SE518802C2 - Ionizing radiation detector for use in planar beam radiography, includes chamber filled with ionizable gas, and electron avalanche amplifier, and read-out arrangement - Google Patents

Abstract

An ionizing radiation detector includes a chamber filled with ionizable gas, an electron avalanche amplifier, and a read-out arrangement. The radiation enters a first chamber section having a first distance and then enters a second chamber section having a second distance. The electron avalanches, and/or corresponding produced ions, in the respective chamber section are separately detectable. An ionizing radiation detector (9) comprises a chamber (13), a radiation entrance (33), an electron avalanche amplifier (15), and a read-out arrangement (29). The chamber is filled with ionizable gas, and includes a first electrode (17, 19) and a second electrode (21), which are in parallel with each other. A first voltage is applicable between the electrodes, for drifting electrons created during ionization to the amplifier. The radiation entrance is parallel with the electrodes, for ionization of the gas. The amplifier includes an avalanche cathode (25) and an avalanche anode (27). A second voltage is applicable between these avalanche arrangements for amplification of electrons. The read-out arrangement detects electron avalanches and/or corresponding produced ions. The radiation (1) enters the first chamber section having a first distance (d1) between the electrode arrangements and then enters a second chamber section having a second distance (d2) between the electrode arrangements. The first and second distances are different. The electron avalanches, and/or corresponding produced ions, derived from ionization in the respective chamber section are separately detectable. Independent claims are also included for the following: (A) a planar beam radiography device comprising an X-ray source (3), a collimator window (5) for forming a planar X-ray beam, and a detector; and (B) a method for detecting ionization radiation in the detector comprising introducing a radiation beam into a chamber between and parallel with the two electrodes, and avalanche amplifying the created electrons. The radiation beam is introduced to first section of the chamber, and then to the second chamber section. The electron avalanches and/or corresponding produced ions are separately detected by a read-out arrangement.

Description

70 15 20 25 30 35 5 1 sso 2 _2__ When designing a detector for lower energy photons and using X-ray irradiation with an energy range that includes both lower and higher energies, higher energy electrons will only be absorbed to a lesser extent in the detector. depending on the relatively short length of the detector.

In some situations, however, it is desirable to also detect high energy electrons, as these may carry information that differs from that of the low energy electrons.

For example, in the field of radiography, the absorption coefficients for bone or tissue vary completely differently with the photon energy.

SUMMARY OF THE INVENTION It is thus an object of the present invention to provide a detector for detecting ionizing radiation which utilizes avalanche amplification and which can operate in a wider energy spectrum, in particular even at higher energies, of incoming radiation than previously known detectors.

In this regard, a particular object of the invention is to provide such a detector which provides for improved spatial resolution and in particular for improved energy resolution. Yet another object of the present invention is to provide a ionizing detector which is efficient, fast, reliable, radiation, accurate, easy to install and use and inexpensive.

A further object of the invention is to provide a detector for detecting ionizing radiation which is sensitive and thus can operate with very low X-ray currents. 70 15 20 25 30 35 518 soz -. u. . Yet another object of the invention is to provide a detector for detecting ionizing radiation, which can be given a length in the direction of the incoming radiation to achieve a desired stopping ability which makes it possible to detect a major part of the incoming radiation. . Yet another object of the invention is to provide a detector for detecting ionizing radiation in which electrons released by the interaction between photons and gas atoms can be taken out in a direction substantially perpendicular to the incoming radiation. This makes it possible to obtain a particularly high spatial resolution. Yet another object of the invention is to provide a detector for detecting ionizing radiation which can operate at high X-ray fluxes without degrading performance and has a long life. Yet another object of the invention is to provide a detector for detecting any kind of ionizing radiation including electromagnetic radiation as well as incident particles including elementary particles.

According to one aspect of the present invention, these objects, together with others, are achieved with a detector according to claim 1.

A further object of the invention is to provide a device for use in plane beam radiography, for example slit or scan radiography, which comprises at least one detector according to the first aspect of the invention.

In this regard, it is a particular object of the present invention to provide such a device which can be used so that an object to be imaged need only be irradiated with a low dose of X-ray photons, while obtaining a high quality image. A further object of the invention is to provide a device for use in plane beam radiography, in which a major part of the X-ray photons incident on the detector can be detected for further counting or integration in order to obtain a value for each pixel in the image.

A further object of the invention is to provide a device for use in plane beam radiography, in which noise is greatly reduced, which is caused by radiation scattered by the examined object. Yet another object of the invention is to provide a device for use in plane beam radiography, in which noise is reduced, which is caused by the scattering of the X-ray energy spectrum. Yet another object of the present invention is to provide a device for use in plane beam radiography which can operate at high X-ray fluxes without degrading performance and has a long life.

These and other objects are achieved according to a second aspect of the invention by a device according to claim 18.

A further object of the invention is to provide a method for detecting ionizing radiation which utilizes avalanche amplification and is effective in a wider energy spectrum, especially even at higher energies, of incoming radiation are previously known methods.

In this regard, it is a particular object of the invention to provide such a method which has both improved spatial resolution and improved energy resolution. 10 15 20 25 30 35 »a -. u: o | Q -. . now . Yet another object of the present invention is to provide a method for detecting ionized radiation, reliable, easy to perform and which can be realized in a simple and cost-effective manner. which is efficient, fast, accurate, Yet another object of the invention is to provide a method for detecting ionizing radiation which is sensitive and thus can operate with very low X-ray currents. Yet another object of the invention is to provide a method for detecting ionizing radiation which detects a major portion of the incoming radiation.

A further object of the present invention is to provide a method for detecting ionizing in which electrons, radiation released by the interaction between photons and gas atoms, can be taken out in a direction which is substantially perpendicular to the incident radiation. This makes it possible to achieve a particularly high spatial resolution. Yet another object of the invention is to provide a method for detecting ionizing radiation which can be performed using high X-ray fluxes.

A further object of the invention is to provide a method for detecting all kinds of ionizing radiation including electromagnetic radiation as well as incident particles including elementary particles.

According to a third aspect of the present invention, these and other objects are achieved by a method according to claim 22.

Further features and advantages of the invention will become apparent from the detailed description below of preferred embodiments of the invention, which are shown in the drawings. BRIEF DESCRIPTION OF THE DRAWINGS This invention will be better understood from the detailed description below of embodiments of the invention and taken by way of Figures 1 to 7, which are merely illustrative and not limiting of the invention.

Fig. 1 schematically shows in an overall view a device for plane beam radiography according to an embodiment of the invention.

Fig. 2 is a graph showing the average wavelength of X-rays depending on absorption in argon, krypton and xenon as a function of the energy of the X-rays at normal temperature and normal pressure (STP).

Fig. 3 is a diagram showing the practical range of interaction of electrons due to scattering in argon, krypton and xenon as a function of electron energy at normal temperature and normal pressure (STP).

Fig. 4 is a diagram showing the absorption coefficient of X-rays due to absorption in tissue or bone in the human body as a function of the energy of the X-rays.

Fig. 5 is a schematic, partially enlarged cross-sectional view taken along A-A in Fig. 1.

Fig. 6 is a schematic view of an X-ray source and an electrode formed by segmented read strips according to the present invention.

Fig. 7 is a schematic cross-sectional view of a device with stacked detectors for use in plane beam radiography according to yet another embodiment of the invention. Detailed Description of Preferred Embodiments For explanatory and non-limiting purposes, the description below sets forth particular details, such as certain techniques and applications, to provide a thorough understanding of the present invention. However, it will be apparent to one skilled in the art that the present invention may be practiced in other embodiments which differ from those particular details. In other examples, detailed descriptions of well-known methods and apparatus are omitted so as not to burden the description of the present invention with unnecessary details.

Fig. 1 is a sectional view of a plane perpendicular to the plane of a plane X-ray of a plane beam radiography apparatus according to an embodiment of the present invention. The device comprises a source 3 for X-rays, which together with a first narrow collimator window 5 provides a flat, fan-shaped X-ray 1 for irradiating an object 7 to be imaged. The first narrow collimator window 5 can be replaced with other means for forming a substantially flat X-ray beam, for example a diffraction mirror or a lens etc. for X-rays.

The beam transmitted through the object 7 enters a detector 9. A narrow slit or a second collimator window 11, which is aligned with the X-ray beam, alternatively forms the input of the X-ray beam 1 to the detector 9.

A main part of the incident X-ray photons is detected in the detector 9, which comprises a chamber 13, means for electron avalanche amplification 15 and a reading device 29. The detector 9 is oriented so that the X-ray photons enter from the side between a first 17, 19 and a second 21 electrode device, between which a voltage U1, U2 is applied to drive electrons and ions into the chamber 13. 70 15 20 25 30 35 nun: nu nn nn v »novo un oo un q nn nu n nn nn nn n nu n, nn nn gnnnn nu: en ß uno unnnnc | nn n nn u un! v I n u n u. n o n n n n n n n n n ann; u n n n n n - n n n nn _8_.

The first drive electrode device 17, 19 is constituted by a first 17 and a second electrode plate 19, respectively, and the second drive electrode device is constituted by an anode plate 21. A voltage U1 is applied between the cathode plate 17 and the anode plate 21, and a voltage U2 is applied between the cathode plate 19 and the anode plate 21 U1 and U2 may be equal, but U2 is preferably larger than U1 to create an electric field of constant amplitude in the chamber 13.

The electrode devices are preferably mutually substantially parallel and separated by a short distance d1 in a first section of the chamber 13, i.e. between the cathode plate 17 and the anode plate 21, and by a longer distance d2 in a second section of the chamber 13, i.e. between the cathode plate 19 and the anode plate 21 This will be discussed in more detail below.

The chamber 13 is a conversion and drive volume and it is filled with an ionizable gas. The radiation entering the chamber 13 ionizes the gas, and the electric field formed by U1 and U2 results in a driving field in the region 13, which causes the electrons to be driven towards the anode plate 21 and thus towards the avalanche amplifying means 15, and which causes the ions to be driven towards the cathode plates 17 and 19.

The chamber 13 is filled with a gas, which may 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 may be under pressure, preferably in the range of 1-20 atm.

The detector therefore comprises a gas-tight housing 31 with a slit entrance window 33, through which the X-ray beam 1 enters the detector. The window is made of a material that is transparent to the radiation, such as Mylar® or a thin aluminum foil. This is a particularly advantageous additional effect of the invention in detecting side-incident beams in a gas avalanche detector 9, compared to previously used gas avalanche chambers which were designed for radiation incident perpendicular to the anode and now 518 802. the cathode teats and requires a window covering a large area. The window can be made thinner in this way, whereby the number of X-ray photons absorbed in the window is reduced.

In operation, the incident X-rays 1 enter the detector through the optional narrow slit or collimator window 11, the cathode plate 17 and the anode plate 21, preferably in one if necessary, and in between the middle planes between them, as shown in Fig. 1. The incident X-rays 1 then travel through the gas volume in one direction, the electrode plates 17, gas molecules are ionized in the chamber 13. which are preferably parallel to 19 and 21 and absorbed, the electron avalanche amplifier means 15 being arranged in such a manner, in an electron avalanche amplification volume 23 of the released electrons being driven towards and entering Preferably by an avalanche cathode device 25, where they are multiplied by means of a voltage Ua applied between the avalanche cathode device 25 and the avalanche anode device 27.

The voltage Ua is applied between the avalanche cathode so that electrons from the chamber 13 passing the avalanche cathode 23 are accelerated towards the avalanche anode 27, resulting in thus reaching multiple 27. electron multiplication reading device, and avalanche electrodes avalanche anode Adjacent to the avalanche anode 27 there is a pulse, induced by the electron avalanches.

In this way, one-photon detection can be realized.

Preferably, the reading device 29 also constitutes the avalanche anode electrode 27, and consequently the devices 29, 27 are integrated into a single element. Alternatively, the reading device 29 may be formed in connection with other components of the electron avalanche amplifier means 15 or in connection with the drive cathode plates 17, 19. It may also be formed. is formed separately from the anode or cathode devices by means of a dielectric layer or substrate in which case it is necessary to anode or (not shown in Fig. 1). In the cathode devices are semi-transparent for induced In connection with Fig. 6, various possible reading devices are pulses, for example designed as strips or cushions. further.

The source 3 of X-rays, the first narrow collimator window 5, the optional collimator window 11 and the detector 9 are interconnected and fixed by suitable means, such as a frame or base (not shown in Fig. 1).

The radiography device designed in this way can be moved as a unit to scan an object to be examined. In a system with a single detector, such as that shown in Fig. 1, the scanning can be performed with an oscillating movement by rotating the unit about an axis through, for example, the source 3 of X-rays or the detector 9.

The location of the shaft depends on the application or use of the device, and possibly the shaft can also run through the object 7 in certain applications. You can also use a translation movement where the detector and the collimator are moved or the imaged object is moved. In a multi-line embodiment, where a number of detectors are stacked as will be explained below in connection with Fig. 7, the scan can be performed in different ways. In many cases it can be advantageous for the device for radiography to be fixed and the imaged object to be moved.

As already mentioned, the X-rays enter the detector in a direction which is preferably parallel to the cathode plate 17 and the anode plate 21. In this way, the detector can easily be made with an interaction path long enough for a major part of the incident X-ray photons to interact and be detected. . The length of the detector 9 and of the cathode plates 17 and 19 in a direction which is parallel is 10 75 20 25 30 35 518 8 0 2 'Åk-Ei.¿f =: f'§..§fII =' f =: f _ _ with the incident radiation will be discussed in more detail below.

When an X-ray photon hits and interacts with a noble gas atom, a hole is created in the K or L shell and a photoelectron with the kinetic energy E = hv - E fi æn is released. Where inside the chamber this interaction takes place is statistically controlled by the rate of absorption of the X-rays in the gas or gas mixture used. Fig. 2 is a graph showing the free mean path Ä of X-rays due to absorption in argon Ar, krypton Kr and xenon Xe, respectively, as a function of the X-ray energy hv at normal temperature and normal pressure (STP). In the diagram, the free mean path varies from 0 to 70 cm when the X-ray photon energy varies between 0 to 50 keV.

The detector of the present invention is particularly suitable when using broadband X-rays.

This releases photoelectrons with a large range of kinetic energy. Electrons with lower kinetic energy are called short-distance electrons, because such electrons have a relatively short free mean path, and electrons with higher kinetic energy are correspondingly called long-distance electrons.

Thus, a long-range electron is an electron with high (up to 100 keV), relatively long path (1 ~ 20 mm at l atm) kinetic energy that will travel before being braked and stopped (loses its kinetic energy) in the gas. Fig. 3 is a diagram showing the practical interaction interval Öe for electrons due to scattering in argon Ar, krypton Kr and xenon Xe, respectively, as a function of the electron energy Ee at normal temperature and normal pressure (STP). interaction length from 0 to 5 cm when the electron energy In the diagram the electrons vary from 0 to 50 keV. 10 15 20 25 30 35 ~ «. u nu n 518 802 _12_ I n. n | . u | | n. - ~ n n n nu When the hole in the K or L shell is filled with an electron from a higher level, an Auger electron and / or a fluorescent (X-ray) photons appear. An Auger electron is a short-distance electron.

A short-range electron is an electron with low kinetic energy (typically 1-5 keV), (typically 0.01-0.1 mm at 1 atm) its kinetic energy) in the gas or gas mixture. which travels relatively short distances before being stopped (loses During a single interaction between an X-ray photon and a gas atom such as Kr or Xe, several electrons will therefore be released simultaneously in many cases, both long-distance and short-distance electrons. Furthermore, fluorescent photons can be emitted during the interaction.

Both long-range and short-range electrons generate electron traces of secondary ionized electrons, which are generated by the electrons during the movement in the gas. When an electron has a fairly high kinetic energy (eg 20-30 keV), the number of released electrons (secondarily ionized electrons) is small per unit length of the orbit. For electrons with low kinetic energy (typically 1-4 keV), the number of released electrons (secondarily ionized electrons) is higher per unit length of the orbit.

As mentioned, the length of the groove of a long-distance electron is about 1-20 mm at atmospheric pressure. There is therefore a physical limit to the spatial resolution. The emitted fluorescent photons also create ionization as mentioned above.

This ionization takes place far away from the primary interaction with a typical attenuation length of 1.5-250 mm at atmospheric pressure. This ionization also degrades the spatial resolution and creates background noise.

By detecting all electrons, including those released by ionization of fluorescent photons, which currently occurs in all existing gas detectors, the spatial resolution is drastically degraded at photon energies above 10 keV. All electrons are detected because the electrode aperture is large.

Typically between a few hundred and a thousand secondarily ionized electron / ion pairs are generated from a single 20 keV X-ray photon in this process.

The secondary ionization electrons (together with the primary ionization electron) are driven towards the electron avalanche amplifier means 15 depending on the electric field in the conversion and drive volume 13. When the electrons enter areas with focused field lines of the electron avalanche amplifier means 15, they undergo avalanche amplification.

The motions of the avalanche electrons and ions induce electrical signals in the electron avalanche detector 29. The reading device 29 is preferably located in the vicinity of the electron avalanche anode 27, but the signals can be captured elsewhere, for example near the cathode plates 17 and 19 or at the avalanche cathode 25 or at a combination of two or more of these locations. The signals are further amplified and processed by reading circuits (not shown in Fig. 1) to obtain precision measurements of the X-ray photons.

The reading device 29 is arranged in such a way that the electron avalanches are detectable separately, which originate mainly from ionization in the respective chamber section (ie between the cathode plate 17 and the anode plate 21 eg at 35, and between the cathode plate 19 and the anode plate 21 eg at 37).

Thus, a detector 9 for energy-resolved detection of X-rays is obtained, in which at least from a statistical point of view lower energy X-rays are absorbed in the first section of the chamber 13, which has a short distance d1 between the electrode plates, and higher energy X-rays are absorbed in the second section 13. which has the distance dg between the electrode plates. 10 15 20 25 30 35 | v - u. -. 518 802 _ 14 _ The electrode spacing dl in the first chamber section is preferably selected in such a way that detection of low energy photons with high spatial resolution is possible.

Thus, d1 should preferably be less than the attenuation distance of the fluorescent (X-ray) photons. As a result, a large number of fluorescent photons do not cause ionization in the conversion and drive volumes.

A further improvement of the spatial resolution is achieved if the detector is provided with a first chamber section, which has the electrode gap d1 which is less than the length of the electron grooves (ie the interaction length) of This results in an even larger and a large number of long distance electrons will not undergo full long distance electrons. number of fluorescent photons, energy loss in the first chamber section.

The distance d1 can preferably be chosen so that a major part of the fluorescent photons and / or the long-distance electrons are discriminated. In other words, a major portion of the fluorescent photons will not cause ionization in the conversion and drive orifice and / or a major portion of the long-distance electrons will not be slowed to the energy level at which they generate a large number of secondary ionizing electrons per unit length .

A further improvement of the spatial resolution is obtained if the detector is provided with a first chamber section, which has an electrode gap d1, which is substantially a few times the length of the electron tracks (ie the interaction length) of short-distance electrons, eg 1-5 times the length of short-distance electron tracks.

As a result, a larger number of fluorescent photons and a larger number of long-distance electrons do not undergo complete energy loss in the conversion and drive volumes. Of course, the distance dl can be less than 70 75 20 25 30 35 -. - - o n o the length of the electron track of short-distance electrons, but then the efficiency decreases because even short-distance electrons will reach the electrode plates before the ionization is complete.

By means of such geometric discrimination of long-distance electrons and fluorescent photons, it is achieved that a larger number of the detected avalanches are caused by short-distance electrons than without such discrimination. This improves the spatial resolution because the long-range electrons and the fluorescent photons cause avalanches far from the interaction site of the incident X-ray. The more long-range electrons and fluorescent photons that are discriminated against, the greater the ratio between detected avalanches caused by short-range electrons and detected avalanches caused by long-range electrons and by fluorescent photons.

This achieves higher spatial resolution.

The length 11 of the first chamber section in the direction of the incident X-rays is such that a substantial part of the radiation with lower energy is absorbed in the first chamber section.

The electrode distance d2 in the second chamber section is preferably selected so that detection of higher energy photons is possible. Thus, d2 should preferably be substantially larger than d1 to allow primary long-range electrons to retard and gas molecules to be ionized, thereby releasing short-range electrons.

The efficiency of detecting higher energy photons is thus improved at the cost of deteriorating spatial resolution.

Preferably, however, the electrode distance d2 in the second chamber section is shorter than the fluorescent ones. This results in the (X-ray) photons. 10 15 20 25 30 35 518 802 _16_ large number of fluorescent photons do not cause ionization in the conversion and drive volume.

Further improvement of the spatial resolution is achieved if the detector is provided with a second chamber section, which has an electrode gap d2, which is substantially a few times the length of the long-distance electrons 'electron trace (ie interaction length), eg 1-5 times the length of the long-distance electrodes' trace.

The length 12 of the second chamber section in the direction of the incident X-rays is such that a substantial proportion of the radiation with higher energy is absorbed in the second chamber section.

Certainly, the electrode distances d1 and dg and the lengths l1 and l2 of the chamber sections are selected for each particular application and for each particular radiation spectrum used to achieve the desired detection efficiency and required spatial and energy resolution.

Electron discrimination can also be used to improve spatial resolution and energy resolution. As mentioned, short distance electrons generate a large number of secondary ionization electrons per unit length of the path, and high energy electrons generate a lower number of secondary ionization electrons per unit length of the path.

Avalanche-amplified pulses at the reading device are therefore high and / or wide for short-range electrons, while they are small for long-distance electrons in the first chamber section.

By providing the readout electronics with a threshold function with respect to any suitable, signal-derived parameter, such as amplitude or integrated value, the pulses from the long-distance electrons in the first chamber section can be discriminated electronically, thus further improving both spatial resolution and energy resolution. 70 15 20 25 30 35 518 802 íšilÉï-šfÄšïffl-IÅÉÉÉÉ? Similarly, in the second chamber section, the high energy electrons are allowed to be completely retarded, and consequently a greater number of electrons are released from such electrons than from short-range electrons, and therefore the detected avalanche-amplified pulses are higher and / or wider for long-distance electrons than for short-range electrons. Thus, since higher energy photons generate long distance electrons, the energy resolution in the second chamber section can be further improved by electronically discriminating pulses which have a signal-derived parameter, such as amplitude or integrated value, which is below a certain threshold value.

In yet another embodiment of the present invention, the first and second chamber sections are separated in a fluid-tight but radiation-permeable manner (not shown in Fig. 1), ionizable gas with preselected pressure. The chamber sections can and each chamber is individually filled with one in which there is one (similar to being separated by a fluid-tight wall, suitably radiation-transparent window window 33), through which window the X-ray beam 1 enters the other chamber section. By regulating the content of the gas mixture and the gas pressure in each chamber individually, further improved energy and spatial resolution can be achieved.

Referring to Fig. 4, a particular embodiment of the present invention will now be described in which it may be used to measure bone mineral density in humans. Fig. 4 is a diagram showing the absorption coefficient a of X-rays due to absorption in tissue or bone in a human body as a function of X-ray energy. In the diagram, the absorption coefficient varies from 1 to roughly 0.4 when the X-ray photon energy varies from 0 to 150 keV. The difference absorption coefficient (designated tissue bone) is also shown, which is an indication of 10 75 20 25 30 35 n. u n oo 518 802 _ 18 _ a n n. ; u | | n - - -. . ø ua the absorption selectivity between tissue and bone as a function of the photon energy.

Normally, the bone is surrounded by an unknown amount of tissue. A commonly used method for estimating the amount of bone is to detect emitted X-rays at two different X-ray energies. An exposure is made at X-ray energies where bone and tissue absorb X-rays very differently, typically at 40 keV where the difference absorption coefficient has a maximum, see Fig. 4. Another exposure is made at energies where bone and tissue absorb X-rays in a similar way, typically at energies above 100 keV , see fig 4. From these two measurements the amount of bone is estimated. However, this requires that the detector be able to detect both (around 40 keV) (> 100 keV), which is often a problem. In order to further achieve beams with narrow low energy X-rays and high energy X-rays energy distribution at low energy, the incoming X-ray stream, which has a wide spectrum, needs to be heavily filtered, which places a large load on the X-ray tube. The two separate exposures, taken at different times, also give rise to problems caused by the object moving between the two exposures.

The present invention describes a detector and a method that can measure X-rays over a wide energy range.

Low energy X-rays are preferably detected in the first narrow chamber section of the detector due to their short absorption distance. High energy X-rays mainly pass through the first chamber section of the detector and are detected in the second thicker chamber section.

The invention allows the same detector to be used to efficiently detect X-rays with sufficiently different energies in the same detector using the commonly used double exposure method. 10 15 20 25 30 35 I a 1 »u- 518 802 u | n ø n n I Q a ~; . o now _.l9_ Alternatively, the two measurements can be made simultaneously using a wide X-ray spectrum of the incident X-rays. This second option requires less filtering of the X-rays and loads the X-ray tube less. By simultaneously detecting the two energy levels, the problems with movements between the exposures are further reduced.

Fig. 5 shows a schematic, partially enlarged cross-sectional view along A-A in Fig. 1 and shows in particular the avalanche amplifier means in more detail. Of course, however, the present invention is not limited to such a construction. It should also be noted that the avalanche amplifier means may be a solid state type device or comprise a liquid gain region. The first electrode device 17 comprises a dielectric substrate 41 and a conductive layer 43 which is a cathode electrode, device such as the avalanche cathode 25. The avalanche anode 27 comprises a dielectric substrate 45 and 47. and a second electrode device 21 is the same A dielectric 49 is arranged between the avalanche cathode 25 and the avalanche anode 27. This may be a gas or a solid substrate 49, shown in the figure. A first voltage is applied between 25 by means of one carrying the cathode 25 and the anode 27 such as the device 17 and the device 21, direct voltage source (not shown in Fig. 5), and a second voltage is applied between the cathode 25 and the anode 27, so as to obtain a electric field, which is very strong in gas-filled avalanche amplifier areas 53. The avalanche areas 53 are formed in an area between and around the edges of 25, 25 and the avalanche anode 27, the avalanche cathode facing each other, and between the avalanche cathode where a concentrated electric field occurs due to the applied voltages. 10 75 20 25 30 35 v u u c oo 518 soz ajg; = n a n u ø ø n v. The applied voltages are selected so that a weaker electric field, the drive field, the section of the chamber 13 is created over the first electrons (primary and secondary electrons), the avalanche amplifier means. which are released by interaction eg at will drive depending on the drive field towards They will enter the very strong avalanche amplifier field and accelerate. The accelerated electrons will interact with other gas atoms in one of the regions 53 and cause additional electron / ion pairs to be generated. This process continues during the movement of the electrons in the avalanche region towards the anode 27, and an electron avalanche is formed.

The avalanche areas 53 consist of openings or channels in the cathode 25 and the dielectric substrate 49, if any. The openings or channels may be of any shape, for example circular or square seen from above or be contiguous and extend longitudinally between two edges of the substrate 49, if any, and the cathode 25.

In case the openings or channels are circular seen from above, they are arranged in rows, each row of openings or channels comprising a plurality of circular openings or channels. A plurality of longitudinal openings or channels or rows of channels are located next to each other and are parallel to each other or to the incident X-rays. Alternatively, the openings or channels can be arranged in other patterns.

Conductive layer strips 47 also form the reading elements of the reading device 29. The strips are arranged adjacent to the openings or channels, which constitute the avalanche areas 53. or channel or row of openings or channels.

Preferably there is a strip for each opening. The strips may be divided into sections along their length, a section may be provided for each opening or channel or for a plurality of openings or channels, in the form of cushions as described in more detail below. The strips or sections, if any, are electrically insulated from each other. Each detector electrode element, i.e. strip or section, is preferably separately connected to processing electronics (not shown in Fig. 5).

By arranging a plurality of read strips 47 as shown in Fig. 5, a detector is obtained in which electron avalanches, which originate mainly from ionization by means of separated portions of the planar radiation beam, can be detected separately. In this way, one-dimensional imaging can be performed using the detector 9.

Alternatively, the readout elements may be located on the back of the substrate. In that case, it is necessary that the anode electrodes 4, 19 be semi-transparent for induced pulses. In connection with Fig. 6, various arrangements of reading elements are shown below.

For example, the longitudinal channels may have a width in the range 0.01-1 mm, the channels may have a width in the range 0.01-1 mm and the thickness of the dielectric 49 (separation between the avalanche cathode 25 and the anode 27) is in the range 0.01-1 mm. l mm.

Alternatively, the conductive layers 43 and 47 may be replaced by a resistive support of, for example, silicon, conductive glass or diamond, and the dielectric layers 41 and 45 may be replaced by conductive layers.

A cross-sectional view parallel to A-A in Fig. 1 but in the second section of the chamber 13 would be identical to that shown in Fig. 5, but the gap forming the chamber 13 would be considerably larger. By arranging separate reading strips under the respective sections of the chamber 13, a detector 9 is obtained in which electron avalanches, which mainly originate from ionization by means of X-rays with different energies, can be detected separately. In the above-described embodiment, special locations of the detector anode and cathode devices have been described. However, there are a number of other locations that are equally useful in connection with the present invention.

Fig. 6 shows a possible design of an avalanche anode device 27 (and reading device 29). The device consists of conductive strips 47 and 63.

A plurality of strips 47 are placed next to each other, and a plurality of strips 63 are placed next to each other, the strips 47, 63 in each position extending in directions parallel to the incident X-ray photons 65, which originate from the source 3. Preferably, the strips 47 are arranged below the first chamber section and the strips 63 arranged below the second chamber section. The strips are applied to a substrate and are electrically insulated from each other in that there is a space between them (not shown in Fig. 6). The strips can be formed by photolithographic methods or electroforming.

The spacing and width of the strips 47 are adapted to the detector in question in order to achieve the desired (optimal) spatial resolution. Thus, the strips 63 are wider than the strips 47 because the achievable spatial resolution is still inferior to high energy X-rays.

The strips 47 should be placed below the openings or channels or rows of openings or channels, as shown in Fig. 5. Similarly, the strips 63 should be placed under the corresponding openings or channels or rows of openings or channels located in the avalanche amplifier means below the second chamber section.

Each of the strips 47, 63 is connected to the processing electronics (not shown in Fig. 6) by means of a separate signal conductor, the signals from each strip being preferably processed separately. If the anode or cathode strips constitute the detector strips, the signal conductors also connect the respective strip to the high-voltage direct voltage source with suitable connections for separation.

As can be seen from Fig. 6, the strips 47, 63 are aligned with the X-ray source 3, which compensates for parallax errors in the detected images.

In case the reading device 29 is a separate device, the anode electrode 27 can be designed as a simple electrode without strips and gaps.

An alternative embodiment of the reading device (not shown) has the strips 47 and / or 63 further divided into segments in the direction of the incident X-rays, the segments being electrically isolated from each other.

Preferably, there is a small gap extending perpendicular to the incident X-rays between each segment of the respective strip. Each segment is connected to the processing electronics by means of a separate signal conductor, the signals from each segment preferably being processed separately.

This reading device can be used when additional energy-resolved detection of the X-rays is required.

Using statistical methods, one can restore the spectrum of the incident photons with good resolution. See, e.g., E. L. 208 (1983) 637 and G.

Instr and methods, 217 (1983) 56.

Instr and methods, Nucl.

Kosarev et al., Nucl.

F. Karabadjak et al., For all embodiments, it generally applies that each incident X-ray photon causes an induced pulse in one (or more) processed in the processing electronics, detector electrode elements. The pulses that possibly form the pulses and integrate or count the pulses from each (pad or set of pads) strip SOm representing a pixel.

The pulses can also be processed to give an energy measure for each pixel.

Where the detector electrode is on the cathode side, the area of an induced signal is wider (in a direction perpendicular to the direction of incidence of the X-ray photons) than on the anode side.

Therefore, weighing of the signals in the processing electronics is preferable.

Fig. 7 schematically shows a device 91 according to an embodiment of the present invention, which has a plurality of detectors 9 according to the invention, which are stacked next to each other. By means of this embodiment, multi-line scanning can be achieved, which reduces the total scanning distance as well as the scanning time. The device according to this embodiment comprises a source 3 for X-rays, which together with a number of collimator windows 5 generates a number of flat, fan-shaped X-rays 1 for irradiating the rays, object 7 to be imaged. which passes through the object 7 optionally enters the individual, stacked detectors 9 through a number of other collimator windows 11, which are aligned with the X-rays. The first collimator windows 5 are arranged in a first rigid structure (not shown in Fig. 7), and the optional second collimator windows 11 are arranged in a second rigid structure (not shown in Fig. 7), which is attached to the detectors 9 or arranged separately on the detectors.

The X-ray source 3, the first rigid structure and the possible second rigid structure comprising the collimator windows 11 and the stacked detectors 9, which are fixed to each other, are interconnected and mutually fixed by means of an unspecified means such as a frame. The radiography device thus formed can be moved as a unit for or base (not shown in Fig. 7). scanning of an object to be examined. In this multi-line design, the scan can be performed in a transverse motion perpendicular to the X-ray beam as mentioned above. It can also be advantageous if the device for radiography is fixed and the imaged object is moved. Alternatively, the collimator and detector can be moved synchronously with a jointly controlled motor.

Another advantage of using a stacked configuration over large single volume gas detectors is that the background noise is reduced due to X-ray photons scattered in the object 7. These scattered X-ray photons, which travel in directions not parallel to the incident X-ray, can cause "false" signals. or avalanches in one of the other detectors 9 in the stack, if they pass through the anode or cathode plates and enter such a chamber. This reduction is achieved by significant absorption of (scattered) X-ray photons in the material in the anode and cathode plates or in the collimator 11.

This background noise can be further reduced by placing thin absorption plates (not shown) between the stacked detectors. These absorbent plates or discs can be made of a material with a high atomic number, such as tungsten.

For all embodiments, it generally applies that the gas volumes are very thin, which results in ions being removed quickly, which leads to little or no accumulation of space charges. This enables high speed operation.

For all embodiments it also generally applies that small which results in advantageous distance leads to low operating voltages, in low energy in any sparks, which for the electronics.

The focus of the field lines in the embodiments is also favorable for suppressing flow formation. This leads to a reduced risk of sparks. 70 75 20 518 802 _ 26 _ As an alternative to all embodiments, the electric field in the conversion and drive gap (volume) can be kept high enough to cause electron avalanches, which are consequently used in a pre-amplification mode.

Furthermore, all electrode surfaces can be covered with a resistive material to reduce the risk of sparking, which affects the measurement and can destroy the detector's electronic equipment.

Of course, the invention can be varied in many different ways.

For example, the voltages can be applied in other ways as long as the described electric fields are created.

Such variations are not to be construed as being obvious to one skilled in the art, are intended to be included within the scope of the invention. All such modifications, as part of the scope of protection of the claims.

Claims (25)

10 15 20 25 30 35 518 802 ZZ Patent claim A detector for detecting ionizing radiation comprising: - a chamber (13), which is filled with ionizable gas, and which (17, 19) electrode device, which are substantially mutually comprised, a first and a second (21) parallel, and between which a first voltage (U1, U fl can be applied; a radiation input (33) (1) and substantially parallel to the first and the second which are arranged in such a way that radiation can enter the chamber between the electrode device for ionizing the ionizable gas comprising ((27), between which a second voltage (Ua) can be applied, and (29), the first voltage can be applied to drive below (15), (25) and an avalanche anode device an electron avalanche amplifier means an avinc cathode a readout device wherein ionization formed electrons in the direction of the electron avalanche amplifier means, the second voltage can be applied for avalanche amplification of said electrons, and the reading device is arranged to detect electron avalanches. and / or ions generated by said avalanche amplification, characterized in that the chamber is arranged in such a way that radiation entering through the radiation input first enters a first chamber section having a first distance (d fl between the first and the second the electrode device, and then enters a second chamber section having a second distance (dz) between the first and the second electrode device, the first (dl) and the second (d fl the reading device being arranged in such a way, the distance being substantially different, and that electron avalanches and / or ions generated by said avalanche amplification, which originate mainly from ionization in the respective chamber section, are separately detectable. 10 15 20 25 30 35 518 802 28 A detector according to claim 1, wherein the first distance (dl) is shorter than the attenuation length of fluorescent photons emitted in the ionizable gas after ionization. A detector according to claim 1 or 2, wherein the first distance (dl) is shorter than the second distance (dz). A detector according to any one of the preceding claims, wherein the second distance (dz) is shorter than the attenuation length of fluorescent photons emitted in the ionizable gas after ionization. A detector according to any one of the preceding claims, wherein the radiation input (33) is arranged in such a way that radiation having a broadband energy spectrum can enter the chamber, wherein short-distance electrons are released by ionization by means of lower energy radiation, and long-distance electrons are released by ionization. by means of radiation with higher energy, and wherein the first distance (dl) is shorter than the interaction length of the long-distance electrons. A detector according to claim 5, wherein the first distance (dl) is shorter than a few times the interaction length of the short-distance electrons. A detector according to claim 5 or 6, wherein the second distance (dz) is shorter than a few times the interaction length of the long-distance electrons. A detector according to any one of claims 5-7, wherein the first and the second chamber sections are separated in a fluid-tight but radiation-permeable manner, and each chamber is individually filled with an ionizable gas of a predetermined pressure. 10 15 20 25 30 35 518 802 29 A detector according to any one of claims 5-8, wherein the ionizable gas and the length (11, 12) of the respective chamber sections in the direction of the incident X-rays are arranged in such a way that a substantial proportion of the lower energy radiation is absorbed in the first chamber section. and a substantial portion of the higher energy radiation is absorbed in the second chamber section, the reading element device (29) being arranged for energy-resolved detection of the radiation. A detector according to any one of claims 1-9, wherein the first and second electrode devices are planar. A detector according to any one of the preceding claims, wherein the radiation input (33) is arranged so that a plane and a plurality of parallel radiation beams (1) can enter the chamber, the reading device (29) is provided with a reading strips (47, 63), which are located together and extend in directions with the directions of said planar beam at each location, so that electron avalanches and / or ions generated at said avalanche amplification, which result mainly from ionization by laterally spaced portions of said plane beam, are separately detectable. A detector according to any one of the preceding claims, further comprising a signal processing means which is arranged to discriminate a signal originating from detected electron avalanches and / or ions generated by said avalanche amplification, depending on a signal-derived parameter. A detector according to any one of the preceding claims, wherein the electron avalanche amplifier means (15) comprises a plurality of avalanche areas (53). A detector according to claim 13, wherein the avalanche electrode device (25) is provided with openings. 70 75 20 25 30 35 518 802 A detector according to claim 13 or 14, wherein the avalanche anode device (27) is in the form of a plurality of strips (47, 63) or cushions. A detector according to any one of the preceding claims, wherein the second electrode device (21) and the avalanche cathode device (25) consist of a single device. A detector according to any one of the preceding claims, wherein the avalanche anode device (27) and the reading device (29) consist of a single device. Device intended for use in planar radiography, characterized in that it comprises a source (3) for X-rays, means (5) for forming a substantially planar X-ray (1), which is located between (7) to be imaged, and a detector (9) according to any one of the preceding claims, wherein the X-ray source and an object are positioned and arranged to detect the planar X-ray beam as it passes through or is reflected by the object. Device according to claim 18, wherein the X-ray source (3), the means (5) for forming a substantially planar X-ray beam (1) and the detector (9) are mutually fixed. Device according to claim 18 or 19, comprising a second and a further detector (9) according to any one of claims 1-17, which detectors are stacked to form a detector unit, and means (5) for forming a substantially flat X-ray beam. (1) for each detector, which means are located between the X-ray source (3) and said object (7), each detector being placed and arranged for detecting respective planar X-ray beams passing through or reflected by the object. 10 75 20 25 30 35 518 802 ä Device according to claim 20, comprising absorbing plates which are arranged between the detectors (9) for absorbing scattered X-ray photons. A method of detecting ionizing radiation in a detector, comprising a chamber (13) filled with ionizable gas and comprising a first (17, 19) and a second (21) electrode device which are substantially parallel to each other, a radiation input ( 33), an electron avalanche amplifier means (15) comprising an avalanche cathode device (25) and an avalanche anode device (27), and a reading device (29), the method comprising: - a radiation beam (1) is introduced into the chamber between and substantially parallel to the first and the second electrode device for ionizing the ionizable gas; a first voltage (Uh th) is applied between the first (17, 19) and the second (21) electrode device to drive electrons generated during ionization in the direction of the electron avalanche amplifier means; a second voltage (Ug) is applied between the avalanche cathode and the avalanche anode device (25, 27) for avalanche amplification of the electrons; and - electron avalanches and / or ions generated by said avalanche amplification are detected by means of the reading device, characterized in that, - the radiation beam (1) is first introduced into a first section (d fl first and the second electrode device, and then carried by the chamber, which has a first distance between the beam of radiation into a second chamber section having a second distance (dz) between the first and the second electrode device, the first (dl) and the second (dz) distance being substantially different, and - electron avalanches and / or ions generated at said avalanche amplification, which mainly originates from ionization in the respective chamber section, is detected separately by means of the reading device. The method of claim 22, further comprising the step of electrically discriminating electrons with a preselected kinetic energy in the chamber (13) by selecting the first (dl) and second (dz) distances depending on said preselected kinetic energy. A method according to claim 22 or 23, further comprising the step of electron avalanches and / or ions generated by said avalanche amplification, arising mainly from radiation with a preselected energy, being discriminated electronically by separating pulses having a signal-derived parameter, in particular amplitude or integrated value, corresponding to said default energy. A method according to any one of claims 22-24, further comprising introducing the beam of radiation into the chamber in the form of a planar beam of radiation (1), and generating electron avalanches and / or ions at said avalanche amplification, which are mainly derived from lateral ionization. separated portions of said planar radiation beam are detected separately by means of the reading device (29).