relates to gaseous-based detection of ionizing radiation.
• Gaseous-based ionizing radiation detectors in general, are very attractive since they are cheap to manufacture, and since they can employ gas multiplication to strongly amplify the signal amplitudes .
• a typical gaseous-based ionizing radiation detector comprises a planar cathode and anode arrangement, respectively, and an ionizable gas arranged between the cathode and anode arrangements.
• the detector is arranged such that a radiation beam from a radiation source can enter the detector for ionizing the ionizable gas.
• a voltage is typically applied for drifting electrons created during ionization of the ionizable gas towards the anode.
• the voltage and the design of the detector electrodes may be adjusted such that multiplication of electrons is achieved to induce an amplified charge at the anode arrangement.
• a read-out arrangement which typically includes a plurality of read-out elements, is arranged adjacent the anode arrangement for detecting the electrons drifted towards the anode arrangement.
• a particular kind of gaseous detector is the one, in which electrons released by interactions between photons and gas atoms can be extracted in a direction essentially perpendicular to the incident radiation.
• an improved spatial resolution is obtained.
• spark discharges can occur in the gas due to the strong electric fields created in the detector. Such events are particularly probable to occur in high amplification detectors.
• Such -spark discharges block the detector for a period of time, and can also be harmful for the detector and particularly for electronics thereof.
• a main object of the invention is to provide a detector for detection of ionizing radiation wherein problems caused by spark discharges are eliminated, or at least reduced.
• a particular object of the invention is to provide such detector wherein the energy in any occurring sparks is low. Thus relatively few charges are released in the gas.
• a further object of the present invention is to provide such detector, which provides for fast recovery subsequent to a spark discharge, and thus provides for faster detection and shorter time periods during which an object under investigation is exposed for ionizing radiation.
• Yet a further object of the invention is to provide such detector, which is effective, accurate, and of low cost.
• Still a further object of the invention is to provide such detector, which is reliable and has a long lifetime.
• Fig. 1 illustrates schematically, in an overall view, an apparatus for planar beam radiography, according to a first embodiment of the present invention.
• Fig. 2 is a schematic plane 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 of a detector according to a third embodiment of the present invention.
• Fig. 4 is a schematic plane view of a cathode arrangement of a detector according to a fourth embodiment of the present invention.
• Fig. 1 is a sectional view in a plane orthogonal to the plane of a planar fan-shaped X-ray beam 1 of a device for planar beam radiography, according to a first embodiment of the present invention.
• the device includes an X-ray source 3, which together with a first thin collimator window 5 produces the planar X-ray beam 1, for irradiation of an object 7 to be imaged.
• a thin slit or second collimator window 11 which is aligned with the X-ray beam, forms the entrance for the X- ray beam 1 to the detector 9.
• the detector 9 is oriented such that the X-ray photons can enter sideways 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.
• a high voltage DC supply unit 7 a voltage 1-J ⁇ can be applied between cathode 17 and anode 19 for drift of electrons and ions in space 13.
• Cathode 17 and anode 19 arrangements are preferably substantially parallel with each other.
• X-ray source 3, thin collimator window 5, optional collimator window 11 and detector 9 are preferably connected and fixed in relation to each other by a suitable means for example a frame or support (not shown in Fig. 1) .
• the ionizable gas or gas mixture comprising for example 90% krypton and 10% carbon dioxide or for example 80% xenon and 20% carbon dioxide.
• the gas may be under pressure, preferably in a range 1-20 atm. Therefore, the detector includes a gas tight housing 31 with a slit entrance window 33, through which the X-ray beam 1 can enter the detector.
• the casing 31 encloses major parts of detector 9. It shall, however, be appreciated that casing 31 may be arranged in other manners as long as the space between the electrodes may be enclosed.
• detector 9 comprises a read-out arrangement for separate detection of the electrons drifted towards anode arrangement 19 and/or ions drifted towards the cathode arrangement 21.
• the read-out arrangement may be comprised of anode arrangement 19 itself as illustrated in Fig. 1, or a separate read-out arrangement may be arranged adjacent anode arrangement 19 adjacent cathode arrangement 17, or elsewhere.
• Anode or read-out arrangement 19 comprises an array of 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 may be formed by photolithographic methods or electroforming, etc.
• strips 35 extend essentially in directions parallel to the direction of incident X-ray photons of beam 1, originating from source 3, at each location.
• read-out strips 35 are arranged in a fan-like configuration. The length and width of strips 35 are adjusted to the specific detector in order to obtain the desired (optimal) spatial resolution and sensitivity.
• Each of the strips 35 is preferably connected to read-out 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.
• the signal conductors also connect the respective strip to the high voltage DC power supply unit 7, with suitable couplings for separation.
• Fig. 1 such provisions are merely indicated by a separate ground connector.
• the above-depicted design of the read-out arrangement provides for capability of separate detection of electrons derivable mainly from ionization by transversely separated portions of planar radiation beam 1 by strips 35. In such manner one- dimensional imaging is enabled.
• anode strips 35 can be formed as a unitary electrode without strips.
• the strips are further divided into segments in the direction of the incident X-rays, the segments being electrically insulated from each other. Preferably a small spacing extending perpendicular to the incident X-rays is provided between each segment of respective strip.
• Each segment is connected to the processing electronics by means of a separate signal conductor, where the signals from each segment preferably are processed separately.
• Such read-out arrangement can be used when energy-resolved detection of radiation is required.
• specific reference is made to our co-pending Swedish patent application Swedish patent application No. 0001167-6 entitled Spectrally resolved detection of ionizing radiation and filed on March 31, 2000, which application hereby is incorporated by reference.
• detector 9 comprises an electron avalanche amplification device 21 for avalanche amplification of electrons drifted within space 13.
• electron avalanche amplification device 21 is suitably connected to high voltage DC supply unit 7.
• the electron avalanche amplification device 21 is comprised of a grid-like conductive sheet or similar, which defines a plurality of holes, through which electrons may pass on their way towards the anode arrangement 19.
• avalanche amplification arrangements or field concentration means may be provided such that electrons freed in space 13 and can be amplified before detection.
• Various such avalanche amplification arrangements are described in our co-pending Swedish patent application No. 9901325-2 entitled Radiation detector, an appara tus for use in planar radiography and a method for detecting ionizing radia tion and filed on April 14, 1999, which application hereby is incorporated by reference.
• avalanche amplification can be achieved simply by keeping the voltage Ui
• the incident X-rays 1 enter the detector through the optional thin slit or collimator window 11, if present, and between cathode 17 and anode 19, preferably in a center plane between them as indicated in Fig. 1.
• the incident X-rays 1 then travel through the gas volume in a direction preferably substantially parallel with electrodes 17 and 19 and get absorbed, thus ionizing gas molecules in space 13.
• X-rays absorbed in space 13 will cause electrons to be released, which will drift towards anode arrangement 19 due to the voltage Ui applied. If the voltages are kept high enough and/or if field concentration means are provided (as discussed above) the freed electrons are avalanche amplified during their travel towards the anode. If an electron avalanche amplification device is provided it is preferably held at electrical potential (s) such that a weak drift field is obtained between the cathode arrangement 17 and the amplification device 21 and strong avalanche amplification field is obtained within amplification device (e.g. between an electrode thereof and anode arrangement 19.
• the electrons induce charges in the strips 35 of the anode/read-out arrangement 19 which are detected. If no avalanche amplification takes place the major part of the signal is due to collection of the liberated charges.
• Each incident X-ray photon causes generally one induced pulse in one (or more) anode strip.
• the pulses are processed in the rad-out and signal processing electronics 14, which eventually shapes the pulses, and integrates or counts the pulses from each strip representing one picture element.
• the pulses can also be processed so as to provide an energy measure for each pixel.
• spark discharges Due to the high electric field strengths that can occur in connection with the electrode plates there is a risk that spark discharges occur in the gas. Such spark discharges blocks the detector for a period of time, and can also damage the anode arrangement 19 and electronics connected thereto.
• the present invention involves providing the cathode arrangement 17 (and optionally the anode strips 35) of a material having a resistivity of at least 5xl0 ⁇ 8 ⁇ m.
• the cathode arrangement 17 is preferably of a material having a resistivity between 5xl0 -8 ⁇ m and IxlO 5 ⁇ m, more preferably between IxlO -3 ⁇ m and IxlO 3 ⁇ m, even more preferably between IxlO -2 ⁇ m and 1 ⁇ m, and most preferably between IxlO -2 ⁇ m and lxlO _1 ⁇ m.
• the material can be a doped or undoped semiconducting material preferably comprising a semiconductor material composed of elements selected from the periodic system groups IV (e.g. the compounds silicon and germanium) and III-V (e.g. the compounds GaAs, InP, and InGaAsP) .
• the cathode arrangement 17 is of undoped or doped silicon.
• the material is an electrically conducting glass or plastic.
• virtually any solid material having a resistivity in the ranges mentioned above may be suitable to employ in the cathode arrangement 17.
• the resistive cathode arrangement 17 faces space 13 and avalanche amplification means 21, where strong electric fields can occur.
• avalanche amplification means 21 where strong electric fields can occur.
• the surface layer 17a of cathode arrangement 17 facing space 13 may be made of a material having a resistivity of at least 5xl0 ⁇ 8 ⁇ m.
• the surface layer may be provided on a conductive substrate or on a dielectric substrate provided with suitable electric connections (not illustrated) .
• the surface layer 17a of cathode arrangement 17 facing space 13 may be partly covered by a plurality of electrically conductive elements electrically connected to each other only by means of said resistive material.
• a cathode arrangement is illustrated in Fig. 2 wherein one of said plurality of electrically conductive elements is denoted by reference numeral 41.
• reference numeral 41 By such provisions a faster detector may be provided, wherein the surface area of high conductivity is still limited to local areas (i.e. the respective elements 41) .
• elements 41 of Fig. 2 are illustrated as elongated stripes they may have other shapes and be arranged in other patterns.
• the electrically conductive elements may be quadratic or rectangular pads arranged in a two-dimensional matrix on the surface 17a of the resistive cathode 17.
• the high voltage DC supply unit 7 is preferably connected to the backside of cathode 17 as indicated at 17b in Fig. 1 (i.e. at the surface opposite to surface 17a) such that the respective electrically conductive elements 41 are each connected to high voltage DC supply unit 7 via the resistive cathode 17.
• the elements 41 shall preferably be oriented with respect to the read-out strips 35 such that an electric field is obtained within the detector that reduces the occurrence of "pockets" within space 13 and amplification device 21, i.e. regions where electrons and/or ions are not drifted further and will thus be accumulated. This is particularly important to avoid close to the anode/read-out arrangement 19.
• the plurality of electrically conductive elements 41 of the cathode 17 are oriented substantially in a first direction
• the plurality of electrically conductive or semiconducting elements 35 of the anode/read-out arrangement 19 are oriented substantially in a second direction, wherein the first and second directions are essentially non-parallel, and conveniently essentially perpendicular .
• cathode 17 in yet a further embodiment of cathode 17, as being illustrated in Fig. 3, such plurality of electrically conductive elements 41 are provided on a surface 42a of a dielectric substrate 42.
• the cathode arrangement is oriented such that electrically conductive elements 41 and surface 42a of dielectric substrate 42 are facing space 13 and anode 19 of the detector 9.
• Each of said plurality of elements 43 is connected to an electrically conductive layer 45 arranged on surface 42b of dielectric substrate 42 opposite to surface 42a by means of a respective resistance 43.
• Fig. 3 these resistance are merely symbolically indicated and it shall be appreciated that they may be implemented in a variety of manners; e.g. as discrete components within or adjacent substrate 42 or as integrated components within substrate 42. In the latter instance the complete cathode may be fabricated in a semiconductor process with the resistances implemented as suitably composed layers between a layer of conductive elements 41 and a conductive layer 45 for connection to high voltage DC supply unit 7.
• each of the plurality of electrically conductive elements and each of the plurality of resistances are provided on surface 42a of dielectric substrate 42 in the form of a stripe 41 having a narrow waist 41b in an end portion thereof, such that the stripe has an elongated portion 41a constituting the electrically conductive element, a narrow waist portion 41b constituting the resistance, and a wider connection portion 41c for connection to high voltage DC supply unit 7.
• each of stripes 41 is inhomogeneous such that each elongated portion 41a has a material composition of a resistivity, which is lower, particularly considerably lower, than the resistivity of the material composition of each narrow waist portion 41b.
• a poor conductor such as chromium
• a good conductor such as gold
• a further aspect of the invention which is relevant when the cathode arrangement includes a surface layer facing the anode arrangement made of a semiconducting material, particularly silicon, is to eliminate any problems occurring due to the semiconducting surface being oxidized. For instance, pure silicon kept at room temperature in ambient air will in a few hours obtain a thermally grown silicon oxide layer, typically 20-40 A thick.
• the present inventors have discovered that the oxide layer is repeatedly charged and subsequently discharged to the semiconducting material via sparks. This may be harmful to the detector and obstructs its operation.
• the oxide grown on the cathode arrangement is removed, typically by means of dipping the cathode arrangement, or at least the surface layer facing the anode arrangement made of the semiconducting material, in an HF bath or similar.
• the cathode arrangement has to be hindered from oxidizing again, and this may be achieved in a plurality of manners.
• a straightforward way is to keep the cathode arrangement, or at least the semiconducting material thereof, in an inert atmosphere, which thus does not contain oxygen.
• such atmosphere may be comprised of vacuum, an inert gas or nitrogen, or a mixture thereof. Then, during use of the detector one has to safeguard that the detector will never be filled with water, air or other oxygen-containing gas.
• a second way to prevent the cathode arrangement from oxidizing is to cover it with a protective layer (not illustrated) of a suitable kind.
• suitable protective layers may be a silicon nitride or any metal suicide, e.g. titanium suicide. Anyhow, it is preferred that that the protective layer shall (i) have suitable resistance, i.e. low enough such that it can be used as a cathode and still high enough to reduce problems with sparks, see embodiments above; (ii) being dense enough not to let silicon atoms to diffuse through the protective layer and cause oxidation at the outer surface of the protective layer; and (iii) naturally not oxidize itself in ambient air if such an oxide is troublesome for the detector operation.
• a third way to reduce the oxidization of the cathode arrangement is to passivate the surface of it by dipping the cathode arrangement, or at least the surface layer facing the anode arrangement made of the semiconducting material, in an HF bath mixed with e.g. ammonia or ammonium fluoride. This slows down the oxidation rate of the semiconductor. Also, the HF bath alone, will probably have a passivating effect.
• a fourth way may be to cool the surface to thereby (i) slow down the oxidation rate; and (ii) slow down the diffusion of silicon atoms to the surface, thus obtaining a thinner oxide layer.
• the present invention relates also to methods for detecting ionizing radiation, wherein any oxide on the cathode is removed and the cathode is thereafter treated in a manner to not oxidize again; and to a method in the fabrication of a radiation detector, wherein the cathode arrangement is treated in a way as described above.
• the gas volumes shall be made thin as this results in a fast removal of ions, which leads to low or no accumulation of space charges. This makes operation at high rate possible.
• inter-electrode distances shall be kept short since this leads to low operating voltages, which results in an even low energy in possible sparks. This reduces also the risk of damaging the electronics.
• the focusing of field lines (which typically is performed in an electron avalanche amplification device) is also favorable for suppressing streamer formations. This leads to a reduced risk for occurrence of sparks.
• the resistances involved at the cathode arrangement should be kept low enough to accept high rate and still high enough to protect the electrodes against sparks.

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• Molecular Biology (AREA)
• High Energy & Nuclear Physics (AREA)
• Measurement Of Radiation (AREA)
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• 2000
  • 2000-10-13 SE SE0003718A patent/SE530171C2/sv not_active IP Right Cessation
• 2001
  • 2001-10-12 AU AU2001296123A patent/AU2001296123B2/en not_active Ceased
  • 2001-10-12 CN CNB018173365A patent/CN100501446C/zh not_active Expired - Fee Related
  • 2001-10-12 EP EP01976971A patent/EP1325356A1/en not_active Withdrawn
  • 2001-10-12 WO PCT/SE2001/002230 patent/WO2002031535A1/en active Application Filing
  • 2001-10-12 KR KR1020037004895A patent/KR100866557B1/ko not_active IP Right Cessation
  • 2001-10-12 AU AU9612301A patent/AU9612301A/xx active Pending
  • 2001-10-12 JP JP2002534867A patent/JP4184075B2/ja not_active Expired - Fee Related
  • 2001-10-12 CA CA002423381A patent/CA2423381A1/en not_active Abandoned

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