WO2002025313A1

WO2002025313A1 - Parallax-free detection of ionizing radiation

Info

Publication number: WO2002025313A1
Application number: PCT/SE2001/002035
Authority: WO
Inventors: Tom Francke; Leif Ericsson
Original assignee: Xcounter AB
Current assignee: Xcounter AB
Priority date: 2000-09-22
Filing date: 2001-09-21
Publication date: 2002-03-28
Anticipated expiration: 2003-03-22
Also published as: SE0003390L; AU2001290436A1; SE0003390D0

Abstract

A detector for detection of ionizing radiation comprises a substantially planar cathode (31; 71) and anode (33; 61), respectively; an ionizable substance (35) arranged between the cathode and anode; a radiation entrance arranged such that radiation (37) from a radiation source (39) can enter the detector and pass through the cathode, for ionizing the ionizable substance; and a read-out arrangement (33) for detection of electrons created during ionization of the ionizable substance, and subsequently drifted towards the anode. The detector is characterized in that the substantially planar cathode and anode, respectively, are patterned (41, 45; 81) and capable of being set to different voltages such that an electrical field within the ionizable substance can be obtained, whose field lines (49) are pointing towards the radiation source, for drifting electrons created during ionization of the ionizable substance in parallel with the field lines towards the anode.

Description

PARALLAX-FREE DETECTION OF IONIZING RADIATION

TECHNICAL FIELD OF THE INVENTION

The present invention generally relates to detection of ionizing radiation, and particularly to the detection of X-rays.

More specifically, the invention relates to a detector and method for detection of ionizing radiation, and to a device and method for two-dimensional imaging radiography.

DESCRIPTION OF RELATED ART AND BACKGROUND OF THE INVENTION

Gaseous detectors, in general, are very attractive at photon lower energies since they are cheap to manufacture compared to solid state detectors, and since they can employ gas multiplication to strongly amplify the signal amplitudes.

In Fig. 1 is schematically illustrated a typical gaseous based radiation detector comprising a substantially planar cathode 1 and anode 3, respectively, and an ionizable gas 5 arranged between cathode 1 and anode 3. The detector is arranged such that a radiation beam 7 from a radiation source 9 can enter the detector by passing through cathode 1 for ionizing the ionizable gas. Further, a voltage V is typically applied for drifting electrons created during ionization of ionizable gas 5 towards the anode. Voltage V and the design of the detector electrodes may be adjusted such that multiplication of electrons is achieved to induce an amplified charge at anode 3. A read-out arrangement 11, which typically includes a plurality of read-out elements 13, is arranged adjacent cathode 1 for detecting the electrons drifted towards the anode.

A severe drawback of such a radiation detector arises from that the radiation beam 7 from radiation source 9 is divergent and that the ionizable gas space between cathode 1 and anode 3 has to be sufficiently thick to absorb substantial portions of radiation beam 7. This leads inevitably to a substantial deterioration of the spatial resolution.

Consider for instance a particular ray bundle 15 of beam 7. The photons of ray bundle 15 are absorbed within the ionizable gas between cathode 1 and anode 3 at a penetration depth which is statistically governed. Thus, some photons are absorbed relatively close to the surface, e.g. at region 17, and some photons are penetrating deeper into the gas and are absorbed e.g. at region 19. Due to the voltage applied between cathode 1 and anode 1 electrons released as a consequence of these absorptions at respective regions 17, 19 are accelerated towards anode 3, and possibly avalanche amplified within the electrical field created. Since the beam divergence regions 17 and 19 are located laterally separated, the correspondingly released electrons are accelerated towards separate portions of anode 3, and will consequently be detected by different read-out elements 21, 23 (shadowed in Fig. 1) . In such manner a parallax error will occur in the detected image.

Note that Fig. 1 is not to scale; e.g. the divergence is strongly exaggerated for illustrative purposes. Typical values of the geometrical parameters may be 0.1 - 2 m for the distance from radiation source 9 to anode 3, 10 - 200 mm for the distance between cathode 1 and anode 1, and 50 - 500 mrad for the divergence of radiation beam 7. Such values would result in a lower limit for the spatial resolution of 0.5 mm.

Such parallax errors occur also in solid state based detectors of the kind that has a similar geometry, but include a solid state wafer of an ionizable semiconducting material between the electrodes instead of a gas. However, here the parallax errors are of slightly less importance due to the relatively thinner ionizable material, which in turn is a consequence of the higher density and higher atomic number of the solid state material. One solution to the described parallax error problem is to provide a detector with electrodes having spherical or cylindrical geometries so as to achieve an electrical field between the electrodes, which is parallel with the radiation beam.

Thus, U.S. 6,011,265 issued to Sauli discloses a radiation detector in which primary electrons are released into a gas by ionizing radiations and drifted towards a collecting electrode for detection by means of an electric field applied between a drift electrode and the collecting electrode. The detector further includes a gas electron multiplier formed by one or several matrices of electric field condensing areas distributed within a solid surface and arranged between the drift and collecting electrodes. The solid surface embodying the gas electron multiplier, the drift electrode, and the collecting electrode may be cylindrical or spherical in shape.

Further, U.S. 4,709,382 issued to Sones discloses a detector assembly includes a plurality of individual detector elements, which collectively generally define a concave curved surface oriented toward an x-ray tube. The curved surface increases uniformity of response of the detector with respect to element position by reducing differences in the lengths of the x-ray paths from the tube to the respective detector elements. The curved surface also substantially eliminates differences in the lengths of the x-ray paths in the detector itself, which also increases uniformity of response. Where a dual energy, dual layer detector is used, the concavity of the detector configuration substantially eliminates any parallax effect which is typically suffered by flat split energy detector arrays.

Nevertheless, such curved detectors suffer from other drawbacks; they are more complicated and expensive to manufacture, they are difficult to align with the radiation source, and they are not very flexible. Large area detectors and detectors having very thin electrode gaps are particularly difficult to manufacture and if electron multiplication is to be used the tolerances have to be kept very small in order not to influence the level of amplification. Further, curved detectors are usable only at a certain distance from the radiation source, i.e. the radius of curvature. Finally, curved detectors are not capable of being produced using common semiconductor processing techniques.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide a detector for detection of ionizing radiation, which is capable of mapping position without parallax distortions, and thus exhibits an improved spatial resolution.

It is in this respect a particular object of the invention to provide such detector that is flexible and can be used at a variety of distances from the radiation source.

A further object of the invention is to provide such detector that can be used with point-like as well as line-like radiation sources .

Yet a further object of the present invention is to provide such detector for detection of ionizing radiation, which is effective, fast, accurate, reliable, easy to manufacture, install and use, and of low cost.

Still a further object of the invention is to provide such detector for detection of ionizing radiation, which is sensitive and can thus operate at very low X-ray fluxes.

Yet a further object of the invention is to provide a device for two-dimensional imaging radiography which comprises a detector for detection of ionizing radiation that fulfills the above said objects . It is in this respect a particular object of the invention to provide such device, which can be operated in a manner such that an object to be imaged only needs to be irradiated with a low dose of X-ray photons, while an image of high quality can be obtained.

Still a further object of the present invention is to provide a method for detection of ionizing radiation, which makes use of a detector for detection of ionizing radiation that fulfills above said objects.

These objects among others are attained by detectors, a device for two-dimensional imaging radiography, and methods as claimed in the appended Claims .

Further characteristics of the invention and advantages thereof will be evident from the following detailed description of preferred embodiments of the invention, which are shown in the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more fully understood from the detailed description of embodiments of the present invention given hereinbelow and the accompanying Figs. 1-7, which are given by way of illustration only, and thus are not limitative of the invention.

Fig. 1 illustrates schematically, in a cross sectional view, a prior art radiation detector.

Fig. 2 illustrates schematically, in a cross sectional view, a radiation detector according to a first embodiment of the present invention.

Fig. 3 is a schematic top view of a cathode as being comprised in the inventive detector of Fig. 2. Fig. 4 is a schematic top view of an anode as being comprised in the inventive detector of Fig. 2.

Fig. 5 illustrates schematically, in a cross sectional view, a radiation detector according to a second embodiment of the present invention.

Fig. 6 is a schematic top view of a cathode as being comprised in a radiation detector according to a third embodiment of the invention.

Fig. 7 illustrates schematically, in a cross sectional view, a radiation detector according to a fourth embodiment of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

With reference to Fig. 2, which schematically illustrates a cross sectional view of a radiation detector, a first embodiment of the present invention will be described.

The radiation detector comprises a substantially planar cathode 31 and anode 33, respectively, and an ionizable solid sate slab 34 arranged between cathode 31 and anode 33. The solid state slab 34 is typically confined together with the electrodes in a casing provided with a radiation entrance window in front of cathode 31 (not shown in Fig. 2) . The solid state slab can be made of silicon but is preferably of a higher Z semiconductor material. Preferably, the wafer consists of high-resistivity CdZnTe, which can operate at room temperature and can be fabricated into detectors, or other semiconductor materials that have high-resistivity and that can be fabricated into detectors. Of course, those skilled in the art will recognize that virtually any semiconductor material may be used in the invention. The detector is arranged such that a divergent radiation beam 37 from a point-like radiation source 39 can enter the detector by passing through the casing entrance window and cathode 31. Preferably, the detector is aligned such that radiation beam 37, being symmetric around a central axis 40, will impinge onto the detector at substantially right angle.

Further, cathode ,31 includes a plurality of circular conductive strips 41 of various diameters arranged on a dielectric substrate 43, which is shown in Fig. 3 in detail. Strips 41 are preferably arranged concentrically with respect to each other and aligned with central axis 40. Anode 33 includes correspondingly a plurality of rectangular or quadratic conductive pads 45 arranged on a dielectric substrate 47. Such arrangement is illustrated in Fig. 4. The anode 33 also constitutes read-out arrangement of the detector and thus conductive pads 45 constitute read-out elements for two- dimensional mapping of electrons drifted or accelerated towards the anode 33. The conductive elements 45 are thus electrically insulated from each other by means of dielectric substrate 47.

Alternatively, a separate read-out arrangement is provided or a separate read-out arrangement may be arranged in vicinity of anode 33, in vicinity of cathode 31, or elsewhere. Typically, such read-out arrangement is separated from the electrode by a dielectric layer, or similar.

Further, the read-out arrangement is connected to a signal processing device (not illustrated) for necessary and/or desired post-processing of collected signal data. Preferably, the readout elements 45 are then separately connected to the signal processing circuit by means of individual signal conduits. A signal display unit (neither illustrated) is provided for displaying the processed signal data. Cathode 31 and anode 33 are held, during use, at selected electric potentials by means of an electrical power supply device (not illustrated) . Preferably, anode 33 is grounded and the various circular strips 41 of cathode 31, are each held at a respective selected electrical potential, preferably all of which being negative with respect to anode 33. The innermost strip is held at the highest absolute voltage value, and then the are decreasing as the strips get wider. The outermost strip is thus held at the lowest absolute voltage value. The exact voltage values of the electrical potentials at which the strips 41 are held are selected such that an electrical field within slab 34 is obtained, whose field lines are pointing towards radiation source 39. Electrical field lines between cathode 31 and anode 33 are schematically by reference numeral 49 in Fig. 2, and corresponding spherical equipotential lines are indicated by reference numeral 51.

The size of the detector and consequently of the read-out arrangement may vary tremendously. In a large area detector, such as would be used for medical imaging purposes, a read-out arrangement may typically have many thousands of read-out elements and have outer dimensions of up to 50 cm x 50 cm. At the contrary, a small area detector used for some applications may be smaller than 1 mm x 1 mm.

In operation, the detector of Fig. 2 is positioned in the path of the radiation desired to be detected. The radiation passes through cathode 31 and ionizes the slab 34. The applied electrical potentials, resulting in electrical field 49, are causing electrons released from ionization (through primary and secondary reactions) to drift parallel with the electrical field lines towards the anode 33. Correspondingly produced holes are drifted with the electrical field lines towards the cathode 31. The electrons induce electric pulses in the anode or read-out elements 45, which are individually detected as each read-out element has its individual signal conduit to the signal processor. The signal processing electronics processes then the pulses; it possibly shapes the pulses, and integrates or counts the pulses from each readout element. Correspondingly, the positive charge carriers induce pulses that may be detected.

By providing a two-dimensional array of read-out elements 45 a detector is obtained, wherein electrons derivable mainly from ionization by transversely separated portions of the incident radiation beam 37 are separately detectable. Hereby, the detector provides for two-dimensional imaging. Similarly, by employing one-dimensional arrays of respective element an apparatus for one-dimensional imaging is obtained.

Consider now a particular ray bundle 53 of beam 37. The photons of ray bundle 53 are absorbed within slab 34 between cathode 31 and anode 33 at a penetration depth, which is statistically governed. Thus, some photons are absorbed relatively close to the surface, e.g. at region 55, and some photons are penetrating deeper into the slab 34 and are absorbed e.g. at region 57. Due to the voltages applied between cathode 31 and anode 33 electrons released as a consequence of these absorptions at respective regions 55, 57 are accelerated towards the very same anode or read-out element 59 (shadowed in Fig. 2) . Consequently, these photons will be detected by this read-out element 59 and thus a parallax-free detection is achieved.

It shall be appreciated that the width of the conductive circular strips 41 of cathode 31 is small, and that the distance between consecutive strips is short. The narrower the strips are and the closer each other they are disposed, the more exactly oriented electrical field may be obtained. In any case the distance between the strips should be shorter than the distance between the cathode 31 and the anode 33.

Further aspects of solid state detectors are found in U.S. 6,037,595 issued to Lingren, in U.S. 5,880,490 issued to Antich et al . , and in references therein, said documents being hereby incorporated by reference.

It shall further be appreciated that other kind of read-out arrangements than the ones depicted above may be used in the present invention. Alternatively, or additionally, the holes created within the solid state slab may be detected, preferably in the vicinity of the cathode. Thus, the holes appear as positive charge carriers, and for the purpose of the present invention such holes are intended to be covered by the expressions charge carriers and positive charge carriers as used herein.

Alternatively, a cross-strip detection arrangement, such as the ones depicted by Lingren, and by Antich et al., respectively, in above said patents, may be provided.

It shall yet further be appreciated that the voltages applied may be sufficiently high so as to achieve avalanche amplification of the primary and secondary electrons released within the ionizable solid state slab.

It shall still further be appreciated that the conductive circular strips 41 of cathode 31 may be segmented into segments electrically insulated from each other, each of which being capable of being set to a selected electrical potential (not illustrated) .

With reference now to Fig. 5, which schematically illustrates an inventive radiation detector, a second illustrated embodiment of the present invention will be described. This embodiment differs from the previous embodiment as regards the following features.

Instead of having a solid state slab an ionizable gas or gas mixture 35 is arranged between the cathode 31 and the anode 33. The ionizable gas 35 is typically confined together with the electrodes in a gas tight casing provided with a radiation entrance window in front of cathode 31 (not shown in Fig. 5) . The ionizable gas or gas mixture comprises 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.

Further, grid- shaped electrode 61 is disposed between the cathode 31 and the anode 33 for achieving avalanche amplification of primary and secondary electrons released between cathode 31 and grid-shaped electrode 61 in a volume delimited by grid-shaped electrode 61, the anode 33 and side walls of the detector.

The grid-shaped electrode or field concentration grid 61 is typically comprised of a grid-like conductive sheet or similar, which defines a plurality of holes 63, through which holes electrons may pass on their way towards the anode 33. Further, this grid-like conductive sheet is suitably connected to an electrical power supply device. The grid may be a thin Ni foil e.g. 10-30 micrometers thick, but it may be as thin a 1 micrometers depending on the size of the detector. In this foil the holes 63 are formed, e.g. by means of etching. The holes may have any suitable shape, e.g. circular, elliptic, quadratic, rectangular, etc.

The electrical potentials of the cathode rings, of the grid-like conductive sheet, and of the anode are selected such that an electrical field between cathode 31 and field concentration device 61 is obtained, whose field lines 49 are pointing towards the radiation source 39, and such that an electrical field between field concentration grid 61 and anode 33 is obtained, whose field lines 65 are substantially parallel with the central axis 40 of the radiation beam 37. Further, the electrical field

49 between cathode 31 and field concentration grid 61, being a drift field, shall be weak for drifting primary and secondary electrons towards the field concentration grid 61, whereas the electrical field 65 between field concentration grid 61 and anode 33, being an avalanche amplification field, shall be strong for avalanche amplifying electrons passing through field concentration grid 61. Thus, field concentration grid 61, and anode 33, may be denoted avalanche cathode and avalanche anode, respectively.

The regions where avalanche multiplication takes place are substantially located between and around the edges of the gridlike conductive sheet, which are facing each other, and between the grid-like conductive sheet and anode 33.

Electrons accelerated in the stronger electrical field will interact with the ionizable gas 35, causing electron-ion pairs to be produced. Those produced electrons will also be accelerated in the field, and will interact repetitively with new materia, causing further electron-ion pairs to be produced. This process continues during the travel of the electrons in the avalanche region towards anode 33 located at the bottom of the avalanche region, and in such manner electron avalanches are formed. These electron avalanches yields a very large and almost noise-free amplification of the induced signals in elements 45.

Preferably the field concentration grid electrode 61 and the anode 33 are aligned with each other such that the respective holes 63 are overlying the respective read-out elements 45.

Electrons released in different regions 55, 57 along a radiation ray between cathode 31 and field concentration grid electrode 61 will thus be passed through a single one of the holes 63 of field concentration grid electrode 61, be accelerated and multiplied, before being detected within a single read-out element 59.

It shall be appreciated that the distance between the cathode 31 and the field concentration grid electrode 61 is much longer than the distance between the field concentration grid electrode 61 and the anode 33 and/or that the gas composition and pressure are selected such that a major part of the incident radiation is absorbed before reaching the field concentration grid electrode 61, such that incident radiation induced ionizations within the avalanche amplification area would not severely affect the spatial resolution obtained.

A dielectric (not illustrated) may be arranged between field concentration grid electrode 61 and anode 33. This can be a solid substrate having through-holes aligned with holes 63 and read-out elements 45, said substrate carrying field concentration grid electrode 61 and anode 33. In such manner, the applied voltages produce a strong electric field in a two- dimensional array of avalanche amplification regions.

It shall further be appreciated that the avalanche amplification arrangement may be designed in a variety of ways, and that the present invention is not limited to the arrangement depicted above, but may be used with all such avalanche amplification arrangements .

Further, avalanche electrodes and voltages may be selected such that a strong electrical field, i.e. avalanche amplification field, is obtained, whose field lines also point towards the radiation source.

With reference next to Fig. 6, which is a is a schematic top view of a cathode as being comprised in a radiation detector, a third illustrated embodiment of the present invention will be described. This embodiment differs from the first embodiment only as regards the cathode structure and potentials. Thus, a cathode 71 is provided, which should replace cathode 31 in Fig. 2. Cathode 71 is substantially planar and includes a plurality of elongated conductive strips 81 arranged side by side on a dielectric substrate 83, each of said strips 81 being capable of being set to a respective selected electrical potential . This detector is preferably used with a fan-shaped radiation beam emanating from a line-like radiation source (not illustrated) . In such instance the detector is oriented such that the strips 81 of cathode 71 are parallel with the line-like radiation source and the voltage values of the electrical potentials at which the strips 81 are held are selected such that an electrical field within the ionizable gas is obtained, whose field lines are pointing towards the line-like radiation source. Thus, electrical field lines between cathode 71 and the anode are achieved, whose equipotential lines are cylindrical.

If the anode is grounded, the various strips 81 of cathode 31, are each held at a respective selected electrical potential, all of which being negative with respect to anode 33. The strips closest the central axis (reference numeral 40 in Fig. 2) are held at the highest absolute voltage values, and then the absolute voltage values are decreasing as the strips are located further from the central axis. The outermost strips are thus held at the lowest absolute voltage value.

By use of such arrangement a parallax-free detection of a divergent fan-like radiation beam from a line-like radiation source is achievable.

With reference now to Fig. 7, which schematically illustrates an inventive radiation detector, a fourth illustrated embodiment of the present invention will be described. This embodiment differs from the Fig. 5 embodiment as regards the following features.

The detector is arranged such that divergent ray bundles 91 from the point radiation source 39 can be entered through the anode 33.

The electric field concentration grid electrode 61 (see description of the second embodiment for further details of the grid) is disposed close to the anode between the cathode 31 and the anode 33 for achieving absorption of the incident radiation substantially in the drift volume 90 between the cathode 31 and the grid 61. The avalanche amplification of primary and secondary electrons is then performed in an amplification volume, being the inter-electrode volume 96 delimited by the anode 33 and the grid 61.

It shall be appreciated that the distance D between the cathode 31 and the field concentration grid electrode 61 is much longer than the distance between the field concentration grid electrode 61 and the anode 33 and/or that the gas composition and pressure are selected such that a major part of the incident radiation is absorbed in the drift volume 90, such that incident radiation induced ionizations within the avalanche amplification volume 96 would not severely affect the spatial and spectral resolutions obtained. Typically the distance D is less than 2 mm, preferably less than 1 mm, more preferably much less than 1 mm. It may be as short as 100 micrometers.

A dielectric 98 may be arranged between the field concentration grid 61 and the anode 33. This can be a solid substrate having through-holes aligned with the holes of the grid 61 and the read-out elements 45, said substrate carrying field concentration grid electrode 61 and anode 33. In such manner, the applied voltages produce a strong electric field in a two- dimensional array of avalanche amplification regions. Alternatively, the dielectric includes a number of wires or similar, the purpose of which being to hold the grid 61 and the anode 33 apart at a controlled distance.

The electrical potentials of the cathode rings, of the grid-like conductive sheet, and of the anode are selected such that an electrical field between cathode 31 and anode 33 is obtained, whose field lines 93 are parallel with the divergent incident ray bundles 91 (i.e. the spherical equipotential surfaces 95 being orthogonal to the incident ray bundles 91) . The electrical potentials of the cathode rings may be determined by the man skilled in the art reading this descrption.

Further, The electrical field 93 between cathode 31 and field concentration grid 61, i.e. in volume 90, being a drift field, shall be weak for drifting primary and secondary electrons towards the field concentration grid electrode 61, whereas the electrical field 97 between field concentration grid electrode 61 and anode 33, i.e. in volume 96, being an avalanche amplification field, shall be strong for avalanche amplifying electrons passing through field concentration grid electrode 61. Thus, field concentration grid 61, and anode 33, may be denoted avalanche cathode and avalanche anode, respectively.

The region where avalanche multiplication takes place are substantially located between the grid-like conductive sheet 61 and the anode 33.

Electrons accelerated in the stronger electrical field will interact with the ionizable gas 35, causing electron-ion pairs to be produced. Those produced electrons will also be accelerated in the field, and will interact repetitively with new materia, causing further electron-ion pairs to be produced. This process continues during the travel of the electrons in the avalanche region towards the anode 33 located at the bottom of the avalanche region, and in such manner electron avalanches are formed. These electron avalanches yields a very large and almost noise-free amplification of the induced signals in the elements 45.

Preferably the field concentration grid 61 and the anode 33 are aligned with each other as in the Fig. 5 embodiment such that the respective holes of grid 61 are overlying the respective read-out elements 45.

By means of the Fig. 7 embodiment an important advantage is achieved. As the absorption of the incident radiation is decreasing exponentially with penetration depth a substantial portion of the incident radiation can be absorbed in the drift volume 90 in vicinity of the grid 61 and thus of the volume 96 (the volume 96 has to be very thin in order not to be responsible for any considerable amount of absorption) . By such provision improved spatial as well as energy resolutions are obtained.

In the embodiments described above particular locations and geometries of cathodes, anodes, avalanche amplification arrangements, and read-out arrangements are described. There are, however, a plurality of other locations and geometries that are suitable in connection with the present invention.

Particularly, the anode and cathode structures of the respective detectors may be swapped provided that the sign of the electric potentials at which they are held, are changed.

Alternatively, the above described cathode structures are used also as anode structures; i.e. in the Figs. 2-5 embodiments both the cathode and the anode may be provided with circular conductive strips, and in the Fig. 6 embodiment both the cathode and the anode may be provided with elongated parallel conductive strips .

Yet alternatively, a detector may be designed and aligned such that a radiation beam, being located off the central axis 40 of Fig. 1, will impinge onto the detector at an angle inclined with respect to substantially perpendicular incidence. In the case a point-like source of radiation is to be located in front of the detector but off central axis 40, the conductive strips of the cathode and/or anode have to be provided as fragments of circles, i.e. as conductive arched strips.

Generally, a detector having a cathode and/or anode with a matrix of conductive pads (possibly similar to the read-out arrangement) can be provided, each of said conductive pads being capable of being individually held at a selected electric potential by means of an electrical power supply device and suitable connecting and/or switching means. In such manner the cathode and/or the anode would be capable of being connectable/switchable to "simulate" the various geometries described above, i.e. circular, arched, and straight strips. Such arrangement may be adapted to appear as virtually any desired geometry. The conformity of the electrical field obtained and the electrical field desired is essentially given by the size of the conductive pads and the distance between them.

It shall further be noted that none of the Figs, are to scale; e.g. the divergence of the radiation beams in Figs. 2 and 5 are strongly exaggerated for illustrative purposes.

It is general for the invention that the inter-electrode volumes are relatively thin, which result in fast removal of ions and thus, low or no accumulation of space charges is obtained. In such manner operation at high rates is made possible. The small distances lead also to low operating voltages, resulting in low energy in possible sparks, which is favorable for the electronics. The focusing of the field lines in the avalanche means is also favorable for suppressing streamer formations, which leads to a reduced risk for sparks.

Further, all electrode surfaces may be covered by a high- resistive or semiconducting material in order to decrease the energy in possible sparks, which will influence the measurement and may destroy electronic equipment of the detector. Alternatively, the complete or portions of the cathode and anode arrangements may be made of a semi-conducting material, e.g. silicon. Finally, some expressions of how the electrical potential values of the electrodes are selected will be given. Three different cases are considered:

(i) A point-like source of radiation is used, the cathode is provided with conductive circular strips concentrically arranged around a center, and the anode is grounded (see Fig. 2-4 embodiment) . The distance between the radiation source and the cathode is z₀ and the distance between the cathode and the anode is assumed to be long, i.e. longer than the distance over which a spherical field is desired. The voltage U(r) of the cathode rings are then selected according -1

U(r) °

V r + z wherein r is the radius of the respective cathode rings. Such arrangement provides for field lines pointing towards the radiation source in a region adjacent the cathode, whereas the field lines are perpendicular to the anode in immediate vicinity thereof. The distance between the electrodes is thus selected such that a major portion of the radiation is absorbed in the "parallax-free" region.

(ii) A point-like source of radiation is used, the cathode is provided with conductive circular strips concentrically arranged around a center, and the anode is grounded (see Fig. 2-4 embodiment) . The distance between the radiation source and the cathode is l-z₀ and the distance between the cathode and the anode is z₀. The voltage U(r) of the cathode rings are then selected according to a Bessel function. However, an approximative solution when zo « 1 and r² « 1 is given by

wherein r is the radius of the respective cathode rings. Such arrangement provides for field lines pointing towards the radiation source in a region adjacent the cathode, whereas the field lines are perpendicular to the anode in immediate vicinity thereof.

(iii) A point-like source of radiation is used, and both the cathode and the anode are provided with conductive circular strips concentrically arranged around a respective center. The distance between the radiation source and the cathode is z_c and the distance between the radiation source and the anode is z_a. The voltage U(r) of the cathode rings are then selected according -1

U{r) i r + z: wherein r is the radius of the respective cathode rings. Correspondingly, the voltage U(r) of the anode rings are selected according 1

U(r) r + z: wherein r is the radius of the respective anode rings.

Such arrangement provides for field lines pointing towards the radiation source in the complete region between the electrodes, and thus all radiation, which is absorbed between the electrodes, is absorbed in a "parallax-free" region.

It will be obvious that the invention may be varied in a plurality of ways. Such variations are not to be regarded as a departure from the scope of the invention. All such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the appended Claims.

Claims

1. A detector for detection of ionizing radiation comprising:

- a substantially planar cathode (31; 71) and anode (33), respectively;

- an ionizable solid state slab (34) arranged between said cathode and anode;

- a radiation entrance arranged such that radiation (37) from a radiation source (39) can enter said detector and pass through said cathode, for ionizing the solid state slab; and

- a read-out arrangement (33) for detection of charge carriers created during ionization of said solid state slab, and subsequently drifted towards the anode or the cathode,

characterized in that

- said substantially planar cathode and anode, respectively, are patterned (41, 45; 81) and capable of being set to different voltages such that an electrical field within said solid state slab can be obtained, whose field lines (49) are pointing towards said radiation source, for drifting charge carriers created during ionization of said solid state slab in parallel with said field lines towards the anode or the cathode.

2. The detector as claimed in Claim 1, wherein

- said radiation entrance' is arranged such that radiation from a point-like source of radiation (39) can enter said detector and pass through said cathode; and

- said substantially planar cathode comprises a plurality of circular conductive strips (41) of different diameters, said circular strips being concentrically arranged and capable of being set to different voltages, all of which are negative with respect to the anode.

3. The detector as claimed in Claim 2, wherein each of said conductive circular strips is segmented into segments electrically insulated from each other.

4. The detector as claimed in Claim 2 or 3, wherein said substantially planar anode (33) comprises a plurality of conductive pads (45), said pads being arranged in a matrix and capable of being set to a voltage, which is positive with respect to the cathode.

5. The detector as claimed in Claim 4, wherein said read-out arrangement comprises said matrix of conductive pads.

6. The detector as claimed in Claim 2 or 3, wherein said substantially planar anode (33) comprises a plurality of circular conductive strips of different diameters, said circular strips being concentrically arranged and capable of being set to different voltages, all of which are positive with respect to the cathode.

7. The detector as claimed in Claim 6, wherein said read-out arrangement comprises a matrix of conductive pads separated from said cathode and anode by a dielectric.

8. The detector as claimed in Claim 1, wherein

- said radiation entrance is arranged such that * radiation from a point-like source of radiation can enter said detector and pass through said cathode; and

- said substantially planar anode comprises a plurality of circular conductive strips of different diameters, said circular strips being concentrically arranged and capable of being set to different voltages, all of which are positive with respect to the cathode.

9. The detector as claimed in Claim 8 , wherein each of said conductive circular strips is segmented into segments electrically insulated from each other.

10. The detector as claimed in Claim 8 or 9, wherein said read- out arrangement comprises a plurality of conductive pads, said pads being arranged in a matrix.

11. The detector as claimed in Claim 1, wherein

- said radiation entrance is arranged such that radiation from a line-like source of radiation can enter said detector and pass through said cathode (71) ; and

said substantially planar cathode or anode comprises a plurality of elongated conductive strips (81) , said strips being arranged parallel with each other, and with said line-like source of radiation, and capable of being set to different voltages .

12. The detector as claimed in any of Claims 1-11, wherein said solid state slab is made of a semiconducting material, preferably of any of silicon, a higher Z semiconductor material, or high-resistivity CdZnTe .

13. A detector for detection of ionizing radiation comprising:

a substantially planar cathode (31; 71) and anode (33) , respectively, and a grid-shaped electrode (61) arranged parallel with and between said cathode and said anode;

- an ionizable gas (35) arranged between said cathode and anode;

- a radiation entrance arranged such that radiation (37) from a radiation source (39) can enter said detector and pass through said cathode, for ionizing said gas; wherein said cathode, anode and grid-shaped electrode are capable of being set to voltages for creating a weak electrical field (49) between said cathode and said grid-shaped electrode for drift of electrons released during ionization and a strong electrical field (65) between said grid-shaped electrode and said anode for avalanche amplification of said drifted electrons; and

- a read-out arrangement (33) for detection of said avalanche amplified electrons, or correspondingly produced ions,

characterized in that

- said substantially planar cathode and anode, respectively, and said grid-shaped electrode are patterned (41, 45; 81) and capable of being set to different voltages such that an electrical field between said cathode and said grid-shaped electrode can be obtained, whose field lines (49) are pointing towards said radiation source, for drifting electrons created during ionization of said gas in parallel with said field lines towards the grid-shaped electrode.

14. A detector for detection of ionizing radiation comprising:

a substantially planar cathode (31; 71) and anode (33), respectively, and a grid-shaped electrode (61) arranged parallel with and between said cathode and said anode;

- an ionizable gas (35) arranged between said cathode and anode;

- a radiation entrance arranged such that radiation (37) from a radiation source (39) can enter said detector and pass through said anode, for ionizing said gas; wherein said cathode, anode and grid-shaped electrode are capable of being set to voltages for creating a weak electrical field (93) between said cathode and said grid-shaped electrode for drift of electrons released during ionization and a strong electrical field (97) between said grid-shaped electrode and said anode for avalanche amplification of said drifted electrons; and - a read-out arrangement (33) for detection of said avalanche amplified electrons, or correspondingly produced ions,

characterized in that

- said substantially planar cathode and anode, respectively, and said grid-shaped electrode are patterned (41, 45; 81) and capable of being set to different voltages such that an electrical field between said cathode and said grid-shaped electrode can be obtained, whose field lines (93) are pointing away from said radiation source, for drifting electrons created during ionization of said gas in parallel with said field lines towards the grid-shaped electrode.

15. The detector as claimed in claim 13 or 14, wherein said grid-shaped electrode is a thin foil having a plurality of through holes (63) , preferably formed by etching.

16. The detector as claimed in claim 15, wherein said grid- shaped electrode is thinner than 0.1 mm, preferably thinner than 50 micrometers, and more preferably between 1 and 30 micrometers or 10 and 30 micrometers.

17. The detector as claimed in any of claims 13-16, wherein the distance (D) between said grid-shaped electrode and said anode is less than 2 mm, preferably less than 1 mm, more preferably much less than 1 mm, e.g. about 100 micrometers.

18. The detector as claimed in any of claims 13-17, wherein a dielectric (98) is arranged between said grid-shaped electrode and said anode, the purpose of which being to hold said grid- shaped electrode and said anode apart.

19. A device for use in two-dimensional imaging radiography, characterized in that it comprises an X-ray source (39) , and the detector as claimed in any of Claims 1-18 located and arranged for detection of X-rays from said X-ray source as transmitted through or reflected off an object to be imaged.

20. A method for detection of ionizing radiation in a radiation detector, characterized by the steps of:

- passing radiation (37) through a substantially planar cathode (31; 71) and entering said radiation into an ionizable solid state slab (34) arranged between said cathode and a substantially planar anode (33) , respectively;

- ionizing the solid state slab by means of said radiation;

- setting said cathode and anode, respectively, to different voltages such that an electrical field within said solid state slab is obtained, whose field lines (49) are pointing towards said radiation source, for drifting charge carriers created during ionization of said solid state slab towards the anode; and

- detecting the charge carriers drifted towards the anode or the cathode by means of a read-out arrangement (33) .

21. The method as claimed in Claim 20, wherein

said radiation entered into said solid state slab is a divergent beam (37) originating from a point-like source of radiation (39) ; and

- a plurality of circular conductive strips (41) , being comprised in the substantially planar cathode and/or anode, are set to different voltages, said circular strips being of different diameters and being concentrically arranged.

22. The method as claimed in Claim 20 or 21, wherein

- said radiation entered into said solid state slab is a divergent beam originating from a line-like source of radiation; and

a plurality of elongated conductive strips (81) , being comprised in said substantially planar cathode and/or anode, are set to different voltages, said strips being arranged parallel with each other, and with said line-like source of radiation.

23. The method as claimed in any of Claims 20-22, wherein the charge carriers drifted towards the anode are detected by means of a read-out arrangement comprising a plurality of conductive pads (45) , said pads being arranged in an array or matrix.

24. The method as claimed in Claim 23, comprising detecting charge carriers created during ionization by a single X-ray photon in a single pad (59) irrespective of the distance said X- ray photon propagates in said solid state slab before ionization (55, 57) .

25. The method as claimed in any of Claims 20-24, wherein the charge carriers created during ionization of said solid state slab are avalanche amplified prior to being detected.

26. A method for detection of ionizing radiation in a radiation detector, characterized by the steps of:

- passing radiation (37) through a substantially planar cathode (31; 71) and entering said radiation into an ionizable gas (35) arranged between said cathode and a substantially planar grid- shaped electrode (61) , and between said substantially planar grid-shaped electrode and a substantially planar anode (33) , respectively;

- ionizing the gas by means of said radiation;

setting said cathode, grid-like electrode, and anode, respectively, to different voltages such that a weak electrical field between said cathode and said substantially planar grid- shaped electrode is obtained, whose field lines (49) are pointing towards said radiation source, for drifting electrons created during ionization of said gas towards the grid-like electrode, and such that a strong electrical field (65) between said grid-shaped electrode and said anode is obtained for avalanche amplification of said drifted electrons; and

detecting said avalanche amplified electrons or correspondingly created ions by means of a read-out arrangement (33) .

27. A method for detection of ionizing radiation in a radiation detector, characterized by the steps of :

- passing radiation (37) through a substantially planar anode (33) and entering said radiation into an ionizable gas (35) arranged between said cathode and a substantially planar grid- shaped electrode (61) , and between said substantially planar grid-shaped electrode and a substantially planar cathode (31; 71) , respectively;

- ionizing the gas by means of said radiation;

- setting said cathode, grid-like electrode, and anode, respectively, to different voltages such that a weak electrical field between said cathode and said substantially planar grid- shaped electrode is obtained, whose field lines (93) are pointing away from said radiation source, for drifting electrons created during ionization of said gas towards the grid-like electrode, and such that a strong electrical field (65) between said grid-shaped electrode and said anode is obtained for avalanche amplification of said drifted electrons; and

28. The method as claimed in claim 26 or 27, wherein said grid- shaped electrode is a thin foil having a plurality of through holes (63) , preferably formed by etching.

29. The method as claimed in claim 28, wherein said grid-shaped electrode is thinner than 0.1 mm, preferably thinner than 50 micrometers, and more preferably between 1 and 30 micrometers or 10 and 30 micrometers.

30. The method as claimed in any of claims 26-29, wherein the distance (D) between said grid-shaped electrode and said anode is less than 2 mm, preferably less than 1 mm, more preferably much less than 1 mm, e.g. about 100 micrometers.