Abstract:
A collimation device for an X-ray beam, an optical device for analyzing a specimen by the scattering of an X-ray beam, and a collimator for an X-ray beam. The collimation device includes an enclosure configured to be under a vacuum or a controlled atmosphere, the enclosure including an inlet and an outlet for the X-ray beam and at least one plate made of a material having a diffracting periodic structure, the plate including two main faces and at least one flared aperture between the faces.

Description:
The present invention pertains to the field of the analysis of a specimen by X-ray scattering. 
     It relates especially to a collimation device for an X-ray beam, an optical device for the analysis of a specimen by X-ray scattering comprising this collimation device and, a collimator for such a beam. 
     Within the framework of the invention, the expression X-ray beam is intended to mean a beam of photons whose energy is between 1 keV and 30 keV. 
     In particular, the invention pertains to the field of the analysis of a specimen by X-ray scattering at small angles. The expression scattering at small angles must be understood to imply that the rays scattered by a specimen traversed by the beam (perpendicular incidence) to be analyzed lie in proximity to the X-ray beam by which the specimen is illuminated, in an angle of generally between 0.1° and 10° with respect to the optical axis of the beam. It is also possible to consider an orientation of the specimen positioned not perpendicularly to the beam but at grazing incidence with respect to the latter. 
     The techniques based on X-ray scattering at small angles are also known by the acronym SAXS signifying “ Small Angle X - Rays Scattering ” (“Small-Angle Scattering of X-rays”, André Guinier and Gérard Foumet, ed. John Wiley and Sons Inc., 1955). 
     By virtue of these techniques, it is especially possible to obtain information about the organization of molecular systems of the specimen. 
     A known optical device for implementing a SAXS technique is represented in  FIG. 1 , according to an exploded perspective view. 
     The device comprises an X-ray source  10 . 
     The beam  1  generated by the source  10  is then directed toward a monochromator mirror  11 , which makes it possible to produce a monochromatic beam, that is to say containing only one X-ray wavelength. Typically, a beam is considered to be monochromatic when the ratio between the wavelength discrepancy and the desired wavelength is less than 1%. 
     It should however be noted that a nonmonochromatic X-ray beam could be used. 
     The beam exhibits a preferential axis of propagation called the “optical axis”. Transversely to the optical axis, the beam exhibits a quasi-uniform cross-section when so-called “collimating” mirrors are used, i.e. convergent toward a distant point when so-called “convergent” mirrors are used. 
     In both cases, the geometric definition of the beam on exit from the monochromator is not sufficient to carry out scattering experiments at small angles. The expression geometric definition is intended to mean the real difference between a perfect geometry (parallel or convergent) of the beam and that which is physically obtained. 
     Better definition of the beam is thus obtained by collimation with a series of obstacles placed along the axis of the beam after the monochromator. The term “obstacle” is understood to mean a device opaque to X-rays at the wavelength employed. 
     In a conventional setup represented in  FIG. 1 , the first “obstacle” generally corresponds to four movable lips opaque to X-rays, referenced  12 . Two parallel lips with a spacing D in the plane perpendicular to the axis of the beam define a “slit”. Two pairs of lips thus arranged, form a hole. A collimator is more generally formed of two “holes” whose centers must be aligned with the optical axis of the beam exiting the monochromator. 
     The first obstacle, taking the form of a plate  12  furnished with two pairs of lips forming these two slits, thus forms a hole. 
     The plate  12  furnished with the two pairs of “lips” may be integrated into the mirror  11 . 
     The plate  12  is generally followed by a calibrated attenuator (not referenced). 
     The beam is thereafter directed toward a second obstacle for collimation, placed some distance from the first obstacle along the optical axis of the beam. This second obstacle also takes the form of a plate  13  comprising two pairs of parallel lips, so as to form two slits whose centers are aligned with the optical axis of the beam. 
     The optical path between the two series of collimation “slits” may be placed under vacuum. Sometimes, it may, as a variant, be placed under a helium atmosphere. 
     The coupling of the two collimation means  12  and  13  makes it possible to delimit the size of the beam that it is desired to obtain at the level of the specimen  16 . 
     On exit from the first evacuated enclosure  14 , the beam passes through a third pair of slits  15 , which are placed along the optical axis just before the specimen to be analyzed. These so-called “anti-scattering” slits do not, properly speaking, form parts of the collimator. Indeed, the anti-scattering slits  15  make it possible to eliminate the spurious scatterings produced by the slits of the collimation means  12  and  13 . 
     Adjustment of the anti-scattering slits  15  is particularly tricky, since it is necessary to skim past the beam without touching it in order to eliminate the spurious scatterings without modifying the size of the beam. 
     The interaction of the beam  1  with the specimen  16  causes scattering of the X-rays, the beam being moreover transmitted at least in part through the specimen. 
     The transmitted beam and the scattered part are then gathered in a second evacuated enclosure  18  at the end of which is a means  19  for halting the beam. The evacuated enclosure makes it possible to limit at one and the same time the additional absorption by air, of the scattered rays and the complementary scattering of the beam  1  likewise by air. 
     A detector  20 , situated downstream of the means  19  for halting the beam  1 , then makes it possible to detect the X-rays scattered by the specimen. 
     Finally, the importance should be noted of the plate  12  furnished with collimation slits (first obstacle), the plate  13  also furnished with collimation slits (second obstacle) and the anti-scattering slits  15 , without which it would be difficult to detect the X-rays scattered by the specimen, in particular the rays scattered at small angles which lie in proximity to the optical axis of the beam. 
     The relative position of the various obstacles  12 ,  13  and  15  is also important in regard to this aim. 
     As mentioned previously, these obstacles  12 ,  13 , are generally four independent lips forming rectangular or square slits. These lips are furnished with vanes which may be displaced to adjust the dimensions of a slit. These vanes are metallic and generally made of steel, tantalum or constructed of tungsten rods. 
     The arrangement of a vane  21  at the level of a slit is for example represented in  FIG. 2 , according to a sectional view. Conventionally, such a vane  21  exhibits a thickness of about 1.5 mm. 
     Recently, it has been proposed that the monocrystalline structure vanes be arranged on the metallic vanes. Hereinafter, these vanes will be referred to as hybrid vanes. 
     The expression monocrystalline structure vane should be understood to imply that the material forming the vane is made of a single solid material exhibiting an elementary mesh cell that repeats in a regular manner, so as ultimately to form an ordered structure. 
     A hybrid vane such as this, comprising a metallic vane  21  and a monocrystalline structure vane  22 , is for example represented in  FIG. 3 , according to the same sectional view as  FIG. 2 . 
     It is for example possible to cite the document “ Scatterless hybrid metal - single crystal slit for small - angle X - ray scattering and high - resolution X - ray diffraction ”, Youli &amp; al., J. Appl. Crystallography (2008), vol. 41, pp. 1134-1139 (D1). 
     The authors of this document have shown that arranging monocrystalline structure vanes formed from a silicon wafer carefully sliced and glued onto the metallic vanes made it possible to reduce the X-ray scattering generated by the slits. 
     Applied to the optical device described hereinabove, the slits furnished with these vanes therefore make it possible to improve the quality of the device. 
     Indeed, the monocrystalline structure which is placed at the vane edge returns the X-rays at well defined angles which depend on the crystalline plane of this structure. These angles are large enough not to merge with the beam. 
     When hybrid slits are installed in the optical device represented in  FIG. 1 , they make it possible to collimate the beam without producing spurious scattering. 
     The slit proposed by Youli &amp; al. therefore makes it possible to simplify the optical device and, therefore, its adjustment. 
     However, the hybrid slit exhibits a more complicated structure than the slits with metallic vanes. 
     Therefore, the displacement of the vanes is also more complex, in particular if the slits are required to be installed under vacuum or in a controlled atmosphere, such as helium (He). 
     Moreover, the fabrication method employed by Youli &amp; al., namely the slicing of a vane from a silicon wafer, generates a surface state of the monocrystalline structure vane which could lead to spurious scatterings: the benefit of the hybrid slit would thus be lost. 
     An objective of the invention is to propose a simplified optical device comprising at least one device for collimating an X-ray beam exhibiting the advantages of a hybrid slit without exhibiting at least one of the drawbacks thereof. 
     Another objective of the invention is to propose a collimation device for an X-ray beam, in particular adapted to be implemented in this optical device. 
     An objective is further to propose a collimator of an X-ray beam, in particular intended to be used in this collimation device. 
     To achieve at least one of these objectives, the invention proposes a collimation device for an X-ray beam, characterized in that it comprises an enclosure intended to be placed under vacuum or controlled atmosphere, the enclosure comprising an entrance and an exit for the beam as well as at least one plate made of a material with diffracting periodic structure, said plate comprising two principal faces and at least one aperture broadening out between said faces. 
     The collimation device will be able to provide other technical characteristics, taken alone or in combination:
         one of the principal faces of said at least one plate being an upstream face, with reference to the direction of propagation of the beam, and the other being a downstream face, the aperture widens out from the upstream face to the downstream face of the plate;   said at least one plate made of material with diffracting periodic structure is arranged at the level of the exit of the enclosure;   there is provided, at the level of the entrance of the enclosure, at least one other plate made of a material with diffracting periodic structure, this other plate comprising two principal faces and at least one aperture broadening out between said faces;   one of the principal faces of said at least one other plate being an upstream face, with reference to the direction of propagation of the beam, and the other being a downstream face, the aperture widens out from the upstream face to the downstream face of the plate;   the two plates are identical;   the two plates exhibit different apertures;   the acute angle θ formed between a direction D of broadening out of the aperture and one of said principal faces is between 10° and 80°;   the angle θ is equal to the angle between two crystalline planes of the material of diffracting periodic structure forming the plate;   the principal faces of the plate correspond to the {100} plane of the monocrystalline material and the faces of the aperture connecting said principal faces of this plate correspond to the {111} plane;   the or each plate is made of a monocrystalline material;   the or each plate is made of a material chosen from among silicon or germanium.       

     The invention also proposes an optical device for analyzing a specimen by scattering of an X-ray beam, characterized in that it comprises a device for collimating the beam according to the invention. 
     The optical device will be able to provide other technical characteristics, taken alone or in combination:
         an X-ray source;   the X-ray source produces a monochromatic beam;   another enclosure intended to be placed under vacuum or controlled atmosphere, this other enclosure, arranged downstream of the specimen, comprising a means for stopping the X-ray beam;   a detector, arranged downstream of the other enclosure.       

     The invention further proposes a collimator for an X-ray beam, characterized in that it comprises several parts, each part, made of a material with diffracting periodic structure, comprising at least one aperture broadening out in the thickness of this part, the faces of the aperture formed by the assembly of apertures of each part of the collimator forming a sawtooth structure along the longitudinal axis of this aperture. 
     The collimator will be able to provide other technical characteristics, taken alone or in combination:
         each of its parts is formed of a plate, the plates being adjoining;   the plates are identical.       

     Finally, the invention proposes a use, in the guise of collimator for an X-ray beam, of at least one plate made of a material with diffracting periodic structure, said plate comprising two principal faces and at least one aperture broadening out between said faces. 
     This use will also be able to provide:
         a use in which the acute angle θ formed between a direction D of broadening out of the aperture and one of said principal faces is between 10° and 80°;   a use of several identical plates adjoining one another.       

    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other characteristics, aims and advantages of the invention will be stated in the description detailed hereinafter given with reference to the following figures: 
         FIG. 1  is a schematic of an optical device for implementing a Small Angle X-Rays Scattering (SAXS) technique; 
         FIG. 2  is a sectional view of a vane at the level of a slit interacting with the x-ray beam for collimation of the x-ray beam; 
         FIG. 3  is a schematic of a hybrid vane including a metallic component and a monocrystalline component; 
         FIG. 4  is a schematic showing an exploded perspective view of an optical device according to the invention including an enclosure through which the x-ray beam transits; 
         FIG. 5  is a sectional view of the enclosure depicted in  FIG. 4 ; 
         FIG. 6  is a magnified sectional view of the enclosure depicted in  FIG. 5 ; 
         FIG. 7(   a ) is a perspective view of a plate, according to the invention, made of a material of monocrystalline structure and having therein an aperture; 
         FIG. 7(   b ) is a sectional view of the plate depicted in  FIG. 7(   a ); 
         FIG. 8(   a ) is a partial sectional view of an enclosure for installation in the device of  FIG. 4 , comprising at the level of its end a collimator including multiple plates; and 
         FIG. 8(   b ) is a magnified view of the collimator of  FIG. 8(   a ). 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     An optical device  100  for analyzing a specimen  105  by X-ray scattering according to the invention is represented in  FIG. 4 . 
     This optical device  100  comprises a source  101 ,  102  of X-rays, producing a monochromatic beam. This source  101 ,  102  comprises, in a known manner, the actual source  101  of X-rays and a monochromator mirror  102 . 
     In this instance, the actual source  101  of X-rays is a point source, but it could be otherwise, for example a line source. Moreover, the source  101 ,  102  need not be monochromatic, in accordance with the definition provided above. 
     Throughout the description which follows, the terms “upstream” and “downstream” will be used with reference to the direction of propagation of the X-ray beam. 
     Downstream of the source  101 ,  102  of X-rays, the device comprises a first enclosure  110  intended to be evacuated or under a controlled atmosphere, such as or helium (He). 
     This first enclosure  110  comprises an entrance and an exit for the beam, at the level of each of which is arranged at least one plate  104 ,  104 ′ made of a material exhibiting a diffracting periodic structure according to the invention. 
     Generally, this diffracting periodic structure will be a monocrystalline structure. 
     These plates  104 ,  104 ′ are preferably mounted against the end walls  120 ,  121  of the enclosure  110 , inside the enclosure  110 . The positioning of these plates  104 ,  104 ′ is therefore easy. These walls  120 ,  121  form moreover, respectively, the entrance for the X-ray beam and the exit for said beam. 
     This enclosure  110  is represented in a sectional view in  FIG. 5 . Moreover, a plate  104  made of a material of diffracting periodic structure according to the invention is represented in  FIG. 7 . 
     Each plate  104 ,  104 ′ comprises two principal faces, and more precisely an upstream face  104   a ,  104 ′ a  and a downstream face  104   b ,  104 ′ b  as well as an aperture  104   c ,  104 ′ c  widening outing out between the upstream face and the downstream face of the plate considered. 
     As is represented in the appended figures, the plate  104 ,  104 ′ is arranged in such a way that the aperture  104   c ,  104 ′ c  broadens out from upstream to downstream, with reference to the direction of propagation of the beam. 
     However, the same plate  104 ,  104 ′ could be arranged in the opposite direction, that is to say so that the aperture  104   c ,  104 ′ c  narrows from upstream to downstream, with reference to the direction of propagation of the beam. 
     The thinning of the plate avoids the reflection of the X-rays of the beam which propagate at small angles, i.e. at grazing incidence. 
     Moreover, the acute angle θ formed between a direction D of widen outing out of the aperture and any one of the upstream or downstream faces of the plate can be between 10° and 80°. The angle θ is for example represented in  FIG. 6 . 
     In particular, the angle θ may be equal to the angle between the crystalline planes {100} and {111} of the material forming the plate  104 . This characteristic may be obtained when the method for fabricating the plate, of chemical nature, is wet anisotropic etching. Indeed, with this method, the chemical attack of the material takes place between the {100} and {111} crystalline planes. The surface state obtained is thus of very good quality. 
     The notations {100} and {111} correspond to the Miller indices. They make it possible to designate the planes in a crystalline material. These indices are well known to a person active in the field of crystallography and commonly accepted. 
     In the case of silicon, it is possible to use a solution of potassium hydroxide (KOH). As a variant, it is also possible to use a process which is less selective relative to etching between the {100} and {111} crystalline planes, by using a solution of tetramethylammonium hydroxide (TMAH). 
     Moreover, the widen outing out of the aperture  104   c ,  104 ′ c  may be referred to as uniform. The expression uniform widening out should be understood to imply that the change of dimension that the aperture undergoes between the upstream face and the downstream face of the plate takes place according to a homothety. The center O corresponds to the intersection between the axis A passing through the centers C 1 , C 2  of the aperture at the level, respectively, of the upstream and downstream faces of the plate with the axis of direction D mentioned hereinabove. It will be possible to refer to  FIG. 7(   a ). 
     Preferably, the upstream faces  104   a ,  104 ′ a  or downstream faces  104   b ,  104 ′ b  of the plate  104  made of a material of diffracting periodic structure correspond to the {100} plane of this structure. The faces of the plate that are inclined with respect to the upstream and downstream faces then correspond to the {111} plane of the structure. 
     As a variant, a mechanical method could be employed to define an angle in the range mentioned hereinabove. 
     By thus arranging two plates, one  104 ′ at the entrance of the enclosure  110 , the other  104  at the exit of the enclosure  110 , an X-ray collimator is then obtained. 
     The plate  104 ′ can for its part be inserted in place of the plate with slits  12  of the device according to the prior art represented in  FIG. 1 , so as to collimate the beam without generating spurious scattering. The plate  104  then avoids any spurious scattering on the collimated beam and can also improve collimation, before the beam strikes the specimen  105 . 
     The plates  104 ,  104 ′ thus exhibit the same functions as a hybrid slit proposed in document D 1 . 
     Downstream of the specimen  105 , the optical device  100  comprises already known means of the optical device represented in  FIG. 1 . This entails a second enclosure  106  also intended to be under vacuum (or under a controlled atmosphere) comprising, at its opposite end from the entrance of the beam in the enclosure  106 , a means  107  for stopping the beam. 
     Finally, the optical device  100  comprises a detector  108 , arranged downstream of the second enclosure  106 . 
     The plates  104 ′,  104  arranged respectively at the entrance and the exit of the first enclosure  110  may be identical. 
     The plates  104 ,  104 ′ can moreover be made of silicon, the angle θ between the {100} and {111} crystalline planes then being about 54.7° if a solution of KOH for example has been used. The shape of the aperture is then defined by the crystalline planes. 
     Here, the aperture of a plate  104 ,  104 ′ may be square or rectangular and the broadening out between the upstream face and the downstream face is given by the angle Θ. For example, when this aperture is square, its side, at the level of the upstream face  104   a ,  104 ′ a  of the plate  104 ,  104 ′, may be 1 mm. 
     Other shapes of apertures are conceivable. It is for example possible to refer to the article “ A flux and Background - optimized version of the NanoSTAR small - angle X - ray scattering camera for solution scattering ”, Jan Skov Pedersen, J. of Applied Crystallography (2004), 37, pp. 369-380. 
     A plate  104 ,  104 ′ can exhibit a dimension of about 10 mm*10 mm, and a thickness of about 1-2 mm. 
     As a variant, they may be different, especially because their apertures  104   c ,  104   c ′ are different. Indeed, the apertures  104   c ,  104   c ′ of these plates can differ by their dimensions and/or by the value of the angle θ. 
     Also as a variant, each plate  104 ,  104 ′ may be made of a material of diffracting periodic structure, other than silicon, in this instance monocrystalline. For example, it may involve a monocrystalline structure like germanium. 
     The optical device represented in  FIG. 4  can form the subject of variant embodiments. 
     A variant embodiment can consist in replacing the assembly formed by the collimation means  13  and the anti-scattering slits  15  of the optical device according to the prior art represented in  FIG. 1  by a plate  104  according to the invention. 
     This plate  104  is then arranged at the exit of an enclosure intended to be under vacuum (or under a controlled atmosphere), as represented in  FIG. 6 , so as to form a device for collimating X-rays. On the other hand, this enclosure does not comprise a plate according to the invention at the level of its entrance, but this entrance is preceded by the slits  12  and, if appropriate, the calibrated attenuator (not referenced) as illustrated in  FIG. 1 . 
     Another variant embodiment of the invention is represented in  FIG. 7  or  8 . 
     According to this variant, there is provided a collimator of the X-ray beam comprising several plates made of a monocrystalline material, adjoining one another so that said at least one aperture of each plate widens out between the upstream face and the downstream face of the plate or the converse. 
     These adjoining plates will generally be identical. 
     The benefit of this arrangement is to limit, or indeed to eliminate, the transmission of the beam  200  through the monocrystalline material, at the level of the outline of the aperture. 
     Indeed, when a single plate is provided, it is understood that the plate thickness e f  encountered by the beam  200  is small at the level of the outline of this aperture. By adjoining several plates, the plate thickness ultimately encountered by the beam  200  at the level of this outline of the aperture, which exhibits a sawtooth shape along the longitudinal axis of the aperture, is thus increased. 
     The collimation of the beam  200  is thereby improved, by transmitting only the beam passing through the space E left by the aperture, on the upstream side of the plate. 
     This is particularly beneficial if the plate is made of silicon. When the plate is made of germanium, which is a denser material than silicon, this arrangement will exhibit particular benefit for the energy range of the X-rays from 15 keV to 30 keV. 
     It should be noted that, in  FIG. 7 , five identical plates adjoining one another have been represented. The person skilled in the art will understand that this is merely an illustration and that the number of plates to be considered will depend especially on the energy of the beam, the thickness of a plate and the nature of the monocrystalline material forming this plate. 
     The applicant has carried out measurements and performed a few calculations. 
     It was found that for an X-ray beam of 8 keV, the superposition of three identical silicon plates each about 1-2 mm thick was equivalent to using a germanium plate, of the same thickness. For an X-ray beam of 17 keV, it is then necessary to adjoin fifteen of these same silicon plates to obtain behavior equivalent to a germanium plate of the same thickness. 
     The adjoining of plates may be envisaged at each end of the enclosure  110  represented in  FIG. 5 . This can also be envisaged solely at the entrance or solely at the exit of this enclosure  110 , in particular if this exit alone comprises a plate  104  in accordance with the invention. 
     Alternatively, it is possible to provide a collimator not comprising adjoining plates, but made from a single piece each of whose various parts  104   1 ,  104   2 ,  104   3 ,  104   4 ,  104   5  can be regarded as a plate  104  such as described above. Thus, the faces of the aperture  10 C formed by the assembly of apertures of each part of the collimator forms a sawtooth structure along the longitudinal axis A 104  of this aperture  104 C. The shape of this aperture  104 C, for example represented in  FIG. 8 , is thus similar to that obtained by adjoining several plates  104 , as is represented in  FIG. 7 . 
     The plate  104 ,  104 ′ used within the framework of the invention ultimately presents several advantages with respect to a hybrid slit such as presented in document D 1 . Indeed, the structure is simple, made from a single crystal. Moreover, this plate will usually be fixed at the ends of an enclosure under vacuum or under a controlled atmosphere, so that the manipulator will not be required to perform adjustments: the sole adjustment being the initial positioning of the plate. Furthermore, the fabrication method generally employed, chemical, generates an excellent surface state, which limits the risks of spurious scatterings.