Multichannel analytical x-ray system on the basis of bright microfocus source
he methods of X-ray diagnostics of materials and nanostructures often provide the basis for the analytical framework of the up-to-date high-tech production and one of the major tools in the development of new materials and products based on them. In this context, one of the key objectives associated with diagnosing materials is to obtain the most complete information about the properties of the tested or controlled object by various methods and compare the results. The modern X-ray instrument making industry proposes to use dedicated analytical systems for solving specific measuring tasks, e.g. X-ray fluorescence spectrometers to determine the elemental composition; small-angle scattering to determine the parameters of inhomogeneous nano objects; absorption spectrometry to study the energy structure of the deep levels in the materials; diffractometry to determine the structural parameters and phase analysis; the projection microscopy and imaging for visualising the internal structures of objects; reflectometry for studying the layered nanostructures and interfaces. Each of these systems has its own light source of radiation, measurement platform and software; it is not possible to combine them into a single measurement system for structural reasons.
This paper proposes a new concept, which is to create a multi-channel X-ray analytical system to ensure the implementation of all major X-ray diagnostic methods based on the total bright microfocus source and a single automated control system.
Choosing the source
We consider the basic requirements to the X-ray source for the proposed multi-channel system. They should not be interpreted as essential but their failure would lead to a sharp deterioration of characteristics of the main user of the system in terms of functionality, performance, measurement sensitivity and accuracy, size and power consumption.
First, the source should be microfocal that allows focusing of the radiation for the local control of parameters and obtaining the directed flows of high intensity. As practice shows, for the effective application of X-ray optics it is essential that the size of the focus meets the requirement Rs ≤ 10 µm. The brightness of the source should provide the direct or focused stream of the monochromatic radiation in a solid angle (Ωs) about 1 mrad at least 109-1010 photons/s. It is necessary for measurements in a wide dynamic range of signal intensity, high sensitivity as well as research and control performances.
Secondly, the source should provide the generation of intense spectral lines and the continuous part of the spectrum, or should be able to reconfigure the power of the generated radiation. Monochromatic lines are required for the accurate determination of various structural parameters; the continuous part of the spectrum or the possibility of readjustment are necessary particularly in the X-ray absorption spectroscopy to determine the orientation and parameters of the reciprocal crystal lattice.
Third, the maximum energy of the Emax range should be around 100 keV which allows visualisation and 3D-reconstruction of the internal structure of both organic and inorganic objects thereby significantly expanding the scope of the system. At an energy range of E <6 keV achieved is efficient excitation of the X-ray fluorescence of light elements, and it is possible to obtain high-contrast images with high organic objects of a small size. Therefore, it is correct to take the energy range of 6 keV to 100 keV as a sufficient operating range. To accurately determine the structural material parameters, the range of the source should have intense spectral lines with the power of about 10 keV that corresponds to the emission wavelength of 0.1 nm, i.e. a typical order of inter-planar spacing in various materials.
Fourthly, it is necessary that the dimensions of the X-ray source allow placing it in a common laboratory room and set at a distance of 5–50 cm from the output window the collimating, focusing and measuring equipment. This provides compactedness for stations and management arrangements.
Fifth, it will be optimal, if the radiation of the X-ray source is directed in the wide solid angle of Ωi = (2–4)π or there is a possibility of quick adjustment of the generation direction in this range of solid angles. This is necessary to simultaneously connect to a source of a set of workstations.
Finally, the source should provide continuous generation of radiation within at least one shift (6–8 hours).
The modern industrial X-ray sources based on standard X-ray tubes and the tubes with rotating anodes do not satisfy the requirements listed above. The stationary anode tubes used for X-ray microscopy and microtomography fail to provide the desired radiation fluxes, and special sources with rotating anodes have the focus size of about 100 µm, which prevents the efficient radiation focusing and obtaining the images with a high spatial resolution. Therefore, it is advisable to consider a number of advanced systems that are at the experimental stage of development.
One of the available laboratory facilities to generate the X-ray spectrum in a solid angle of 2π in a wide spectral range is the irradiation of the target by a focused beam of a femtosecond laser [3,4] that will ensure the high-temperature plasma in the excitation of the target material by the optical pulse. An advantage of such a generation tool is the ease of changing the spectrum by changing the target and the ability to study fast processes in synchronising the X-ray pulse with a predetermined time interval after the initiation of the object. However, the conversion efficiency of transformation of the optical radiation in the X-ray one is low, and it is mainly impulse radiation, unstable and is accompanied by the ablation of material. Therefore, the source based on laser excitation by high-temperature plasma can only be regarded in the future as an independent addition to the analytical system. Petawatt lasers, which are also being developed at the moment and can be used to generate directed X-rays in plasma, are of particular interest; however, they cannot be considered as the available laboratory sources of radiation.
A comparatively effective tool of generating X-rays is the installation using Z- and X-pinch [5–7]. The effect of compression of the current channel under the influence of a magnetic field induced by the strong current in the thin metal wires produces high-temperature plasma emitting the full solid angle in a wide X-ray spectrum with the effective focus size of about 10 µm. Since the radiation pulse is accompanied by complete evaporation of the target, such a system cannot operate in a steady state, and, as in the case of excitation by a femtosecond laser, it should be regarded only as a possible supplement to the analytical system.
The high values of the radiation flux up to 5·1011 photons/s and spectral brightness 2·1012 photons/(s·mm2·mrad2·0.1%) obtained at the present time on the plants, which use the effect of inverse Compton scattering generated by the laser of optical photons with an energy of about 1 eV in the high-energy electrons [8-10]. Calculations show [9-11] that with the help of this type of sources in addressing a number of technical issues achieved can be the flows of 1013–1014 photons/s in the solid angle ωc ~ E0/E, where E and E0 – energy of the electron colliding with an optical photon and the rest energy of the electron respectively. To convert the optical radiation into the X-ray spectrum with a wavelength of about 0.1 nm in inverse Compton scattering it is enough to have the energy of electrons in the range of 10 MeV; that is why such sources are based on compact accelerators that may be placed in laboratory premises. An important advantage of the laser and electron source is the ability to change the generated spectrum band in a wide energy range thereby allowing you to put into practice a variety of spectrometric methods. The reached focus size less than 10 µm makes it possible to use focusing optics and obtaining images of the internal structure with the high spatial resolution. However, at which a typical angle a stream of X-ray photons with the energy about 10 keV is emitted is typically less than 0.01 cp and therefore less than 0.001 share of the total solid angle, therefore this source cannot be used to connect multiple workstations for a variety of complex research by various methods.
The microfocus X-ray source recently developed by the Swedish company Excillum [12, 13] satisfies the above requirements to a source of the multi-channel system. The novelty of the proposed design is that a liquid metal stream based on the alloy Ga (95%) and In (5%) is used as the target irradiated with electrons. The said alloy is in a liquid state at room temperature and has a relatively low vapour pressure at temperatures up to 1000°C enabling direct exposure of the metal stream by a focused electron beam. In this context, due to the large area of contact of the metal with the cooling surface of the heat exchange device, the alloy temperature can be reduced sooner. Thus, it is possible to obtain the energy flux density, when radiation is excited by an electron beam up to 2.5 mW/mm2 and ensure the record spectral brightness of the source.
A simplified diagram of source on the liquid anode is shown in fig.1. The pump creates a continuous circulation of the liquid metal in the direction indicated by arrows. The pressure is 20 MPa and the flow rate of metal is 80 m/s. In the gap shown by the dotted line, the stream with a diameter of 0.2 mm and is taken from the pipeline and is irradiated by an electron beam which is focused by the magnetic lens. Radiation is generated in the solid angle Ωi about 4π. Since the elements of the design of the radiator do not allow the full output from the source of the X-ray flux, then, like in the conventional X-ray tubes, radiation is output through the window of a given size. It should also be noted that part of the X-ray spectrum with the energy of E <30 keV is absorbed into the metal stream, and the source radiates into a solid angle Ωi about 2π in this part of the spectrum. The key technical characteristics of the source are given in table.
Choosing workstation layout
For ease of use of the multichannel analytical system, it is optimal to place the equipment on the horizontal platforms, whereby the metal stream is directed vertically, and the axis of the X-ray beams are focused in the horizontal plane. The area of the energy spectrum E <30 keV is most often used for the measurement of structural materials. However, as mentioned above, in this area of the spectrum the source emits in the solid angle Ωi about 2π, therefore the permissible angular positions of the axes of subject beams should be displaced from the tangent line to the metal stream at the point of excitation by an electron beam in the direction of the source of excitation (fig.2). The angular interval α between axes of the beams 1-6 is formally limited to the need to place at the output windows of the source collimation, alignment and focusing elements of the structure. The technical restrictions on the number of channels are associated with the presence in the radiator body of elements of the equipment, i.e. the electron gun, pipes, valves and gauges of the source parameters. In the hard part of the spectrum E > 50 keV radiation can be additionally put out from the metal stream in the direction of the beams 7 and 8. However, the operational practice shows that for the measurement, the instrument setup and installation of various types of samples it is necessary to allow staff access to any part of the station’s equipment; so it is optimal to place four stations around the source according to the diagram shown in fig.3.
The proposed diagram is also due to the development of workstations in a protective housing as shown in fig.4. The radiation source, as shown in fig.3, is in the central part of the system. The sliding panels of the housing provide the necessary access to the measuring equipment and the ability to install and transfer samples. Individual stations can be controlled with the computers installed in the operating room and outside in the remote access mode. However, the overall coordinated management of the source and individual stations is effected by the computer of the system.
Scheme of formation
of X-ray beams
The main parameters of the beams used in modern measurement systems are their spectrum, shape and intensity. The microfocus X-ray source along with the focusing and collimation systems and the means of spectrum selection allow you to create the most complete set of beams required for a comprehensive diagnosis. Fig.5 shows the main types of beams that can be created in the system:
the cone polychromatic beam (1) for a quick visualisation of the projection images and computer tomography;
a fan-shaped monochromatic beam (2) for the visualisation of the projection images and computer tomography in a predetermined spectral range;
the converging focused monochromatic or polychromatic beam (3) for the local diagnosis of material parameters;
the focussed quasi-parallel beam (4) for the diagnosis of structural materials;
the tightly collimated monochromatic beam (5) for precision measurements of the structural parameters and X-ray measurements.
The cone-shaped and fan-shaped beams 1 and 2 are created by the controlled diaphragms. A fan-shaped beam can be preliminarily monochromated with multilayer X-ray mirrors  or a refracting prism. . The convergent focused monochromatic beam 3 is created by the curved multilayer mirrors  built according to the Montel  or Kirkpatrick-Baez schemes . The Kumakhov polycapillary optics can help obtain a focused polychromatic beam with the focal diameter df of about 10 µm [19, 20]. It should also be noted that in the photon energy of about 10 keV with the source focus diameter of ds ≤ 5 µm the refractive optics in the form of compound refractive lenses can be used [21, 22]. However, the preliminary monochromatisation of the primary radiation is necessary. The focused quasi-parallel beam 4 of high intensity is generated by the parabolic X-ray mirrors. The rigidly collimated monochromatic beam 5 with an angular divergence of Δψ ≤ 10˝ is created by successive reflections from the dislocation-free crystals, such as silicon crystals. Additional schemes of the beam generation and spectrum selection can be obtained by the curved crystals and semi-transparent monochromators  and dispersion prism optics .
Creation of the multi-analytical system proposed in this paper will help solve the urgent problems as follows:
ensure access to a wide range of experts in developing new materials and nanostructures to advanced tools of X-ray diagnostics;
combine basic X-ray diagnosis techniques based on a single automated system;
improve the accuracy and reliability of measurement results;
provide for savings associated with the purchase and maintenance of equipment.
In the following study we will examine various types of workstations of the analytical system and present a number of experimental results also obtained with a source on a liquid anode.