WO2013093334A1

WO2013093334A1 - Device for filtering x-rays

Info

Publication number: WO2013093334A1
Application number: PCT/FR2012/052986
Authority: WO
Inventors: Philippe TROUSSEL; Bruno VILLETTE
Original assignee: Commissariat A L'energie Atomique Et Aux Energies Alternatives
Priority date: 2011-12-20
Filing date: 2012-12-19
Publication date: 2013-06-27
Also published as: FR2984584A1

Abstract

A device for filtering x-rays comprising: a Bragg mirror able to filter x-rays by reflection depending on their energy, said interference filter comprising a stack of refracting layers forming a multilayer structure, characterised in that the multilayer structure has an aperiodic structure, the spectral response of the Bragg mirror being dependent on the characteristics of the multilayer structure.

Description

DEVICE FOR FILTERING X-RAYS

The present invention relates to an X-ray filter device.

It further relates to a method of manufacturing an X-ray filtering device.

The invention applies to X-ray instrumentation domains such as spectrometry and X-ray imaging. By way of example, the following applications can be cited in a nonlimiting manner:

the physics of the dense and hot plasmas produced, for example, by the interaction of a high power laser beam with the material, the plasmas returning a large part of the absorbed energy in the form of X-radiation;

- medical imaging;

- non-destructive X-ray imaging

- X-ray analysis on synchrotron sources.

A device for filtering an X-ray beam, used in these applications, must meet two specifications: having a bandwidth with a given width and a spectral response with a specific profile.

In the X-ray field, it is known to filter X-radiation using screens of absorbent materials. Indeed, in this area, below an energy of 15 keV, the dominant phenomenon for X-ray filtration is photoelectric absorption. This photoelectric absorption is manifested by discontinuities in the transmission of screens, at energies characteristic of the atomic levels of the material.

The oldest broadband spectrometry method, still in use today, involves this principle of filtration. It has been used by P.A. Ross and is called "Ross Filters" or "Balanced Screens". Figure 1 shows the diagram of an example of such a Ross filter. A band of energy is analyzed using two measurement channels 1 1, 12, each consisting of a screen, respectively F1 and F2; and a detector, respectively D1 and D2. The two detectors D1 and D2 have identical responses and the screens F1 and F2 set the width of the spectral band. The thicknesses of the screens F1 and F2 are chosen to allow the same transmission so that the difference of the signals is significant of the energy contained in the analyzed band. This method has some disadvantages. First, it uses two channels for each measurement. In addition, it is a measure of difference therefore unreliable when the discontinuity energies are close. Finally, the balancing of energies is not possible at all energies. We also see that there remains a spectral component of high energy that comes from the filter and which is not eliminated because the transfer function of such a system (filter + detector) is not completely zero beyond the cutoff energy selected. It rises, presenting a parasitic response of triangular shape as illustrated in Figure 2.

A second interesting filtering method consists in adding an X-ray mirror because the mirror eliminates the X-hard parasitic component transmitted by the filter and acts as a low-pass energy filter. Each path then results from three elements (filter + mirror + detector). This measurement principle is therefore an improvement over the previous system.

In these two methods, the spectral response of the measurement channel must integrate that of the detector and applies in the case where we can find materials whose absorption threshold is suitable. Few elements have discontinuities and, therefore, the scope is limited to the small number of existing materials. The energy position of this spectral band is therefore dependent on the filter. Moreover, the shape of this response is imposed, non-constant and a de-convolution of the signal is essential, which introduces a measurement uncertainty.

The realization of periodic multilayer interference mirrors that can be used as Bragg reflectors makes it possible to be more selective or adjustable depending on the energy used, and to define a spectral band (monochromator) located in a broad spectral range. As a reminder, a Bragg reflector or a Bragg grating is a structure in which layers of two different refractive index materials alternate. The layers make it possible to reflect, thanks to constructive interference phenomena. This alternation of layers causes a periodic variation of the effective refractive index in the reflector.

The main interest of Bragg reflectors lies in the flexibility of choice of the nature and constituents of the X-ray mirror. This choice is optimized according to the characteristics of the source X to be analyzed. Such a device is described, for example, in US4684565. Bragg reflectors can be made on supports of various shapes to make selective and focusing mirrors. These devices are known as the "Goebbels" mirror if they are placed on a parabolic support. They can be subject to various optimizations such as having a lateral gradient or depth to adapt the Bragg grating to the local curvature of the support to optimize the yield. However, there are three limitations for these systems:

the spectral width of the maximum possible band is limited and incompatible with a broad band operation (that is to say of spectral resolution ΔΕ such that Ε / ΔΕ is close to 1),

these reflective Bragg systems diffract (at the same angle) some higher order harmonics of the energy out of the bandwidth concerned and reduce the spectral purity arriving at the detector,

wavelength adaptability only works by changing the angle of incidence.

To try to remedy the limitations of Bragg mirrors, devices called "supermiroirs" have been designed. They are interferential mirrors constituted by stacking several Bragg mirrors. They make it possible to reflect several incident wavelengths simultaneously by "adding" the contribution of several Bragg peaks as illustrated in FIG. 5. These "supermirors" were applied in the case of visible light with very little absorbent materials, in the case of neutrons and in the field of X-hard.

In this technique, stacks of different thickness are produced, corresponding to each of the wavelengths that one wants to reflect. However this is done to the detriment of a decrease in reflectivity. These devices have therefore been developed by multiplying the number of layers necessary for making the stacks of Bragg filters. The spectral profiles of the mirrors obtained show strong oscillations due to interference between the Bragg filters. It can be seen that these systems make it possible to target some energy lines (those corresponding to the Bragg filters), but do not make it possible to create a device whose response is a continuum.

This technique, initially developed with low-absorbency materials that can be used in the visible range, is even more difficult to implement in the X-domain because the multiplication of the number of layers required to make Bragg grating stackings leads to either absorption too much. strong, that is to say a too limited number of lines of reflected energies. In the X domain, the necessary use of highly absorbent materials has therefore not allowed the realization of continuous broad spectrum supermirrors to date. At best, those skilled in the art have been able to make Bragg grating stacks with the limitations indicated above, or Bragg mirrors having a gradient in order to adapt a Bragg grating to a non-plane support. .

These devices do not make it possible to obtain a mirror with a continuous wide spectral band with a specific profile in the X-ray field.

One of the aims of the present invention is to overcome disadvantages of the state of the art mentioned above.

It proposes for this purpose an X-ray filtering device in an energy range of 100eV to 50keV, preferably 400eV to 20keV comprising: an interference mirror capable of filtering X-rays, by reflection of Bragg, according to their energy, said interference mirror comprising a stack of refraction layers forming a multilayer structure, characterized in that the multilayer structure has an aperiodic structure, the spectral response of the interference filter being dependent on the characteristics of the multilayer structure, and in that the multilayer structure comprises at least one set (120_1 ..120_m; 220_1 ... 220_m) composed of at least one layer of a heavy material (A) and at least one layer of a light material (B), said layers being superimposed on one another and further comprising an absorbent filter capable of filtering X-rays due to the total reflection on the interference mirror by absorption at energies lower than an energy defining the lower limit of the bandwidth of the spectral response of the device.

The use of an aperiodic structure makes it possible to considerably broaden the spectral response of the mirror, while limiting the number of layers necessary in order to minimize the loss of reflectivity of the mirror with respect to a multi-Bragg solution.

According to one embodiment, the interference mirror is capable of filtering grazing incidence X-rays, for example X-rays having an angle of incidence between 0.2 ° and 10 °.

According to one embodiment, the heavy material (A) is chosen from the group comprising Co, Cr, Cu, Fe, Bi, V, Ni and Zn and the light material (B) is chosen from the group comprising B ₄ C, SiC, Se, Ti.

According to one embodiment, the multilayer structure comprises a plurality of sets (120), the respective total thickness of each set varying as a function of the position of said set in the depth of the multilayer structure.

According to one embodiment, the respective ratio of the thickness (d _A ) of the layer of a heavy material (A) to the total thickness (d) of each set is chosen to define the spectral response of the filter (1). 10, 210).

According to one embodiment, at least one of the layers is composed of several different materials. According to one embodiment, the characteristics of the multilayer structure on which the spectral response of the interference filter (110; 210) depends comprise at least one of the number of layers, the thicknesses of the layers and the materials constituting the layers.

According to one embodiment, the width ΔΕ of the spectral band of the device is such that 0.5 <Ε / ΔΕ <4, E representing the central energy of the reflected X energy band.

A device according to one embodiment further comprises a support on which the multilayer structure is disposed, the carrier being configured to function as a focusing optic.

A second aspect of the invention relates to a method of manufacturing an X-ray filtering device comprising identifying the desired spectral response of the filtering device; the configuration, according to the identified spectral response, of a multilayer structure comprising a plurality of refraction layers, the multilayer structure having an aperiodic structure characterized in that the configuration comprises the choice of characteristics of the multilayer structure to obtain the response spectral identified.

In one embodiment, the choice of characteristics of the multilayer structure comprises the choice of at least two materials including a heavy material and a light material for the layers of the multilayer structure.

In one embodiment, the choice of characteristics of the multilayer structure comprises the number of layers of the multilayer structure and / or the thicknesses of the layers.

According to one embodiment, the method further comprises a preliminary definition of a preliminary multilayer structure comprising a superposition of a plurality of periodic stacks producing interference at the energies of the respective Bragg peaks in the band of the identified spectral response.

In one embodiment, the method according to the second aspect comprises a step of equalizing the spectral response of a system of analysis and in which the spectral response identified is calculated from the spectral response of the elements of the analysis system in order to obtain a constant spectral response.

Other features and advantages of the invention will appear in the description below, illustrated by the accompanying drawings, in which:

- Figure 1 schematically shows a Ross-type filter of the prior art;

FIG. 2 represents an example of a spectral response of the prior art;

FIG. 3 schematically represents an X-ray filtering device according to a first embodiment of the invention;

FIG. 4 diagrammatically represents the preliminary definition according to one embodiment of the invention of a filter composed of several periodic stacks;

FIG. 5A represents the spectral response of the filter of FIG.

4;

FIG. 5B schematically represents a possible spectral response of an X-ray filtering device according to one embodiment of the invention;

FIG. 6 schematically represents an X-ray filtering device according to a second embodiment of the invention.

An X-ray filtering device according to a first embodiment is shown diagrammatically in FIG. 3. This filtering device 100 comprises an interference mirror 110 able to filter selectively by Bragg reflection, X-rays according to their energy. . The interference filter consists of a stack of a plurality of layers 11 1 to 1 1 1 n of different refractive indices. The layers 11 1_1 to 11_n are stacked to form a multilayer structure having an aperiodic structure. The spectral response of the filtering device 100 can be chosen according to the characteristics of this multilayer structure. The multilayer structure in this first embodiment consists of sets of monolayers 120_1 ... 120_m. Each set is composed of two individual monolayer materials A and B alternately heavy and light and of respective thickness dA, of. The total thickness of each set d = d _A + d _B changes with its depth in the multilayer structure 1 10. Thus, for example in the embodiment of FIG. 3 d ₃ > d ₂ > di.

The choice of the number n of layers of the multilayer structure 1 10, and their respective thickness is made according to the desired spectral response of the filtering device 100.

A method for determining the multilayer structure according to at least one embodiment of the invention will now be described.

In a first step, the shape of the desired spectral response for the filtering device is identified.

Then a choice of at least two monolayer materials A and B, respectively heavy and light, is made. A heavy material may be selected from the group consisting of Co, Cr, Cu, Fe, Bi, V, Ni and Zn and a light material may be selected from the group consisting of B ₄ C, SiC, Se, Ti. For example, the multilayer structure may consist of sets of layers 120 to two materials of type Co / B ₄ C; Co / SiC; Cr / B ₄ C; Cr / Sc; Cu / B ₄ C.

Then a preliminary definition is made of a Bragg-type supermirror as illustrated in FIG. 4, consisting of the superposition of several periodic stacks with a given period d1> d2> ...> dn producing interference at the energies of the respective Bragg peaks. E1 <E2 <... <In the requested bandwidth.

An exemplary theoretical response of this mirror illustrated in FIG. 5A is calculated by adjusting the parameters d1, d2, ... dn in order to make a first approximation of the shape of the desired spectral response.

Then the cyclic ratio (weight ratio heavy element on period (total thickness of the assembly)) of each period with Π, Γ ₂ , ... Γ _η the respective ratios of each bilayer assembly 120 1, ..., is modified. 120 m. Thus, an optimization of 2n parameters is obtained which offers possibilities of adjusting the shape of the spectral response of the mirror such as its selectivity, its flatness, or its optimization with respect to a defined profile. To do this an optimization algorithm can be used, such as, for example, that published by AA Tikhonravov Applied optics 32, 5417-5426 (1993) transposed in the X domain (but of course other methods can to be applied). In the X-ray domain between 50 eV and 100 keV, the photoelectric effect is predominant: the transposition in this area is done by taking into account the scattering parameters of the elements instead of conventional optical indices in the visible range.

The preliminary design of departure is a thick layer close to the total thickness of the final stack of the multilayer structure 1 10. The algorithm determines the best place to insert a new layer of very low thickness and reiterates the process. It makes it possible to optimize the thickness of the layers but also their number and their order.

In some embodiments of the invention, it may be desirable to use more than two different materials to achieve the multilayer structure to improve the reflectivity of the filter device.

Two approaches are possible. In a first approach layers may consist of a mixture of several different materials. In this case, the method described above is applied, in which the characteristics to be taken into account will be those of the mixture, rather than a pure material, for the layers concerned.

In a second approach, the method according to another embodiment includes making assemblies having a number of layers greater than 2. Figure 6 illustrates an X-ray filtering device 200 according to another embodiment of the invention. In this embodiment, the multilayer structure 210 comprises sets 220_1 to 220_n of tri-layers consisting of 2 layers of light materials B and C and a layer of heavy material A. The multilayer structure 210 may consist of sets 220 of three layers of different materials such as: Cr / B ₄ C / SiC or Cu / Co / B ₄ C. Of course any other combination can be used.

The advantage of this approach is to better control the interfaces between certain materials that are likely to be disturbed by an interdiffusion phenomenon.

In this case, the initial thicknesses of the dA, d _B , d _c , ... layers of each of the n initial periodic stacks are fixed and Π, Γ _2ι ... Γ _η are defined as being the respective ratios of the thickness of the (or heavy) material (s) over the period d. The method described above for the first embodiment can then be applied with the parameters thus defined.

In another embodiment the two approaches can be combined if necessary, the only limits being those of the technological mastery of the realization of the layers. In practice, the use of mono or bi-material bilayers or tri-layers already gives many degrees of freedom to meet many needs.

In certain embodiments of the invention, the multilayer structure may be deposited on a flat support or on a non-planar support which may for example act as a focusing optic.

In other embodiments of the invention, the filtering device may further comprise a thin absorbent filter to better limit the spectral response of the device between two energies, [E1, E2]. The lower terminal E1 is provided by one of the elements of the multilayer mirror whose material is chosen to have an absorption discontinuity corresponding to this value. The low energy components <E1 reflected by the mirror effect of the filtering device (total reflection phenomenon) can be eliminated by a judicious choice of the angle of attack (or the angle of incidence of the X-rays) allowing to push the bottom of the field of this reflection to the level of the cut of the mirror. For example, the interference mirror may be configured to operate at an angle of attack between 0.2 ° and 10 °.

The thin filter strengthens this elimination. The upper bound E2 is given by the absorption discontinuity of one of the elements of the filtering device. Known evaporation or vacuum spraying methods can be used for the manufacture of the filtering device. The improvement of these techniques makes it possible today to manufacture this type of complex spectral function component.

Example of a practical case:

An X mirror operating at an angle of attack of 1.5 ° gives the desired profile for a spectral analysis channel of an X-ray spectrometer. Cr / Sc-type supermirror deposits were used with nearly 200 alternations of layers. to reflect 10% of the X-rays in the energy band of 2 to 4 keV and with a reflectivity close to zero outside the spectral range targeted thanks to the combination with a 33μΐτι filter with a thickness of C ₁₀ H80 ₄ . In this example, the periods (the total thicknesses of the layer sets) of the multilayer vary between 60 A and 92 A.

The X-ray filtering device according to the various embodiments of the invention can be used in broadband X-ray spectrometry or imaging from polychromatic sources for energies of between 100 eV and 50 keV, and in particular including energies. between 400 eV and 20 keV

The advantage of this system according to some embodiments of the invention which can be considered as "a spectral response equalizer" is to reflect a spectral window with a wide band and to allow the composite system that integrates it (for example a spectrometer or an imager) to have a constant spectral response or responding to a desired profile. A device according to embodiments of the invention may thus be sized to have a flat spectral response over a wide range of wavelengths, or to have a spectral response compensating the spectral response of other elements of a system analysis, or to have a specific response to select one or more spectral bands and to exclude others, always in the X-ray field mentioned above. In spectrometry it is thus possible to directly restore the radiated power of an X source in a broad spectral band, corrected by the response of the constituent elements of the spectrometer.

In X-ray imaging (or radiography) the control of the bandwidth makes it possible to improve the quality and the contrast of the image and the control of the spectral profile at the output of the system makes it possible to improve the homogeneity of the image. The use of several spectral bands makes it possible to simultaneously obtain several images at different energies of the same object with different contrasts (different transmissions of the object).

On a source X, the device according to the embodiments of the invention allows a better knowledge of the source and better control thereof by controlling its emission spectrum. This is important in the medical field where the control of received doses of radiation is fundamental.

Claims

1. An X-ray filtering device (100; 200) in an energy range of 400eV to 20keV, comprising:

an interference mirror (1 10; 210) capable of filtering X-rays by Bragg reflection as a function of their energy, said interference mirror comprising a stack of refraction layers (1 0 1 1 1 1 n;

210_1 ... 210_n) forming a multilayer structure;

wherein the multilayer structure has an aperiodic structure, the spectral response of the interference mirror (110; 210) being dependent on the characteristics of the multilayer structure and the multilayer structure comprises at least one set (120_1 .. 120_m; 220_1 ... 220_m ) composed of at least one layer of a heavy material (A) and at least one layer of a light material (B), said layers being superimposed on one another and further comprising an absorbent filter capable of filtering X-rays due to the total reflection on the interference mirror by absorption at energies lower than an energy defining the lower limit of the bandwidth of the spectral response of the device.

2. A device according to claim 1 wherein the interference mirror is capable of filtering X-rays having an angle of incidence between 0.2 ° and 10 °.

3. A device according to claim 1 or 2 wherein the heavy material (A) is selected from the group consisting of Co, Cr, Cu, Fe, Bi, V, Ni and Zn and the light material (B) is selected from the group consisting of group comprising B ₄ C, SiC, Se, Ti.

4. A device according to any one of claims 1 to 3 wherein the multilayer structure comprises a plurality of sets (120), the respective total thickness of each set varying according to the position of said set in the depth of the multilayer structure.

5. A device according to any one of claims 1 to 4 wherein the respective ratio of the thickness (dA) of the layer of a heavy material (A) to the total thickness (d) of each set is chosen. to define the spectral response of the interference mirror (1 10, 210).

6. A device according to any one of the preceding claims wherein at least one of the layers is composed of several different materials.

A device according to any one of the preceding claims wherein the characteristics of the multilayer structure on which the spectral response of the interference mirror (110; 210) depends comprise at least one of the number of layers, the thicknesses of the layers and the materials constituting the layers.

8. A device according to any preceding claim wherein the width ΔΕ of the spectral band of the device is such that 0.5 <Ε / ΔΕ <4, E representing the central energy of the X energy band reflected.

9. A device according to any one of the preceding claims further comprising a support on which the multilayer structure is disposed, the carrier being configured to function as a focusing optics.

A method of manufacturing an X-ray filtering device according to any one of claims 1 to 9 comprising

identifying the desired spectral response of the filtering device; configuring, according to the identified spectral response, a multilayer structure having a plurality of refractive layers, the multilayer structure having an aperiodic structure;

wherein the configuration comprises the choice of characteristics of the multilayer structure to obtain the identified spectral response.

1. A method according to claim 10 wherein the choice of characteristics of the multilayer structure comprises the choice of at least two materials including a heavy material and a light material for the layers of the multilayer structure.

12. A method according to claim 10 or 11 wherein the choice of characteristics of the multilayer structure comprises the number of layers of the multilayer structure and / or the thicknesses of the layers.

A method according to any one of claims 10 to 12 including a preliminary definition of a preliminary multilayer structure comprising a superposition of a plurality of periodic stacks producing interference at the energies of the respective Bragg peaks in the band of the spectral response identified.

A method according to any one of claims 10 to 13 including a step of equalizing the spectral response of an analysis system and wherein the identified spectral response is calculated from the spectral response of the elements of the analysis system. analysis to obtain a constant spectral response.