[0002] The invention concerns an X-ray or neutron-optical system with an X-ray or neutron source from which associated radiation is guided as a primary beam to a sample under investigation, and with an X-ray or neutron detector for receiving radiation diffracted or scattered from the sample, wherein the source, the sample and the detector are substantially disposed on one line (=z-axis), and wherein a beam stop is provided between the sample and the detector, whose cross-sectional shape is adjusted, perpendicularly to the z-direction, to the cross-section of the primary beam.
[0003] An X-ray optical system of this type is disclosed e.g. in the company document “HR-PHK for NanoSTAR” Instruction Handbook, Anton Paar GmbH, Kärntner Str. 322, A 8054 graz (Austria), 1998, in particular, on page 16.
[0004] X-ray and neutron-optical methods are used to investigate the properties, i.e. material properties, of samples. Towards this end, a focussed X-ray or neutron beam is directed onto the sample where it interacts with the sample in a plurality of ways, in particular through scattering and/or diffraction. The X-ray or neutron radiation after the interaction process is registered by a detector and subsequently evaluated to obtain information about the properties of the sample.
[0005] In many of these methods, only a small part of the X-ray or neutron radiation is deflected in direction; the major portion of the radiation passes the sample without deflection. The non-deflected part of the radiation is called the primary beam, both in front of as well as behind the sample. Detectors for registering diffracted or scattered radiation must usually be protected from direct influence of the primary beam to prevent irreversible damage to the detector. Towards this end, so-called beam stops are used which partially shield the detector to prevent impingement of primary radiation. A beam stop can also shield disturbing divergent parasitic radiation (e.g. through Fresnel diffraction on collimator edges).
[0006] A conventional beam stop is described in the company document of Anton Paar GmbH loc. cit. The beam stop consists essentially of a gold plate which is fixed in a steel ring using nylon threads. The position of the gold plate in the annular plane (xy plane) can be adjusted with two micrometer screws. The steel ring is flanged to the detector.
[0007] The shape of the primary beam, in particular its diameter, depends on various factors. First of all, the components used such as diaphragms or the beam optics have production tolerances. Secondly, there are temporally varying properties of the beam optics, such as e.g. temperature influences, aging effects, or varying experimental structures.
[0008] To provide sufficient and reliable protection of the detector under these circumstances, a relatively large beam stop must be used which also shields part of the radiation in the region of small angle scattering (approximately 0.1 to 5° beam deflection), and information about the sample can be lost. Alternatively, the beam stop can be iteratively adjusted to a given beam optics. In this case, varying properties of the beam optics cannot be corrected.
[0009] In contrast thereto, it is the underlying purpose of the present invention to provide a beam stop which protects the detector from the influence of the primary beam and divergent parasitic interfering radiation and at the same time permits passage of a maximum selectable part of diffracted or scattered radiation to the detector, wherein the beam stop can be easily adjusted to temporally varying properties of the beam optics.
[0010] This object is achieved in a surprisingly simple but effective fashion with an X-ray or neutron optical system of the above-mentioned type in that the beam stop is disposed to be displaceable along the z-direction to optimally set the ratio of useful radiation to interfering radiation reaching the detector.
[0011] After penetration through the sample, the primary beam is generally divergent, i.e. the beam diameter increases with the propagation path along the beam axis (z-axis). The inventive feature that the beam stop can be displaced in the z-direction, i.e. towards the detector or away from the detector, permits displacement of the beam stop to exactly that position along the beam path, where the fixed diameter of the beam stop and the spatially varying diameter of the primary beam (and of the parasitic stray radiation) coincide. This geometry keeps the primary beam and parasitic stray radiation away from the detector and at the same time diffraction phenomenon close to the beam can be largely detected by the detector.
[0012] In other words, in accordance with the invention, the diameter of the shielding projection of the beam stop in the detector plane (perpendicular to the beam axis, z-direction) can be set as desired. When the shadow cast by the beam stop exactly covers the beam spot of the primary beam and optionally parasitic radiation at the detector plane, the position of the beam stop is optimum. The diameter of the shielding projection can be adjusted to the experimental conditions, in particular to the exact dimensions of the components. Change of the shielding projection is easy to adjust in response to time-dependent changes of the properties of the beam optics.
[0013] In a particularly preferred embodiment of the inventive system, the system is adjusted to measure small-angle scattering, in particular between 0.1° and 5°. In this case, exact blanking of the interfering radiation of the primary beam and divergent parasitic radiation is particularly advantageous to guarantee maximum information content of the detected useful radiation, since the useful radiation of small-angle scattering experiments is mainly radiation diffracted close to the beam.
[0014] In a preferred embodiment, the beam stop can be adjusted in an xy-plane, perpendicular to the z-direction which permits setting of the diameter and also of the position of the shielding projection of the beam stop at the detector plane.
[0015] In one additional advantageous embodiment, the beam stop has a round, preferably circular cross-section. The cross-sections of the primary beam and parasitic stray radiation are also round such that in this case, the cross-section of the beam stop has a shape adapted to the standard situation.
[0016] One embodiment of an inventive system is also preferred, with which the beam stop has a shape similar to a truncated cone. The cone axis is thereby oriented on the beam axis and the broader truncated cone side faces the source or the sample. In this case, the broad truncated cone side edge defines a precise border of the shadowed region in the path of rays. Interaction between radiation and the cone surface is largely eliminated.
[0017] In a further advantageous embodiment of the inventive system, the beam stop is formed from a material having good radiation-absorbing properties, in particular from Au and/or Sb and/or Pb and/or W and/or Bi. In this case, the beam stop may be relatively thin and light, which facilitates its adjustment.
[0018] One embodiment is also advantageous with which the beam stop can be displaced in the z-direction by a motor to permit highly precise mechanical adjustment in the z-direction.
[0019] In one particularly preferred further development of this embodiment, the system can be automatically adjusted in accordance with predetermined criteria. Automatic adjustment is possible, in particular, after each change of the experimental structure or before each measurement. The measurements are carried out under optimum conditions. Typical criteria are e.g. keeping below a certain upper power limit for radiation on the detector.
[0020] One embodiment of the inventive system is also preferred with which the surface of the beam stop facing the impinging beam is concave. The radiation impinges approximately perpendicularly to the surface of the beam stop, achieving good radiation absorption.
[0021] In another preferred embodiment, the detector is a one-element detector (zero-dimension detector) which can scan a defined angle region about the z-axis. One-element detectors are particularly inexpensive and reliable.
[0022] In an alternative embodiment, the detector is a one-dimensional detector which can increase the measuring speed for measuring an angular or solid angular region.
[0023] In a further, particularly preferred alternative embodiment having even larger measuring speeds for measuring a solid angle region, the detector is a two-dimensional area detector, wherein the detector surface is disposed substantially perpendicular to the z-direction. Area detectors are particularly sensitive.
[0024] Further advantages of the invention can be extracted from the description and the drawing. The features mentioned above and below can be used in accordance with the invention individually or collectively in arbitrary combination. The embodiments shown and described are not to be understood as exhaustive enumeration but have exemplary character for describing the invention.
[0025] The invention is shown in the drawing and is explained in more detail with reference to embodiments.
[0026]
[0027]
[0028]
[0029]
[0030]
[0031] A sample
[0032] A beam stop
[0033] If the beam stop were disposed further to the left, i.e. at a lower z-position closer to the sample
[0034]
[0035] A beam stop
[0036]
[0037] To adjust the beam stop
[0038] Instead of a magnetic device, mechanical structures may be used for moving the beam stop
[0039] To adjust the beam stop
[0040]
[0041] Beam stops which can be adjusted in all three spatial directions, can also be used to shadow or blank individual diffracted beams in a diffraction spectrum. In this way, combinations of several beam stops are possible within the scope of this invention.