EP2592626A1 - Monochromateur efficace - Google Patents

Monochromateur efficace Download PDF

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Publication number
EP2592626A1
EP2592626A1 EP11195252.9A EP11195252A EP2592626A1 EP 2592626 A1 EP2592626 A1 EP 2592626A1 EP 11195252 A EP11195252 A EP 11195252A EP 2592626 A1 EP2592626 A1 EP 2592626A1
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Prior art keywords
laue
crystals
monochromator
crystal
lens
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EP11195252.9A
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German (de)
English (en)
Inventor
Dietrich Habs
Marc Günther
Michael Jentschel
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Max Planck Gesellschaft zur Foerderung der Wissenschaften eV
Ludwig Maximilians Universitaet Muenchen LMU
Institut Max Von Laue - Paul Langevin
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Max Planck Gesellschaft zur Foerderung der Wissenschaften eV
Ludwig Maximilians Universitaet Muenchen LMU
Institut Max Von Laue - Paul Langevin
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Application filed by Max Planck Gesellschaft zur Foerderung der Wissenschaften eV, Ludwig Maximilians Universitaet Muenchen LMU, Institut Max Von Laue - Paul Langevin filed Critical Max Planck Gesellschaft zur Foerderung der Wissenschaften eV
Priority to EP11195252.9A priority Critical patent/EP2592626A1/fr
Priority to PCT/EP2012/004495 priority patent/WO2013068079A1/fr
Publication of EP2592626A1 publication Critical patent/EP2592626A1/fr
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    • G—PHYSICS
    • G21—NUCLEAR PHYSICS; NUCLEAR ENGINEERING
    • G21K—TECHNIQUES FOR HANDLING PARTICLES OR IONISING RADIATION NOT OTHERWISE PROVIDED FOR; IRRADIATION DEVICES; GAMMA RAY OR X-RAY MICROSCOPES
    • G21K1/00—Arrangements for handling particles or ionising radiation, e.g. focusing or moderating
    • G21K1/06—Arrangements for handling particles or ionising radiation, e.g. focusing or moderating using diffraction, refraction or reflection, e.g. monochromators
    • G—PHYSICS
    • G21—NUCLEAR PHYSICS; NUCLEAR ENGINEERING
    • G21K—TECHNIQUES FOR HANDLING PARTICLES OR IONISING RADIATION NOT OTHERWISE PROVIDED FOR; IRRADIATION DEVICES; GAMMA RAY OR X-RAY MICROSCOPES
    • G21K1/00—Arrangements for handling particles or ionising radiation, e.g. focusing or moderating
    • G21K1/06—Arrangements for handling particles or ionising radiation, e.g. focusing or moderating using diffraction, refraction or reflection, e.g. monochromators
    • G21K1/065—Arrangements for handling particles or ionising radiation, e.g. focusing or moderating using diffraction, refraction or reflection, e.g. monochromators using refraction, e.g. Tomie lenses
    • G—PHYSICS
    • G21—NUCLEAR PHYSICS; NUCLEAR ENGINEERING
    • G21K—TECHNIQUES FOR HANDLING PARTICLES OR IONISING RADIATION NOT OTHERWISE PROVIDED FOR; IRRADIATION DEVICES; GAMMA RAY OR X-RAY MICROSCOPES
    • G21K2201/00—Arrangements for handling radiation or particles
    • G21K2201/06—Arrangements for handling radiation or particles using diffractive, refractive or reflecting elements
    • G21K2201/064—Arrangements for handling radiation or particles using diffractive, refractive or reflecting elements having a curved surface
    • G—PHYSICS
    • G21—NUCLEAR PHYSICS; NUCLEAR ENGINEERING
    • G21K—TECHNIQUES FOR HANDLING PARTICLES OR IONISING RADIATION NOT OTHERWISE PROVIDED FOR; IRRADIATION DEVICES; GAMMA RAY OR X-RAY MICROSCOPES
    • G21K2201/00—Arrangements for handling radiation or particles
    • G21K2201/06—Arrangements for handling radiation or particles using diffractive, refractive or reflecting elements
    • G21K2201/067—Construction details

Definitions

  • the present invention is in the field of radiation physics.
  • the present invention relates to an efficient monochromator comprising refractive deflection means and diffractive crystals.
  • ⁇ -photons or low energy neutrons each referred to as “particles” in the following for brevity
  • the physical effect of "Bragg-reflection” or "Laue-diffraction” at a single crystal can be used.
  • Laue-diffraction is equivalent to "Bragg-reflection", except that the diffracted beam passes through the crystal rather than being reflected at the crystal surface.
  • a Laue-diffraction can be regarded as a Bragg-reflection at crystal planes that are vertical to the crystal surface.
  • a Laue-crystal may be placed in the beam path of a primary beam of particles such that the Bragg-condition is fulfilled for particles of the desired energy.
  • the particles of the desired energy are Laue-diffracted and leave the crystal under the diffraction angle while the remaining fraction of the beam is transmitted without diffraction.
  • the diffracted beam is hence spatially separated from the primary beam and comprised of particles of a distinctive energy, i.e. it is monochromatized.
  • Monochromatization of a beam of particles can also be achieved in double crystal or Laue spectrometers. This is described for a beam of ⁇ -photons in E.G. Kessler et al., Nucl Inst. Meth. A 457, 187 (2001 ).
  • Such a Laue spectrometer is shown in Fig. 1 in two different geometries. The left part shows the two-crystal spectrometer 10 in the non-dispersive geometry, in which the first and second Laue-crystals 12, 14 are parallel to each other.
  • the portion of the incoming particle beam that satisfies the Bragg-condition with regard to angle and wavelength, i.e.
  • the crystal planes within the two crystals 12, 14 are parallel, too, and consequently all wavelengths that are diffracted by the first Laue-crystal 12 will simultaneously satisfy the Bragg-condition at the second Laue-crystal 14 and will be diffracted likewise.
  • the second Laue-crystal 14 is inclined with regard to the first Laue-crystal 12 by twice the Bragg angle of a given wavelength ⁇ . This means that at the second Laue-crystal 14, only particles with the corresponding wavelength ⁇ will be Laue-diffracted. However, particles deviating from ⁇ that have been Laue diffracted by the first Laue-crystal 12 do no longer meet the Bragg-condition at the second Laue-crystal 14 and will therefore be transmitted without diffraction by the second Laue-crystal 14 (undiffracted transmitted beam not shown).
  • the two-crystal spectrometer 10 acts as a monochromator, since the particles which are Laue-diffracted by the second Laue-crystal 14 are within a very narrow energy band.
  • the monochromatization which is equivalent to the relative energy width ⁇ E/E of a ⁇ -beam in a Laue spectrometer in dispersive geometry is typically 10 -6 .
  • the intensity profile of the twice diffracted beam can be recorded as a function of the rotation angle.
  • This measured intensity profile is also referred to as "rocking curve” due to the "rocking" movement of the crystal.
  • the width of the rocking curve I( ⁇ ) is proportional to ⁇ meaning that the intensity-angle-profile is very narrow for large energies.
  • Example calculations of I( ⁇ ) for a 2.5 mm thick single crystal of Si in [220] orientation are shown in Fig. 2 for photons of 100 keV, 500 keV and 1000 keV. As can be seen, the width of theses curves decreases with energy and can be as small as a few tens of nanoradiants (nrad).
  • the width of I( ⁇ ) can be called the "acceptance witdh” of the single crystal, since it defines an angular range for particles of the given wavelength ⁇ that will be "accepted” by the crystal in the sense that they are Laue-diffracted thereby. Conversely, this means that radiation impinging at an angle outside this acceptance range will not be Laue-diffracted by the first Laue-crystal 12, even if it has the desired energy (the appropriate wavelength ⁇ ) and will hence be lost for the monochromator of Fig. 1 .
  • the best collimated ⁇ -beams typically have a divergence in the range of tens of microrad, which means that actually the biggest part of the impinging ⁇ -beam will fall outside the angular acceptance range and hence will be lost for the monochromator, resulting in a very low efficiency.
  • the problem underlying the invention is to provide a monochromator of improved efficiency for monochromatizing a beam of ⁇ -photons, x-ray photons having an energy higher than 10 keV and neutrons of an energy below 1 eV.
  • This problem is solved by the monochromator of claim 1. Preferred embodiments are defined in the dependent claims.
  • the monochromator according to the present invention comprises a plurality of pairs of Laue-crystals, each pair of Laue-crystals having
  • the first Laue-crystals of all pairs of Laue-crystals are parallel to each other and the second Laue-crystals of all pairs of Laue-crystals are parallel to each other as well.
  • the first Laue-crystals of the pairs of Laue-crystals are arranged in series with refractive deflection means arranged inbetween such that the part of the beam that is transmitted without diffraction by one of said first Laue-crystals is deflected prior to impinging on the next first Laue-crystal in said series of first Laue-crystals.
  • the part of the beam that is transmitted without diffraction by one of the first Laue-crystals ⁇ because it may be outside the angular acceptance range of said first Laue-crystal ⁇ is deflected prior to impinging on the next first Laue-crystal in said series. This deflection will lead to a shift of the transmitted undiffracted beam in angular space before impinging on the next first Laue-crystal.
  • the refractive deflection means arranged between consecutive first Laue-crystals in said series of first Laue-crystals are adapted to deflect the beam by an angle ⁇ that is larger than but still reasonably close the angular acceptance width of the first Laue-crystal for the desired particle energy.
  • the angle ⁇ is given by 5 nrad ⁇ ⁇ ⁇ 200 nrad, preferably 10 nrad ⁇ ⁇ ⁇ 100 nrad.
  • the number of pairs of Laue-crystals employed in the monochromator is at least 5, preferably at least 10 and more preferably at least 50.
  • the first and second Laue-crystals of each pair of Laue-crystals are either parallel to each other, which corresponds to the non-dispersive mode, or inclined with respect to each other such as to allow for an energy selection by two consecutive Laue diffractions, and in particular inclined by twice the Bragg-angle of the desired wavelength.
  • the dispersive geometry is preferably used for monochromatization, while the non-dispersive mode may be used to align all optical elements, in particular the deflection means.
  • the first and second Laue-crystals are shiftable between the two configurations. This will for example allow precisely adjusting the refractive deflection elements in the non-dispersive (parallel) mode and then decreasing the energy width by shifting the first and second Laue-crystals with respect to each other to the dispersive geometry.
  • all first Laue-crystals are part of one fixed first unit, and all second Laue-crystals are part of one fixed second unit.
  • the first and second units are preferably each made from a single crystal block, as this ensures that all first crystals and all second crystals are mutually parallel with each other.
  • both blocks are taken from the same ingot, which allows for a perfect match of the first and second Laue-crystals.
  • the Laue-crystals may in particular consist of Si and/or Ge.
  • a collimating lens, lens stack or lens array is arranged upstream of the monochromator such as to parallelize or at least reduce the divergence of the beam prior to entering the monochromator. This way, the efficiency of the monochromator can be dramatically increased.
  • collimating lenses, lens stacks or lens arrays may be employed.
  • the geometry will rather be such that a beam that has already been Laue-diffracted by a first (second) Laue-crystal of one pair passes a first (second) Laue-crystal of another pair of Laue-crystals, becauseit is difficult to keep these Laue-crystals of the further pair out of the way of the diffracted beam.
  • all first (second) Laue-crystals are parallel to each other, this would imply that the already Laue-diffracted beam is Laue-diffracted at the fist (second) Laue-crystal of this other pair of Laue-crystals again.
  • this is prevented by ensuring that a beam that has already been Laue-diffracted by a first (second) Laue-crystal of one pair is deflected using a refractive optical element prior to passing a first (second) Laue-crystal of another pair of Laue-crystals. This way, it can be ensured that the beam is diffracted only once at a first Laue-crystal and once at a second Laue-crystal forming the abovementioned pair of Laue-crystals.
  • the refractive optical elements arranged between two adjacent first Laue-crystals for deflecting a beam transmitted without diffraction by the previous first Laue-crystal in said series are arranged to also deflect the beam that is Laue-diffracted by said previous first Laue-crystal. Accordingly, this way it can be ensured that the Laue-diffracted beam is not Laue-diffracted at any further first Laue-crystal in said series, because after deflection, i.e. an angle change, it no longer obeys the Bragg-condition.
  • the first and second Laue-crystals are arranged in the monochromator such that the first and second Laue-crystals of each pair are adjacent to each other.
  • first and second Laue-crystals are arranged in the monochromator such that all first Laue-crystals are arranged in a series and all second Laue-crystals are arranged in a further series that is arranged downstream of the series of first Laue-crystals with regard to the propagation direction of the beam. Further, first refractive deflection means are placed between each two neighbouring first Laue-crystals, and second refractive deflection means are placed between each two neighbouring second Laue-crystals.
  • the second refractive deflection means in the n th gap between neighbouring second Laue-crystals when counted in opposite propagation direction of the beam is adapted to compensate for the deflection provided by the first refractive deflection means in the n th gap between neighbouring first Laue-crystals when counted in propagation direction of the beam.
  • each beam that is Laue-diffracted at any first Laue-crystal may pass through all downstream first and second Laue-crystals while it is still ensured that it is only Laue-diffracted by the corresponding second Laue-crystal of the pair.
  • the most upstream one of the first Laue-crystals and the most downstream one of the second Laue-crystals form a pair
  • the second most upstream one of the first Laue-crystals and the second most downstream one of the second Laue-crystals form a further pair and so on.
  • refractive deflection means are employed in preferred embodiments of the monochromator to ensure that at a distinctive Laue-crystal the Bragg-condition is or is not fulfilled.
  • Different refractive optical elements for example a collimating lens, may be arranged upstream of monochromator for a further increase of efficiency as mentioned above. While it is known to manipulate x-ray beams using refractive optical elements, it is so far generally accepted that at photon energies above say 200 keV, this is no longer possible. The reason is that according to the present understanding in the art, the index of refraction, which even in the x-ray regime is already very close to 1, rapidly converges even closer to 1 with increasing energy.
  • ⁇ photo the physical effect giving rise to ⁇
  • ⁇ photo the virtual photo effect (Rayleigh scattering)
  • ⁇ photo the index of refraction n is smaller than 1.
  • a typical value of ⁇ photo at 80 keV and aluminum is -0.8 x 10 -7 .
  • the use employs the fact that the index of refraction n of the optical material has a real part > 1, or, in other words, that ⁇ > 0.
  • the design of the optical elements will be different from refractive x-ray optical elements and conceptually in fact more similar to ordinary light optics.
  • refractive optical elements for ⁇ -photons having an energy of more than 700 keV a focusing lens would have a convex shape, whereas an x-ray focusing lens has a concave shape.
  • Suitable novel means for collimation and deflection which may be used with the monochromator in an embodiment for specific use in the ⁇ -regime will be described below.
  • the refractive deflection means for deflecting the beam is comprised by an array of prisms, wherein the array of prisms comprises at least one series arrangement of prisms allowing for being consecutively passed by a beam.
  • a number N of prisms are arranged in series, such as to be consecutively passed by a beam.
  • N may be ⁇ 2, preferably ⁇ 10 and more preferably ⁇ 100.
  • the suitable number of N also depends on the angle of the prism. However, if desired, the number of prisms arranged in series can be easily increased to hundreds or even thousands, in view of the comparatively small absorption of ⁇ -radiation in matter.
  • At least the majority of prisms in the array of prisms has a wedge-shape with a base surface having a triangular shape.
  • the height of the triangular shape is preferably smaller than 200 ⁇ m, preferably smaller than 50 ⁇ m.
  • the base surface may further have an isosceles triangle shape, wherein the angle ⁇ between the two equal sides of said isosceles triangle is preferably between 5° and 120°, more preferably between 15° and 90°.
  • the array of prisms may also comprise a number M of series arrangements of prisms arranged in parallel, wherein M ⁇ 2, preferably M ⁇ 4 and more preferably M ⁇ 10. This allows deflecting a large diameter beam with a suitably large number of series arrangements of comparatively small prisms arranged in parallel.
  • the array of prisms is preferably at least in part made from one or more wafers, in particular Si and/or Ge wafers, in which the prisms are formed by etching.
  • the one or more wafers has/have a thickness between 20 ⁇ m and 200 ⁇ m, preferably between 50 ⁇ m and 100 ⁇ m, and the array of prisms may at least in part be made from a stack of a plurality of identically etched wafers wherein said wafers of said stack are preferably grown or fused together such as to yield a total thickness of the stack of 5 mm or more, preferably 8 mm or more.
  • the above mentioned collimating lens to reduce the divergence of the beam prior to entering the monochromator has at least one, preferably two lens surfaces having a convex shape in two dimensions, and in particular, a rotation-ellipsoid shape.
  • Such lens is referred to as a 2-D-lens, as it can collimate a ⁇ -beam in two dimensions.
  • the convex lens is made from an embossed foil comprising one of Be, Al, Ni, Ta or Th as its main constituents. By embossing the foil, two-dimensional lenses can be manufactured easily and efficiently and with great precision of about 5 nm.
  • 2-D- ⁇ -lens could also be made by micromachining.
  • the tangential radius R at the apex of the lens is ⁇ 2000 ⁇ m, preferably ⁇ 1000 ⁇ m and more preferably between 5 ⁇ m and 500 ⁇ m.
  • the tangential radius R at the apex of the lens is ⁇ 2000 ⁇ m, preferably ⁇ 1000 ⁇ m and more preferably between 5 ⁇ m and 500 ⁇ m.
  • radii down to 1 ⁇ m can be used.
  • a number N of said convex lenses are arranged in series, for example stacked one behind the other in a lens holder.
  • N is preferably between 2 and 10000, more preferably between 10 and 200.
  • the body of said lens has a hole for ventilation, to thereby prevent the formation of air cushions and to avoid bending of the lens when mounting the lens array.
  • the shape of the hole is not limited, as long as it allows for sufficient ventilation.
  • the divergence of the ⁇ -beam prior to entering the monochromator is reduced by an array of lenses, wherein said array of lenses comprises at least one series arrangement of lenses allowing for being consecutively passed by a ⁇ -beam, and wherein at least the majority of the lenses has at least one, preferably two lens surfaces having a convex shape in at least one dimension.
  • the "series arrangement” of lenses could be an arrangement of lenses along their optical axes.
  • the term “lens surface” refers to the "entrance surface” and "exit surface” of the lens.
  • the mean radius of curvature of the convex shape or the tangential radius at the apex of the convex shape is preferably between 1 ⁇ m and 500 ⁇ m, preferably between 10 ⁇ m and 80 ⁇ m.
  • the convex lens surface may have a conical shape (in case of a 2-D-lens) or a triangular prism-like shape (in case of a 1-D-lens), in which case no tangential radius at the apex of the convex shape is defmed, but a prism angle instead.
  • we refer to the mean radius of curvature of the convex shape which is defined as the radius of an arch or a sphere containing the apex and an edge portion of the lens surface.
  • a number N of lenses are arranged in series such as to be consecutively passed by a ⁇ -beam, wherein N ⁇ 10, preferably N ⁇ 100 and more preferably, N ⁇ 300. In some applications N may even be ⁇ 1000.
  • N may even be ⁇ 1000.
  • the array of lenses comprises a number M of series arrangements of lenses arranged in parallel, wherein M ⁇ 2, preferably M ⁇ 4 and more preferably M ⁇ 10.
  • M ⁇ 2 preferably M ⁇ 4 and more preferably M ⁇ 10.
  • the lens array is at least in part made from one or more wafers, in particular Si and/or Ge wafers, in which the lens surfaces are formed by etching.
  • Si and/or Ge provide a sufficient index of refraction to construct useful refractive optical elements therefrom.
  • the advantage of using Si and/or Ge is that one can resort to well-established lithography and etching technology, in particular electron beam lithography, to efficiently and precisely manufacture miniature structures, thereby allowing to manufacture arrays of very large numbers of lenses with a very small radius of curvature in a cost efficient way. Also, due to this manufacturing, the individual lenses can be aligned very precisely.
  • the one or more wafers has/have a thickness between 20 ⁇ m and 200 ⁇ m, preferably between 50 ⁇ m and 100 ⁇ m. If the thickness is below 100 ⁇ m, it is possible to etch precise vertical walls constituting the lens surfaces, for example by ion beam deep etching or the like. Note that alternative manufacturing methods, including improved methods that will become available in the future are also possible.
  • the lens array is at least in part made from a stack of a plurality of identically etched wafers, wherein the wafers of the stack are preferably grown or fused together. This allows obtaining a total thickness of the stack of wafers of for example 5 mm or more, preferably 8 mm or more, thereby allowing to shape a ⁇ -beam having a corresponding beam width. If desired, even larger stacks of identically etched wafers can be formed.
  • the lens surfaces have a convex shape only in one dimension, i.e. are 1-D lenses only.
  • the lens array can only focus a ⁇ -beam in the plane of the wafer but not in a plane perpendicular to the wafer plane.
  • the ⁇ -beam is focused or collimated by two arrays of lenses according to one of the embodiments described above, which are arranged in series and are oriented with respect to each other such that each of the two lens arrays focuses or collimates a ⁇ -beam within different planes, such as two perpendicular planes.
  • Fig. 3 is a schematic representation of a monochromator 16 according to an embodiment of the present invention.
  • the monochromator 16 comprises three pairs of Laue-crystals, 18a/18b, 20a/20a, 22a/22b which in combination each form a two-crystal spectrometer 10 in the dispersive geometry as shown on the right hand side of Fig. 1 . All the first Laue-crystals 18a, 20a, 22a of all pairs of Laue-crystals 18a/18b, 20a/20a, 22a/22b are parallel to each other.
  • first Laue-crystals 18a, 20a, 22a of the pairs of Laue-crystals 18a/18b, 20a/20a, 22a/22b are arranged in series with refractive deflection means 24 arranged inbetween neighbouring first Laue-crystals 18a, 20a, 22a such that the part of the beam that is transmitted without diffraction by one of the first Laue-crystals 18a, 20a is deflected prior to impinging on the next first Laue-crystal 20a, 22a in the series.
  • refractive deflection means 24 is symbolically represented by a wedge prism 24 in Fig. 3 , it is understood that in practice the wedge arrays as described for example further below with reference to Fig. 9 will be employed.
  • Panel A of Fig. 4 shows the intensity-angle profile of the incoming beam impinging on the first Laue-crystal 18a of the first pair of Laue-crystals 18a/18b. Only the portion of the beam that fulfils the Bragg-condition with respect to the first crystal 18a will be Laue-diffracted at the first Laue-crystal 18a. However, most of the beam will actually not meet the Bragg-condition and hence be transmitted without diffraction through the first crystal 18a of the first pair of Laue-crystals 18a/18b. This is indeed seen from panel B of Fig.
  • FIG. 4 which shows the intensity-angle profile at location B of Fig. 3 , i.e. the intensity-angle profile of the beam that was transmitted without diffraction through the first Laue-crystal 18a.
  • panel B of Fig. 4 a small angular band of the intensity is missing, corresponding to the narrow angular band that has been Laue-diffracted by the first Laue-crystal 18a.
  • Panel C of Fig. 4 shows the intensity-angle profile at position ⁇ just behind the refractive deflection means 24.
  • the shape of the intensity-angle profile has not changed, but it has been shifted in angle space due to the refractive deflection means 24.
  • the beam with the shifted intensity-angle profile impinges onto the first Laue-crystal 20a of the second pair of Laue-crystals 20a/20b.
  • the beam now contains a portion obeying the Bragg-condition with regard to angle and energy again, allowing for a further Laue-diffraction at the first Laue-crystal 20a of the second pair of Laue-crystals 20a/20b.
  • the angular acceptance range for Laue-diffraction is always the same.
  • This procedure can be repeated with many pairs of Laue-crystals, where in each case the part of the beam that is transmitted without diffraction by the previous first Laue-crystal is deflected prior to impinging on the next first Laue-crystal in the series. Since only the Laue-diffracted portion of the beam adds to the output of the monochromator 16, it is seen that the efficiency of the monochromator 16 is thereby increased.
  • the efficiency is generally proportional to the number of pairs of Laue-crystals.
  • Fig. 5 schematically shows the energy and angular distribution of the beam entering the monochromator 16. If only a single pair of Laue-crystals was used, only a small section of the angle-energy range of the impinging beam will be Laue-diffracted, which is schematically indicated by the small square 26 in Fig. 5 . Namely, the small area 26 corresponds to the part of the angle-energy distribution that satisfies the Bragg-condition at the first Laue-crystal 18a, while the rest of the beam is transmitted without diffraction by the first Laue-crystal 18a and would be lost in an ordinary double-crystal spectrometer.
  • the beam that is Laue-diffracted at the second Laue-crystal 18b of the first pair of Laue-crystals 18a/18b does not pass through the second Laue-crystal 20b of the second pair of Laue-crystals 20a/20b.
  • each beam of the desired energy is Laue-diffracted exactly once at a first Laue-crystal 18a, 20a, 22a and once at a corresponding second Laue-crystal 18b, 20b, 22b.
  • Fig. 6 An example of such an arrangement is shown in the monochromator 30 of Fig. 6 .
  • Fig. 6 again three pairs of Laue-crystals 18a/18b, 20a/20b, 22a/22b and refractive deflection means 24 are shown.
  • all first Laue-crystals 18a, 20a, 22a are part of a (fixed) first unit 32 that is made from a single crystal block.
  • all second Laue-crystals 18b, 20b and 22b are part of one fixed second unit 34 and are also made from a single crystal block.
  • both units 32 and 34 are taken from the same ingot, so that the lattice structures ideally match.
  • the refractive deflection means 24 arranged between two adjacent first Laue-crystals 18a/20a, 20a/22a for deflecting a beam transmitted undiffracted by the previous first Laue-crystals 18a, 20a in the series is arranged to also deflect the beam that is Laue-diffracted by the previous first Laue-crystal 18a, 20a. This way, it is avoided that a beam that has been Laue-diffracted at one of the first Laue-crystals 18a, 20a is diffracted at another first Laue-crystal 20a, 22a in the monochromator 30 again.
  • the geometry of the monochromator 30 ensures that a beam that has been Laue-diffracted at one of the second Laue-crystals 18b, 20b is deflected once before passing another second Laue-crystal 20b, 22b, thereby avoiding a further Laue-diffraction at a second Laue-crystal.
  • the tilt angle between the first Laue-crystals 18a, 20a, 22a and the second Laue-crystals 18b, 20b, 22b does not correspond to twice the Bragg-angle, since the angular shift of the refractive deflection means 24 needs to be taken into account.
  • the beams that are Laue-diffracted at the plural second Laue-crystals 18b, 20b, 22b diverge when leaving the monochromator 30, since they have passed different numbers of refractive deflection means 24.
  • these individual beams extracted from the monochromator 30 can be focused to a single spot using an appropriate lens system.
  • a further monochromator 36 is shown, which has the currently most preferred geometry.
  • the monochromator 36 also comprises two units 32, 34, where the first unit 32 contains three first Laue-crystals 18a, 20a, 22a and the second unit 34 contains three second Laue-crystals 18b, 20b, 22b. Note that in the monochromator 36, the first and second units 32, 34 are arranged in series, while in the monochromator 30 of Fig. 6 , they were arranged in an interleaved relationship. That is to say, in the monochromator 36 of Fig.
  • all first Laue-crystals 18a, 20a, 22a are arranged in a series and all second Laue-crystals 18b, 20b, 22b are arranged in a further series that is arranged downstream of the series of first Laue-crystals 18a, 20a, 22a with regard to the propagation direction of the beam.
  • the order of the second crystals 18b, 20b, 22b in the second unit 34 is reversed in the sense that the most upstream first Laue-crystal 18a of the first unit 32 forms a Laue-crystal pair with the most downstream second crystal 18b of the second unit 34 and so on.
  • Refractive deflection means 24 are placed between each two neighbouring first Laue-crystals 18a, 20a, 22a of the first unit 32, in order to provide for the desired angular shift.
  • Further refractive deflection means 38 are placed between each two neighbouring second Laue-crystals 18b, 20b, 22b of the second unit 34.
  • the further refractive deflection means 38 compensate the effects of the refractive deflection means 24 arranged in the first unit 32.
  • the refractive deflection means 24 and 38 could be identical wedge arrays similar to those shown in Fig. 9 but with reversed orientation, as is symbolically shown in Fig. 7 .
  • the reflection means 38 in the n th gap between neighbouring second Laue-crystals 18b, 20b, 22b when counted in opposite propagation direction of the beam is adapted to compensate for the deflection provided by the deflection means 24 in the n th gap between neighbouring first Laue-crystals 18a, 20a, 22a when counted in propagation direction of the beam.
  • each of the monochromators 16, 30 and 36 is further increased if the inherently diverging beam is made less diverging or even parallelized prior to entering the monochromator 16, 30, 36.
  • the collimation lenses, lens stacks or lens arrays discussed below with reference to Figs. 10 , 11 can be ideally used. Note in this regard that a focusing lens acts as a collimation lens if the source of the diverging beam is placed in the focal point of the lens.
  • a schematic perspective view of a Laue-crystal unit 40 is shown, that could be used as one of the units 32 or 34 of Fig. 7 .
  • the unit shows three Laue-crystals 18, 20, 22 which are made from one ingot. Further shown are the refractive deflection means 24, which are formed by wedge arrays 24 in the present example. Note that Fig. 8 is also highly schematic and not drawn to scale. Further, the wedge arrays 24 can be adjusted with respect to each other by means of a flexure cut 42 and a piezo actuator 44. By controlling the piezo actuator 44, the relative orientation of the wedge arrays 24 can therefore be adjusted such as to tune the monochromator 36.
  • Fig. 9(a) a plan view and in Fig. 9(b) a perspective view of an array 46 of triangular prisms 48 is shown which may be used as the aforementioned refractive deflection means 24, 38.
  • the triangular prisms 48 have an isosceles triangle shape, wherein the height (h) of the triangle shape is smaller than 200 ⁇ m, preferably even smaller than 100 ⁇ m and more preferably even smaller than 50 ⁇ m, as miniaturizing allows for increasing the number of prisms 48 to accommodate in the array and hence for increasing the total refractory power.
  • the prisms 48 are also referred to as "wedges" or "microwedges" in the following.
  • the wedge array 46 of Fig. 9 is etched from a semiconductor wafer, in particular Si and/or Ge. Accordingly, the thickness of the wedges of Fig. 9 corresponds to the thickness of the wafer, which is typically between 20 ⁇ m and 200 ⁇ m. A plurality of identical wedge arrays 46 can be stacked on top of each other, to thereby produce a thicker wedge array 46 having a thickness of several millimetres or even beyond a centimeter.
  • the wedge array 46 of Fig. 9 is comprised of a number M of series arrangements 50a, 50b, 50c of wedges 48 that are arranged in parallel, where the number M can again be chosen as desired. Further, each series arrangement 50a, 50b, 50c of wedges 48 contains a number N of wedges 48 arranged in series such as to be consecutively passed by a beam. Herein, the number N will depend on the total deflection angle that is intended. From a manufacturing point of view, hundreds of or even a thousand wedges 48 can be easily provided in each arrangement of prisms 50a, 50b, 50c.
  • a cross section ( Fig. 10(a) ) and a perspective view ( Fig. 10(b) ) of a 2-D focusing ⁇ -lens 52 according to an embodiment of the invention is shown.
  • the ⁇ -lens 52 is "two-dimensional" in the sense that it is optically active in two dimensions, meaning that the lens surfaces 54, 56 are curved in two orthogonal sections A-A (shown in Fig. 10(a) and B-B (not shown). This means that a ⁇ -beam will be focused in two dimensions, such as to converge to a focal "point". This is to distinguish the lens from 1-D-lenses described below, where the beam is only shaped in one dimension but left unaffected in another dimension such as to, for example focus a circular beam onto a line rather than onto a point.
  • the lens 52 is made from a nickel foil that is squeezed between two profiled pistons to acquire the convex shape that is particularly apparent from Fig. 10(a) . With this embossing technique, the lens 52 can be manufactured comparatively easily and cheaply and to a high precision of about 100 nm.
  • the lens surfaces 54 and 56 of the lens 52 have a rotational ellipsoid shape that is characterized by an inscribed tangential radius at its apex 58.
  • the radius R at the apex 58 is smaller than 2000 ⁇ m, preferably smaller than 1000 ⁇ m and preferably between 5 ⁇ m and 500 ⁇ m. The smaller the radius of curvature (i.e. the larger the curvature), the smaller the focal length.
  • the lens 52 has mounts 60 at two sides, which may have a length of about 10 mm. With these mounts, a plurality of lenses 52 can be stacked one behind the other in a lens holder (not shown), to thereby add up the focusing power of multiple lenses. In practice, several hundreds or even a thousand of lenses 52 can be manufactured and stacked one behind the other.
  • a hole 62 for ventilation is provided in the body of the lenses 52, thereby preventing a deformation of the lenses 52 when the lens stack is evacuated.
  • a schematic plan view ( Fig. 11(a) ) and a schematic perspective view ( Fig. 11(b) ) of an array 64 of 1-D lenses 68 is shown.
  • the lens array 64 is comprised of three series arrangements 66a, 66b, 66c of lenses 68 that are arranged in series for being consecutively passed by a ⁇ -beam.
  • N 6 lenses 68 are arranged in series in each of the series arrangements 66a, 66b, 66c, however, the number N may in practice be much larger, i.e. N ⁇ 10, preferably N ⁇ 100 and more preferably N ⁇ 300, in order to increase the refractive power of the lens array 64.
  • Each of the lenses 68 constituting the lens array 64 has two lens surfaces 70, 72 that have a convex shape in one dimension only.
  • the convex shape can be seen in the plan view of Fig. 11(a) , the shape having a tangential radius R at the apex of each lens surface 70, 72 of between 1 ⁇ m and 500 ⁇ m, preferably between 10 ⁇ and 80 ⁇ m.
  • This means that ⁇ -beams 74 as shown in Fig. 11(a) will only be focused in the paper plane of Fig. 11(a) , but not within a plane vertical to the paper plane of Fig. 11(a) . Accordingly, when the lens array 64 of Fig.
  • a ⁇ -beam 74 would be focused onto a line rather than onto a focal point.
  • two lens arrays 64 could be arranged in series one after the other and rotated by 90° with respect to each other such as to achieve a focusing in two dimensions.
  • the lens arrays 64 of Fig. 11 can be made from a wafer, such as an Si and/or Ge wafer by vertical etching, thereby leading to the vertical wall parts of the lens surfaces 70, 72.
  • a wafer such as an Si and/or Ge wafer by vertical etching, thereby leading to the vertical wall parts of the lens surfaces 70, 72.
  • first a mask is generated by electron beam lithography.
  • the material between neighbouring lenses 68 is etched for example by ion beam deep etching.
  • the wafer thickness is between 20 ⁇ m and 100 ⁇ m and more preferably between 50 ⁇ m and 200 ⁇ m. With these thicknesses, precise vertical walls can still be etched.
  • a plurality of identical lens arrays 64 as shown in Fig. 11 can be manufactured and then stacked one on top of the other to thereby increase the total thickness of the lens array 64.
  • the wafers in the stack can be grown or fused together. This way, a total thickness of a lens array 64 of more than 5 mm or even more than 8 mm can be achieved.
EP11195252.9A 2011-11-08 2011-12-22 Monochromateur efficace Withdrawn EP2592626A1 (fr)

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