X-ray detector

Abstract

X-ray detector (1) having at least - a photosensitive material (5), - a holder (2) for the photosensitive material (5), wherein the holder (2) is formed from a non-conductor and has a first opening (4) and wherein the photosensitive material (5) is arranged at least partially overlapping with the first opening (4), - a shield (6, 7, 8) against magnetic fields, which encloses the holder (2), wherein the shield (6, 7, 8) has opposing second and third openings (9, 9', 9", 10, 10', 10") which can be arranged congruently with the first opening (4) and wherein - the diameter of the second and third openings (9, 9', 9", 10, 10', 10") ∅ Ö in relation to the diameter of the shielding (6) ∅ A , ∅ Ö : ∅ A, in the direction spanned by the second and third openings (9, 9', 9", 10, 10', 10") ≤ 0.15 and wherein the diameter of all openings (4, 9, 9', 9", 10, 10', 10", 12) is larger than that of a transmitted X-ray beam and wherein a shielding attenuation S is a function of the ratio ∅ Ö : ∅ A according to S ( OO ¨ : OA ) with S(∅ Ö : ∅ A ) x = Shielding attenuation in the middle of the shielding (6) in the direction of an incident X-ray beam (x-direction) and ES0 = normalized shielding attenuation of the shield without openings in the x-direction, not less than 10% of ES0.

Description

Technical field

[0001] The present invention relates to an X-ray detector such as is used in X-ray spectrometers and with which the intensity of an X-ray beam in the energy range from vacuum ultraviolet radiation (-10 eV - 2000 eV) up to the upper range of hard X-ray radiation (~ 120 keV) can be determined. State of the art

[0002] X-ray detectors in the energy range from vacuum ultraviolet radiation (-10 eV - 2000 eV) to the upper range of hard X-ray radiation (~120 keV), hereinafter referred to collectively as X-rays, are known from the prior art. Photodiodes, microchannel plates, CCD sensors (CCD: Charge Coupled Device), and gas absorption chambers are particularly noteworthy examples.

[0003] For many spectroscopic methods, it is necessary to determine the intensity of the X-ray radiation before it strikes a sample, and especially in its immediate vicinity, in order to obtain the most precise knowledge possible of the intensity at the sample location. Furthermore, for some methods, such as X-ray Raman scattering, X-ray dichroism spectroscopy (circular and linear), and generally in scattering experiments as well as in hysteresis in magnetization, it is necessary to determine the intensity of the X-ray beam incident on the sample by the detector simultaneously with the irradiation of the sample by the same X-ray beam. Detectors that exhibit relatively high absorption of the X-ray radiation—for example, due to their opacity—are therefore unsuitable, as they disadvantageously reduce the intensity of the X-ray beam at the sample location.

[0004] In JP H07-318 657 A and in the [reference missing], examples of X-ray detectors are given that detect an X-ray beam by detecting secondary electrons generated by the X-ray beam striking a diamond film or a photosensitive membrane. In JP H07-318 657 A, the X-ray beam passes through the membrane and thus through the X-ray detector.

[0005] US Patent 7,394,083 B2 describes a light source for EUV radiation that also includes a detector for the EUV radiation. The EUV radiation is passed through a medium for photoelectron generation, in this case a gas, and the photoelectrons are detected by a corresponding detector. Enclosing the photoelectron generation point to shield against stray magnetic fields, such as the Earth's magnetic field, is proposed, but only a possible material (micro-metal) is specified in detail.

[0006] Determining the intensity of an X-ray beam incident on a sample is difficult or even impossible if the sample is to be examined in situ under the influence of a magnetic field. The magnetic fields interfere with the detection of the X-rays when they are associated with photoelectric or ionizing effects. The resulting secondary electrons are deflected by the Lorentz force when passing through a magnetic field. Task

[0007] The object of the present invention is to provide an X-ray detector that enables interference-free detection of X-rays due to the formation of secondary electrons in the area of ​​magnetic fields and also allows the passage of the X-ray beam through the detector in order to irradiate a sample located behind it in the beam path with the same X-ray beam.

[0008] The problem is solved by the features of claim one. Advantageous embodiments are the subject of the dependent claims.

[0009] Surprisingly, the Helmholtz Centre Berlin discovered that, contrary to the common misconception that gaps in shielding have a significant negative impact on shielding efficiency, sufficient magnetic shielding can be achieved even with partial gaps. For reference regarding the negative influence of gaps in shielding on its efficiency, see the widely cited review article by A.J. Mager (Magnetic Shields, IEEE Transactions on Magnetics, Vol. 6, 1970, pp. 67-75).

[0010] The X-ray detector according to the invention comprises at least one photosensitive material. Photosensitive materials are defined here as all materials that generate secondary electrons upon exposure to electromagnetic radiation. This includes solid materials, such as the metals gold or tantalum, gases, such as the noble gases argon and xenon, and liquid materials. The secondary electrons are generated by the external photoelectric effect, in the case of gases and liquids by ionization. The photosensitive material may need to be adapted to the energy of the X-ray beam to be detected.

[0011] The amount of photosensitive material introduced into the X-ray beam for detection is optimized to minimize absorption while simultaneously ensuring a sufficient yield of secondary electrons. This optimization is always dependent on the material and the energy to be detected in the X-ray beam. In the case of solid materials, this usually results in the photosensitive material being in the form of films, plates, or grids. The absorption properties of the photosensitive materials can be determined by those skilled in the art using known methods.

[0012] The photosensitive material is interconnected to dissipate secondary electrons. To minimize noise, the interconnection is ideally done without grounding.

[0013] The X-ray detector also has at least one holder for the photosensitive material. The holder is made of a non-conductor, advantageously of a material with a resistance > 5 GΩ. The holder has a first opening whose cross-section or diameter is at least larger than the X-ray beam that must pass through the detector in its intended use, thus preventing absorption of the X-ray beam by the holder. The same applies to all other openings provided for the passage of the X-ray beam. The photosensitive material is arranged to overlap the opening at least partially. This is done to achieve optimal interaction (minimized absorption with optimized generation of secondary electrons, see above) between the photosensitive material and the X-ray beam, and ideally, the material completely covers the opening.The holder is shaped at the opening to accommodate the photosensitive material. For gaseous or liquid materials, appropriate containers with suitable interconnections must be used. Due to the expected increased absorption by the containers, the use of liquid or gaseous materials is ideal for applications at high energies in the X-ray beam to be detected (> 2 keV).

[0014] Ideally, the holder has a groove for guiding wiring connections.

[0015] The X-ray detector according to the invention further comprises a shield against magnetic fields, which encloses the holder. The shield has two opposing second and third openings, which can be arranged congruently with the first opening. If the shield is formed from several layers, this means a sequence of second and third openings that are opposite each other. The diameter of the first and second openings is determined by the outer diameter of the shield, which is also given by the distance between the second and third openings, which is also referred to here as the x-direction. The ratio between the diameter of the openings, ∅ Ö , and the outer diameter of the shielding, ∅ A , ∅ Ö : ∅ AThe value should be less than or exactly 0.15 (≤ 0.15). This results in a reduction of the shielding attenuation of a maximum of one order of magnitude in the direction of the magnetic field (xy), or approximately 10%. This value is acceptable for most experiments / investigations, such as X-ray Raman scattering, X-ray dichroism spectroscopy (circular and linear), and generally for scattering experiments, as well as for hysteresis in magnetization. In addition to experimental validation, this value is also supported by simulations. The simulations were performed using the software COMSOL Multiphysics. ®4.2 was carried out at the Helmholtz Centre Berlin for Materials and Energy GmbH. Besides the attenuation of the shielding attenuation, which still needs to be accounted for, the size of the second and third openings is also determined by the diameter of an X-ray beam, where the diameter of the openings is larger than that of the X-ray beam. In the application, the outer diameter of the shielding and the diameter of the openings must be adapted to the minimum achievable value of the shielding attenuation (which is also determined by the materials used, see below), which can be approximated by the relationship: S(OO¨:OA)x=ES0⋅{1,(OO¨:OA)≤0.1(12⋅(OO¨:OA))−5,(OO¨:OA)>0.1 with S(∅ Ö : ∅ A ) x= Shielding attenuation in the center of the shield in the direction of an incident X-ray beam (x-direction) as a function of the ratio of the diameters of the openings to the diameter of the shield and ES0 = normalized shielding attenuation of the shield without openings in the x-direction, to be determined. Ideally, the shielding and the openings should have a round cross-section, as this ensures the greatest possible shielding.

[0016] All materials suitable for shielding against magnetic fields are appropriate. The specific design of the shield, including its dimensions and thickness, as well as the optional use of multiple materials in layers, as in one embodiment, and the thickness of these layers, are determined by considering the required shielding attenuation with respect to the magnetic fields to be shielded. Classic materials for shielding magnetic fields include mu-metal (µ-metal, a nickel-iron alloy), pure iron, and superconductors at the appropriate temperatures. In this embodiment, the shield consists of three layers: two inner layers of mu-metal and one outer layer of pure iron.

[0017] The geometric shape of the shielding is also designed with an eye toward optimizing shielding attenuation. Spherical or cylindrical shapes are advantageous in terms of achieving high shielding attenuation. For reasons of simple manufacturing, a cylinder is advantageously used, as is the case in one embodiment. The longitudinal axis of the cylinder is perpendicular to the path of the X-ray beam to be detected, with the path of the beam being predetermined by the openings in the holder and the shielding, and oriented perpendicular to the field direction. Advantageously, the ends of the cylinder are also sealed with materials suitable for shielding magnetic fields in order to maximize shielding attenuation in all directions. Advantageously, one side of the cylinder is shaped to allow the X-ray detector to be mounted on a holder or other support.

[0018] In a further embodiment, an electrode for extracting electrons from the photoelectric material is arranged on the holder at the location of the photoelectric material, at a distance from it. The electrode has a fourth opening, which is congruent with the first opening and whose diameter is at least the same as the second and third openings. The electrode enables improved extraction of generated photoelectrons.

[0019] The advantage of the X-ray detector according to the invention lies in the provision of a magnetically shielded detector, whereby an X-ray beam can be detected which can simultaneously be used to irradiate a sample under the influence of a magnetic field. The X-ray detector is simple to manufacture. Example of implementation

[0020] The invention will be explained in more detail using an exemplary embodiment and with reference to figures.

[0021] The figures show: Fig. 1: Schematic drawing of an X-ray detector according to the invention in cross-section, not to scale. Fig. 2: Plotting the intensity of an X-ray beam measured with an X-ray detector without shielding against the strength of an acting magnetic field (■) and plotting the intensity of an X-ray beam measured with an X-ray detector according to the invention with shielding against the strength of an acting magnetic field (▲). Fig. 3: Magnetic fields present inside the detector along the cylinder axis of an X-ray detector according to the invention (simulation). Fig. 4: Dependence of the effective shielding attenuation S, in the direction of the three cylinder axes of the X-ray detector according to the invention, on the ratio of the diameter of the openings in the shielding to the diameter of the shielding (∅ Ö : ∅ A ) (Simulation).

[0022] The Fig. Figure 1 shows a cross-section through an X-ray detector 1 according to the invention, not to scale. The X-ray detector 1 has a holder 2 with a groove (not shown) and a first opening 4. A mesh made of an iron / nickel alloy with a wire diameter of 0.1 mm, coated with a 150 nm thick tantalum layer 5, is stretched over the first opening 4. The tantalum coating shields the iron / nickel alloy and is non-magnetic. The iron / nickel alloy mesh provides the necessary stiffness and covers approximately 20% of the opening area. The opening 4 has a diameter of 6 mm. The tantalum-coated mesh 5 is contacted for the discharge of secondary electrons; the discharge cables (not shown) run in the groove (not shown) of the holder 2. The holder 2 is surrounded by a shield.The shield is cylindrical and consists of an outer layer of pure iron 6 and two inner layers of mu-metal 7 and 8. The layer thicknesses are 2 mm for the pure iron, and 0.8 mm (7) and 1.5 mm (8) for the mu-metal layers. The diameters are 34 mm for the pure iron, 27 mm for the middle layer of mu-metal 7, and 22 mm for the inner layer of mu-metal 8. The shield has two opposing sequences of second and third openings 9, 9', 9", 10, 10', 10", which are arranged congruently with the first opening 4. The path defined by the openings 4, 9, and 10 specifies the x-direction for a measured X-ray beam 11. The diameter of the second and third openings 9, 9', 9", 10, 10', and 10" is 5 mm. The ratio of the diameter of the second and third openings to the diameter of the shielding (diameter of the outer layer) is therefore 0.15. The shielding attenuation S(∅. Ö / ∅ A )x The value is 10,000, which is 10% of the shielding attenuation ES0 without openings in the x-direction. An electrode 11, for discharging electrons from the photoelectric material, with an opening 12, which is also arranged congruently to the first opening 4, is located above the mesh 5.

[0023] In the Fig. Figure 2 shows the intensity of an X-ray beam measured with an unshielded X-ray detector plotted against the strength of an applied magnetic field (■) (normalized to 1). The strong, non-linear dependence, which also changes during operation (indicated by the arrows for moving up → and down ← the magnetic field strengths), is clearly visible. Normalization of the measurement data is not possible in this case. In comparison, Figure 2 shows the intensity of an X-ray beam measured with a shielded X-ray detector according to the invention plotted against the strength of an applied magnetic field (▲) (normalized to 1). No dependence of the measured intensity of the X-ray beam on the magnetic field is apparent. Normalization is therefore readily possible.

[0024] Magnetic fields detected along the cylinder axis of an X-ray detector according to the invention are located inside the detector. Fig. Figure 3 shows a projection across the cylinder axis, z, with values ​​for the y-axis (-) and the x-axis (···), which is equal to the x-direction, and also for the z-axis itself (---). The effects at the location of the openings at z = 0 for the values ​​in the x-direction can be seen. The shielding area of ​​the X-ray detector extends from approximately z = -200 mm to z = 100 mm.

[0025] The dependence of the effective shielding attenuation S, in the direction of the three cylinder axes (x = ■, y = ●, z = ▲) of the X-ray detector according to the invention, on the ratio of the diameter of the openings in the shielding to the diameter of the shielding (∅ Ö : ∅ A ) is in the Fig. Figure 4 shows the dependency. The dependency can be approximately described by the function (I) (see above).

[0026] With a ratio of 0.15 for the beam aperture to the shield diameter, the shielding is reduced to approximately 10% of the original shielding effectiveness, which is considered acceptable and can therefore be defined as a limit value. Other limit values ​​are possible depending on the application and shielding requirements.

[0027] The X-ray detector according to the invention ensures interference-free detection of X-rays due to the formation of secondary electrons in the area of ​​magnetic fields and also the passage of the X-ray beam through the detector in order to irradiate a sample located behind it in the beam path with the same X-ray beam.

Claims

[1] X-ray detector (1) comprising at least - a photosensitive material (5), - a holder (2) for the photosensitive material (5), wherein the holder (2) is formed from a non-conductor and has a first opening (4) and wherein the photosensitive material (5) is arranged at least partially overlapping with the first opening (4), - a shield (6, 7, 8) against magnetic fields, which encloses the holder (2), wherein the shield (6, 7, 8) has opposing second and third openings (9, 9', 9", 10, 10', 10") which can be arranged congruently with the first opening (4) and wherein - the diameter of the second and third openings (9, 9', 9", 10, 10', 10") ∅ Ö in relation to the diameter of the shielding (6) ∅ A , ∅ Ö : ∅ A, in the direction spanned by the second and third openings (9, 9', 9", 10, 10', 10") ≤ 0.15 and wherein the diameter of all openings (4, 9, 9', 9", 10, 10', 10", 12) is larger than that of a transmitted X-ray beam and wherein a shielding attenuation S is a function of the ratio ∅ Ö : ∅ A according to S(OO¨:OA)x=ES0⋅{1,(OO¨:OA)≤0.1(12⋅(OO¨:OA))−5,(OO¨:OA)>0.1 with S(∅ Ö : ∅ A ) x = Shielding attenuation in the middle of the shielding (6) in the direction of an incident X-ray beam (x-direction) and ES0 = normalized shielding attenuation of the shield without openings in the x-direction, not less than 10% of ES0. [2] X-ray detector (1) according to claim 1, characterized by , that the photosensitive material (5) is in the form of a network. [3] X-ray detector (1) according to claim 1, characterized by, that the photosensitive material (5) is in the form of a membrane. [4] X-ray detector (1) according to claim 2, characterized by , that the photosensitive material (5) is tantalum. [5] X-ray detector (1) according to any one of the preceding claims, characterized by , that the shielding (6, 7, 8) is cylindrical. [6] X-ray detector (1) according to any one of the preceding claims, characterized by , that the shielding is formed from three layers (6, 7, 8), wherein the inner two layers (7, 8) are made of mu-metal and the outer layer (6) is made of pure iron. [7] X-ray detector (1) according to any one of the preceding claims, characterized by, that an electrode (11) for drawing electrons from the photoelectric material (5) is arranged on the holder (2), at the location of the photoelectric material (5), at a distance from the same, and wherein the electrode (11) has a fourth opening (12) which is arranged congruently with the first opening (4) and whose diameter is at least the same as the second and third openings (9, 9', 9", 10, 10', 10").

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