EP2887775A1

EP2887775A1 - Apparatus and method of generating X-rays, in particular for X-ray fluorescence imaging

Info

Publication number: EP2887775A1
Application number: EP13006046.0A
Authority: EP
Inventors: Florian Grüner; Reinhard Brinkmann; Christoph Hoeschen
Original assignee: Deutsches Elektronen Synchrotron DESY; Universitaet Hamburg
Current assignee: Deutsches Elektronen Synchrotron DESY; Universitaet Hamburg
Priority date: 2013-12-23
Filing date: 2013-12-23
Publication date: 2015-06-24
Anticipated expiration: 2033-12-23
Also published as: EP2887775B1

Abstract

An X-ray source apparatus (100) for generating an X-ray beam (1), comprises a laser-plasma accelerator (10), which includes a gas channel (12) with an acceleration section for flowing a working gas along a flow direction, wherein the laser-plasma accelerator (10) is configured for irradiating a first pulsed laser beam (2) into the working gas, so that a plasma can be created in the working gas and an accelerated electron beam (4) can be generated, and a Thomson scattering part (20), which is configured such that the X-ray beam (1) can be generated by Thomson scattering of the electron beam (4) with a second pulsed laser beam (5), wherein the gas channel (12) includes a beam forming section for creating a pressure gradient so that the working gas flowing along the flow direction is subjected to a gas density reduction, wherein the pressure gradient is formed such that the transverse emittance of the electron beam (4) is preserved and, due to an adiabatic variation of transverse focusing forces in the plasma channel, the divergence of the electron beam (4) is reduced in the beam forming section relative to the acceleration section. Furthermore, an X-ray fluorescence imaging device including the X-ray source apparatus (100) and method for generating an X-ray beam (1) and for X-ray fluorescence imaging are described.

Description

The present invention relates to an X-ray source apparatus for generating an X-ray beam, in particular to an X-ray source apparatus including a laser-plasma accelerator creating an electron beam accelerated in a plasma wakefield and a Thomson scattering part for subjecting the electron beam to Thomson scattering for generating the X-ray beam (so called Thomson source). Furthermore, the invention relates to an X-ray fluorescence imaging device for X-ray fluorescence imaging of an object, in particular a biological organism, including the X-ray source apparatus. Furthermore, the invention relates to a method of generating an X-ray beam, in particular based on a wakefield acceleration of electrons and Thomson scattering. Furthermore, the invention relates to methods of X-ray fluorescence imaging of an object, in particular using the X-ray source apparatus. Applications of the invention are available in the fields of generating and using X-rays, in particular for imaging or irradiation purposes, e. g. in medicine or material science.
Conventional X-ray based medical imaging uses X-rays with photon energies of a few ten to hundred keV, e. g. for X-ray absorption imaging of the human body. Furthermore, X-ray fluorescence imaging has been proposed as a functional ultrasensitive in vivo imaging method. Typically, X-ray fluorescence imaging is based on detecting Au-nanoparticles being distributed in a region of interest in the human body, e.g. with a concentration below 1 mg/ml. As well, it could be used for mammography investigations.
Conventional X-ray tubes are capable of generating X-rays with photon energies being sufficient for X-ray fluorescence imaging. However, due to the relatively large spectral width of the X-ray tubes, the irradiation dose would be quite high, or the sensitivity and efficiency of imaging would be too low for practical applications. In addition, long imaging times would be required.
The disadvantage resulting from the broadband radiation of conventional X-ray tubes can be avoided by using undulator radiation, which can be created with narrow spectral width and small divergence. However, a conventional undulator plant requires large electron energies, a complex electron beam optic and complex irradiation protection measures, so that the application of undulator radiation is restricted to laboratory experiments. The conventional undulator apparatus is not suitable for routine applications, e. g. for medical imaging with clinical applications.
It has been proposed to replace the undulator plant by a laser-plasma accelerator for creating accelerated electrons. The laser-plasma accelerator includes a gas channel which is arranged in an evacuated space. A working gas flowing in the gas channel is irradiated with a pulse laser beam. In response to the irradiation, a plasma field (so-called plasma wakefield) is created, which accelerates the electrons in the plasma. Subsequently, after an exit into the evacuated space, the accelerated electrons are subjected to Thomson scattering for creating the X-rays. An X-ray source (Thomson source) including the laser-plasma accelerator and Thomson scattering has been described by e. g. J. Faure et al. in "Nature" vol. 43, 2004, p. 541, and P. Catravas et al. in "Meas. Sci. Technol. Vol. 12, 2001, p. 1828.
Thomson sources based on wakefield acceleration have substantial advantages in terms of a compact structure of the laser-plasma accelerator and reduced requirements with regard to radiation protection. However, it has been found that the conventional X-ray sources based on Thomson scattering have an important disadvantage in terms of the divergence of the electron beam and correspondingly of the resulting X-ray beam.
Due to the low electron energies, the divergence of the electron beam is about 10 mrad or higher with conventional Thomson sources. The resulting X-ray beam with a divergence of 10 mrad would have a beam diameter of 1 cm at a distance of 1 m from the X-ray source. However, diagnostic applications of X-ray imaging, e. g. for tumour diagnostic, require a local resolution below 5 mm and correspondingly a divergence of about 1 mrad. The large divergence of the electron beam affects not only the X-ray beam diameter, but also the spectral width of the X-ray radiation. Thomson scattering of divergent electrons with a laser beam results in scattering events within an unacceptable broad range of scattering angles, thus broadening the photon energy spectrum of the X-rays.
An X-ray source generating X-rays with a divergence being sufficient for imaging applications has not yet been obtained with the conventional techniques based on wakefield acceleration. Thus, due to the lack of suitable X-ray sources, X-ray fluorescence imaging has not yet been used in routine applications.
R. Weingartner et al. ("Physical review Topics - Accelerators and beams" vol. 15, 2012, p. 111302) have published investigations on the emittance of the electron beam created with a laser-plasma accelerator. This publication is generally related to laser-plasma accelerators, but not to the generation of X-rays. It has been found that the divergence of an electron beam is reduced after the exit from the gas channel of the accelerator, where the plasma is subjected to a density reduction.
The objective of the invention is to provide an improved apparatus and method for generating an X-ray beam being capable of avoiding disadvantages of conventional techniques based on Thomson scattering. In particular, the X-ray beam is to be generated with reduced divergence compared with the beam divergence of conventional Thomson sources. Furthermore, the objective of the invention is to provide an improved device and method for X-ray fluorescence imaging of an object, being capable of avoiding disadvantages of conventional techniques. In particular, the scanning technique is to be suitable for routine applications of X-ray fluorescence imaging, e. g. in hospitals. According to a particular aspect, the objective of the invention is to provide an improved X-ray fluorescence imaging method having increased sensitivity and allowing an irradiation of the object with a reduced radiation dose.
These objectives are solved with an X-ray source apparatus, a method of generating an X-ray beam, an X-ray fluorescence imaging device and methods of X-ray fluorescence imaging, comprising the features of the independent claims, respectively. Features of preferred embodiments and preferred applications of the invention are defined in the dependent claims.
According to a first general aspect of the invention, the above objective is solved by an X-ray source apparatus comprising a laser-plasma accelerator for generating an electron beam based on wakefield acceleration with a first pulsed laser beam and a Thomson scattering part (Thomson scattering portion) for generating the X-ray beam by Thomson scattering of the electron beam with a second pulsed laser beam. According to the invention, the laser-plasma accelerator includes a gas channel for flowing a working gas, wherein the gas channel has a first gas supply conduit for introducing the working gas and an acceleration section being adapted for the creation and wakefield acceleration of the electrons and a beam forming section being adapted for creating a negative pressure gradient in the flowing working gas. Furthermore, according to the invention, the pressure gradient is formed such that the working gas density and correspondingly the plasma density in the gas channel is reduced along the flowing direction through the beam forming section. The pressure gradient is adjusted such that a transverse emittance of the electron beam is preserved in the beam forming section and, resulting from an adiabatic variation of transverse focussing forces in the plasma, the divergence of the electron beam is reduced in the beam forming section relative to the divergence in the acceleration section. Due to the adiabatic variation of the transverse focussing forces, the divergence of the electron beam is reduced down to a final divergence at an exit end of the gas channel, where the working gas is released to an evacuated surrounding space.
According to a second general aspect of the invention, the above objective is solved by a method of generating an X-ray beam, including generating an electron beam by wakefield acceleration with a first pulsed laser beam and generating the X-ray beam by Thomson scattering of the electron beam with a second pulsed laser beam. According to the invention, the working gas used for creating the wakefield is subjected to a negative pressure gradient along the flow direction of the working gas, wherein the pressure gradient is created such that the transverse emittance of the electron beam is preserved along the preferred radiant and the beam divergence of the electron beam and correspondingly the X-ray beam is reduced. The pressure gradient is created by generating a pressure profile within the beam forming section of the gas channel, e. g. using at least one gas supply conduit opening into the beam forming section.
Contrary to the conventional Thomson sources, wherein the electrons are accelerated in supersonic gas jets having a step-shaped density reduction towards the evacuated surrounding space of the gas channel, the invention teaches the provision of the negative pressure gradient along the flow direction within the gas channel. Furthermore, the pressure gradient is adjusted such that the divergence of the accelerated electron beam can be adapted adiabatically at any time to the local variation of the focussing forces of the plasma wakefield resulting from the gas density reduction. With decreasing gas density, the plasma density is reduced, the so-called plasma period and, thus the size of the plasma wakefield are increased, and the transversal focussing fields are reduced. With the decreasing gas density and plasma density, the size of the plasma wakefield is increased longitudinally and transversally. As the emittance is preserved, the electron beam diameter is increased and the divergence of the electron beam is decreased accordingly. Advantageously, the invention allows an electron beam divergence reduction down to a final divergence below 5 mrad, in particular below 3 mrad, e. g. 1 mrad or even lower. In correspondence with the inventive electron beam divergence along the beam forming section, the X-ray beam is generated with a divergence below 5 mrad, in particular down to 1 mrad or even lower.
Advantageously, the beam forming section with the negative pressure gradient can be obtained by designing the gas channel, which preferably comprises a straight tube or capillary accommodating the flowing working gas. According to a preferred embodiment of the invention, the beam forming section is a downstream portion of the gas channel between a second gas supply conduit and the exit end of the gas channel. The second gas supply conduit opens to the gas channel, and it is adapted for adjusting the pressure gradient in the beam forming section. The second gas supply conduit is connected with a working gas reservoir. By supplying working gas through the second gas supply conduit into the gas channel, the gas pressure as well as the decreasing gas density towards the exit end of the gas channel can be adjusted.
According to a particularly preferred embodiment of the invention, the beam forming section has a channel length, i.e. a distance between the second gas supply conduit opening into the gas channel and the exit end of the gas channel, which is at least 0,5 mm, in particular at least 1 mm. Furthermore, the channel length preferably is at most 5 cm, in particular at most 3 cm, e. g. 1 cm or lower. A channel length of 0,5 mm to 3 mm is particularly preferred. The preferred longitudinal dimensions of the beam forming section, providing a minimum divergence of the electron beam down to a range below 1 mrad, have been found by the inventors by extended practical and numerical tests. With a channel length of the beam forming section below 0,5 mm, an abrupt pressure gradient would be obtained, which would not allow an adaptation of the local divergence to the transversal fields of the focussing wakefield and the divergence could not be further reduced. On the other hand, with a channel length of the beam forming section above 5 cm, the diameter of the laser beam would increase too far over such long distance, hence the laser could not maintain the plasma wakefield any more.
According to a further advantageous embodiment of the invention, the gas channel of the inventive X-ray source apparatus may be provided with a third gas supply conduit, which is arranged between the first gas supply conduit and the second gas supply conduit, i.e. the third gas supply conduit opens into the acceleration section of the gas channel. Advantageously, the third gas supply conduit can be adapted for creating a constant gas density upstream relative to the beam forming section, i.e. before the second gas supply conduit, thus providing a constant wakefield acceleration of the electrons.
Preferably, the working gas comprises hydrogen. Hydrogen has particular advantages in terms of a complete ionization in response to the first pulsed laser irradiation. Alternatively, other working gases could be used, which can be ionized in response to the pulsed laser irradiation, like e. g. Ar.
According to a further preferred embodiment of the invention, the Thomson scattering is obtained with the second pulsed laser beam having a pulse duration longer than the pulse duration of the first pulsed laser beam used for creating the plasma wakefield in the laser-plasma accelerator. Preferably, the pulse duration of the pulsed laser irradiation for Thomson scattering is 30-fold longer than the pulse duration for electron acceleration. Advantageously, this reduces the spectral width of the X-ray beam created by Thomson scattering.
According to a third general aspect of the invention, the above objective is solved with an X-ray fluorescence imaging device for imaging an object, wherein the X-ray fluorescence imaging device includes the X-ray source apparatus according to the above first aspect of the invention. Correspondingly, with a fourth general aspect of the invention, a method of X-ray fluorescence imaging of an object is provided, wherein the X-ray beam for exciting the X-ray fluorescence is generated with the X-ray source apparatus and/or the method of generating an X-ray beam according to the above first and second aspects of the invention, resp..
With more details, the inventive X-ray fluorescence imaging device comprises the inventive X-ray source apparatus, a scanning device, which is adapted for scanning the X-ray beam over a region of interest of the object and an X-ray detector device being arranged for detecting an X-ray fluorescence emission created in the object. With a scanning device, the direction of the X-ray beam relative to the object can be changed, so that the X-ray fluorescence can be excited at multiple locations, thus providing the X-ray fluorescence image to be obtained.
The X-ray fluorescence imaging device is configured for X-ray fluorescence imaging a distribution of Au-particles in the object to be imaged. Preferably, Au-nanoparticles are contained in the object with a concentration below 1 mg/ml, and/or they can be functionalized with substances having a specific binding capability with pre-determined portions within the object. As an example, the Au-nanoparticles can be adapted for a specific binding to tumour tissues within a biological object. The Au-particles have an X-ray absorption at the Kα-line of Au, which corresponds to an absorption energy of about 80 keV, while the emitted fluorescence photon has an energy of 69 keV. Alternatively, the X-ray fluorescence imaging device can be configured for X-ray fluorescence imaging a distribution of another fluorescing substance, like e. g. iodine or gadolinium, in the object.
It is an important advantage of the invention that the X-ray source apparatus has a compact structure, thus facilitating the scanning of the X-ray beam relative to the object. According to a preferred embodiment of the inventive X-ray fluorescence imaging device, the scanning device is arranged for stepwise changing an orientation of the gas channel of the inventive X-ray source apparatus relative to the object. Due to the low mass and simple structure of the gas channel, which comprises e. g. a gas capillary, dynamic scanning of the entire target can be obtained.
According to a particularly preferred embodiment of the invention, the X-ray beam is created with a photon energy being larger than the absorption energy of the substance to be excited. Preferably, the photon energy can be 4-fold larger or more than the absorption energy of the substance to be excited. With practical preferred examples, the photon energy is at least 150 keV, in particular at least 200 keV, particularly preferred at least 250 keV, and up to 400 keV. In particular with the X-ray fluorescence imaging based on exciting Au-nanoparticles, the photon energy is particularly preferred in a range from 250 keV up to 300 keV or up to 400 keV. To this end, the electron beam is accelerated by the laser-plasma accelerator to an energy of at least 150 MeV. With the above increased photon energies of the X-ray beam, background radiation due to multiple Compton scattering can be suppressed. In particular, second and/or third order scattering events are avoided, so that the X-ray fluorescence imaging can be obtained with an increased sensitivity. The photon energy of at least 250 keV is a preferred value. Depending on the application of the invention and/or the fluorescing substance to be excited, a lower photon energy can be used, in particular at least 150 keV or at least 200 keV.
The inventors have found that the X-ray fluorescence imaging using the increased range of photon energies, e. g. a photon energy 4-fold larger or more than the absorption energy of the substance to be excited, can be implemented also with another X-ray source, e. g. a conventional X-ray source, even if it has increased divergence. Thus, according to a fifth independent aspect of the invention, a method of X-ray fluorescence imaging of an object to be imaged is proposed, wherein the object is positioned on a support device, an X-ray beam is generated and directed to the object, the X-ray beam is scanned over a region of interest of the object and the X-ray fluorescence emission created in the object is detected, wherein, according to the invention, the X-ray beam has a photon energy of at least 250 keV. Again, depending on the application of the invention and/or the fluorescing substance to be excited, a lower photon energy can be used, in particular at least 150 keV or at least 200 keV.
Further details and advantages of the invention are described in the following with reference to the attached drawings, which show in:

Figure 1:: a schematic illustration of a preferred embodiment of the X-ray source apparatus according to the invention;
Figure 2:: a detailed illustration of the gas channel included in the X-ray source apparatus of Figure 1;
Figure 3:: a schematic illustration of the pressure gradient in the beam forming section of the gas channel of Figure 2; and
Figure 4:: a schematic illustration of an X-ray fluorescence imaging device according to a preferred embodiment of the invention.

Preferred embodiments of the invention are described in the following with particular reference to the design of a gas channel in a laser-plasma accelerator and the features of an X-ray fluorescence imaging device including the laser-plasma accelerator. It is emphasized that features of accelerating electrons with a laser beam accelerator, like e. g. the creation of pulsed laser beams for the excitation of the working gas or for conducting the Thomson scattering and setting a photon energy are not described as far as they are known from prior art. Furthermore, details of X-ray fluorescence imaging, like e. g. the selection and optional functionalization of Au-particles and the introduction thereof into an object to be imaged are not described as they can be implemented as it is known from conventional X-ray fluorescence imaging methods.
Figure 1 shows a first embodiment of an X-ray source apparatus 100 comprising a laser-plasma accelerator 10, a Thomson scattering part 20, a vacuum chamber 30, a control device 40 and a working gas supply 50. Preferably, the X-ray source apparatus 100 provides the X-ray source of a X-ray fluorescence imaging device 200 for X-ray fluorescence imaging, e. g. according to Figure 4. However, the X-ray source 100 can be used for other applications as well, e. g. for locally resolved X-ray absorption measurements or X-ray irradiations.
The laser-plasma accelerator 10 comprises a first laser source device 11 and a gas channel 12. The laser source device 11 is configured for creating a first pulsed laser beam 2 having a pulse duration of e. g. 25 fs, a pulse power of e. g. 100 TW to 200 TW, and a repetition frequency in a range of some Hz, e. g. 5 Hz, up to the kHz range. Preferably, the first laser source device 11 comprises a combination of a pulse amplifier and pulse compressor as it is known from conventional high power pulse laser systems. The first pulsed laser beam 2 is created within the vacuum chamber 30. Accordingly, at least a compressor part of the first laser source device 11 is arranged within the vacuum chamber 30.
The gas channel 12, which is illustrated with further details in Figure 2, comprises a capillary, which is made of e. g. sapphire, having an inner diameter of at least 50 µm. With practical applications of the invention, the inner diameter comprises a few 100 µm, e. g. 200 µm. The gas channel 12 is provided with a first gas supply conduit 13, which opens to a radial side of the gas channel 12 at an upstream end thereof. Furthermore, the gas channel 12 is provided with a second gas supply conduit 16, which opens to the gas channel 12 with a distance L from an exit end 17 of the gas channel 12. Optionally, a third gas supply conduit 18 is provided, which is arranged between the first and second gas supply conduits 13, 16. All gas supply conduits 13, 18 and 16 are connected with the working gas supply 50 as schematically shown in Figure 1. Working gas 3 is flown through the gas supply conduits 13, 18 and 16 into the gas channel 12.
A first portion of the gas channel 12 extending from the first gas supply conduit 13 to the second gas supply conduit 16 provides the acceleration section 14 of the gas channel 12. The longitudinal length of the accelerator section 14 is about 0.5 mm to 3 mm. Within the acceleration section 14, an accelerated electron beam is created as it is known from conventional laser-plasma accelerators. The first pulsed laser beam 2 is coupled into the accelerator section 14, where a plasma state of the working gas 3 is created. The high intensity pulses of the first pulsed laser beam 2 causes the creation of the plasma wakefield accelerating the electrons.
The second portion of the gas channel 12 extending from the second gas supply conduit 16 to the end exit 17 of the gas channel 12 provides the beam forming section 15. The longitudinal channel length L of the beam forming section is e. g. 2 mm to 4 mm. Working gas is supplied to the beam forming section 15 via the second gas supply conduit 16 in order to create and adjust a negative pressure gradient within the beam forming section 15. To this end, the working gas supply 50 is controlled for supplying a pre-determined gas flow as outlined below.
The Thomson scattering part 20 comprises a second laser source device 21, which is adapted for directing a second pulsed laser beam 5 to a scattering area 22 at a downstream side of the gas channel 12. The second pulsed laser beam 5 comprises pulses e. g. with a duration of 1 ps to 3 ps and a pulse power of about 1 TW. Figure 1 shows separate first and second laser source devices 11, 21. Alternatively, a common laser plant could be provided for generating the first and second pulsed laser beams with appropriate pulse parameters. The accelerated electron beam 4, which exits from the gas channel 12, in particular from the beam forming section 15 thereof, is irradiated with a second pulsed laser beam 5, so that the X-ray beam 1 is obtained due to Thomson scattering. The X-ray beam 1 is coupled out of the vacuum chamber 30 via a window 31, made of e. g. Be.
The vacuum chamber 30 generally is a container accommodating at least parts of the first and second laser source devices 11, 21, the gas channel 12 and parts of the gas supply conduits 13, 16 and 18. The vacuum chamber 30 can be evacuated with a conventional vacuum pump (not shown). Preferably, the pressure within the vacuum chamber 30 is below 10^-4 mbar.
The control device 40 comprises a microcontroller, which is connected in particular with the first and second laser source devices 11, 21 and the working gas supply 50. Pulse parameters of the pulsed laser beams 2, 5 and flow rates through the gas supply conduits can be controlled with the control device 40. The X-ray source apparatus 100 can be provided with detector elements (not shown) sensing at least one of the first and second pulsed laser beams 2, 5, the accelerated electron beam 4 and the X-ray beam 1. At least one of the detector elements can be connected with the control device 40 in order to create a control loop, wherein at least one of the laser-plasma accelerator 10, the Thomson scattering part 20 and the working gas supply 50 is controlled in dependency on the current parameters of the first and second pulsed laser beams 2, 5, electron beam 4 and/or X-ray beam 1.
The pressure gradient in the beam forming section 15, in particular the flow rates through the first and second gas supply conduits 13, 16, can be adjusted on the basis of the following considerations. The emittance of the electron beam is preserved and the divergence can be reduced along the pressure gradient, when the following condition (1) is fulfilled. $K_{pl} (ρ (z), ϕ (z), L (z)) = K_{o} / {(1 + a \cdot z)}^{4}$
The focussing strength K_pl in the plasma is a function of the plasma density p(z), the phase φ(z) (distance of the electron beam behind the laser pulse of the first laser source device 11) and the laser pulse envelope L(z), with K_o being the focussing strength in the acceleration section 14, z being the coordinate of the longitudinal direction of the gas channel 12, and a being a free parameter, depending on the energy width and emittance of the electron beam.
Based on condition (1), the minimum plasma density p(z) required for preserving the emittance can be calculated numerically. With the calculated minimum plasma density gradient d/dz[p(z)] or a smaller plasma density gradient, the gas density profile in the beam forming section 15 can be provided, e. g. a linear pressure gradient as shown in Figure 3, or an exponential pressure gradient. With a practical example, the #gas density pressure is reduced from about 10¹⁸ cm^-3 down to vacuum level outside the gas channel 12.
Additionally, the pressure gradient can be influenced by the inner shape of the beam forming section 15, e. g. by an inner diameter of the beam forming section 15 conically decreasing from the second gas supply conduit 16 to the exit end 17.
With modified embodiments of the invention, the gas channel 12 can be provided with more gas supply conduits in the acceleration section and/or in the beam forming section, in order to influence the focussing strength in the acceleration section and/or the pressure drop in the beam forming section, resp..
Figure 4 schematically illustrates a preferred embodiment of an X-ray fluorescence imaging device 200 for imaging an object 6. The device 200 includes the X-ray source apparatus 100 as described with reference to Figure 1. Additionally, the X-ray fluorescence imaging device 200 comprises a support device 210, which is adapted for accommodating the object 6 to be imaged, a scanning device 221, 222, which is adapted for scanning the X-ray beam 1 created by the X-ray source apparatus 100 over a region of interest within the object 6, and an X-ray detector device 230, which is arranged for detecting X-ray fluorescence emission excited in the object 6.
The support device 210 comprises e. g. a support table with a platform, where the object 6, like e. g. a biological organism, in particular a human patient, can be positioned. The position of the object 6 relative to the X-ray beam 1 can be adjusted and changed by a first drive unit 221 of the scanning device. The first drive unit 221 comprises e. g. an electric motor or a piezoelectric translation drive.
Furthermore, the scanning device comprises a second drive unit 222, which is arranged for moving the gas channel 12 within the vacuum chamber 30. The second drive unit 222 comprises e. g. a piezoelectric translation drive, which is configured for stepwise varying the orientation of the gas channel 12 in space.
The X-ray detector device 230 comprises a sensing unit as it is known from prior art, like e. g. X-ray detectors.
As an optional feature, Figure 4 additionally shows a collimator device 240, which is arranged between the support device 210 and the X-ray detector device 230. The collimator device 240 is capable of suppressing a background signal of multiple Compton scattering events inside the object 6.
With a practical example, the object 6 comprises a small animal, like a mouse including Au nanoparticles with a concentration of 0,001 mg/ml to 0.1 mg/ml. The mouse is irradiated with the X-ray beam 1 having a photon energy of 150 keV or 250 keV, and the characteristic Kα line of the Au nanoparticles is emitted. During scanning, the Kα line is detected with local resolution as the image signal to be obtained. High spatial resolution is obtained by the inventive low divergence X-ray source apparatus 100, with a maximum signal-to-noise-ratio and signal-to-background-ratio.
While fluorescence of the low concentrations of Au nanoparticles can be detected by the detector device 230 there could be in parallel a detection of a standard absorption image of the object 6.
The features of the invention disclosed in the above description, the drawings and the claims can be of significance both individually as well as in combination for the realization of the invention in its various embodiments.

Claims

X-ray source apparatus (100), being adapted for generating an X-ray beam (1), comprising:
- a laser-plasma accelerator (10), which includes a first laser source device (11) and a gas channel (12) having a first gas supply conduit (13) and an acceleration section (14) being adapted for flowing a working gas (3) along a flow direction, wherein the laser-plasma accelerator (10) is configured for irradiating a first pulsed laser beam (2) from the first laser source device (11) into the working gas (3), so that a plasma with an accelerating plasma field can be created in the working gas (3) and an electron beam (4) can be generated by electrons accelerated in the plasma field, and

- a Thomson scattering part (20), which includes a second laser source device (21), wherein the Thomson scattering part (20) is configured such that the X-ray beam (1) can be generated by Thomson scattering of the electron beam (4) with a second pulsed laser beam (5) from the second laser source device (21),
characterized in that
- the gas channel (12) further includes a beam forming section (15) being arranged downstream from the acceleration section (14) and being configured for creating a pressure gradient so that the working gas flowing along the flow direction is subjected to a gas density reduction, wherein

- the pressure gradient is formed such that, resulting from the density reduction, the transverse emittance of the electron beam (4) is preserved and, due to an adiabatic variation of transverse focusing forces in the plasma channel, the divergence of the electron beam (4) is reduced in the beam forming section (15) relative to the acceleration section (14).
X-ray source apparatus according to claim 1, wherein
- the beam forming section (15) comprises a portion of the gas channel (12) between a second gas supply conduit (16) and an exit end (17) of the gas channel (12), wherein

- the second gas supply conduit (16) is arranged for adjusting the pressure gradient in the beam forming section (15).
X-ray source apparatus according to claim 2, including at least one of the features
- the beam forming section (15) has a channel length (L) between the second gas supply conduit (16) and the exit end (17) of at least 0,5 mm, in particular at least 1 mm,

- the beam forming section (15) has a channel length (L) between the second gas supply conduit (16) and the exit end (17) of at most 5 cm, in particular at most 3 cm.
X-ray source apparatus according to one of the claims 2 or 3, wherein
- the gas channel (12) has a third gas supply conduit (18) which is arranged between the first gas supply conduit (13) and the second gas supply conduit (16) for adjusting a gas density in the acceleration section (14).
X-ray source apparatus according to one of the foregoing claims, including at least one of the features
- the working gas comprises hydrogen,

- the second laser source device (21) is adapted for creating the second pulsed laser beam (5) with a pulse duration longer than the pulse duration of the first pulsed laser beam (2).
X-ray fluorescence imaging device (200), being adapted for X-ray fluorescence imaging of an object (6) to be imaged, comprising:
- a support device (210) being adapted for accommodating the object (6),

- an X-ray source apparatus (100) according to one of the foregoing claims, wherein the X-ray source apparatus (100) is arranged for directing the X-ray beam (1) to the object (6),

- a scanning device (221, 222) being adapted for scanning the X-ray beam (1) over a region of interest of the object (6), and

- an X-ray detector device (230) being arranged for detecting X-ray fluorescence emission (7) created in the object (6).
X-ray fluorescence imaging device according to claim 6, wherein
- the scanning device (221, 222) is arranged for stepwise changing an orientation of the gas channel (12) relative to the support device (210).
X-ray fluorescence imaging device according to one of the claims 6 to 7, wherein
- the X-ray source apparatus (100) is arranged for creating the X-ray beam (1) having an energy of at least 150 keV, in particular at least 200 keV, and up to 400 keV.
X-ray fluorescence imaging device according to claims 8, wherein
- the X-ray source apparatus (100) is arranged for creating the X-ray beam (1) having an energy of at least 250 keV.
X-ray fluorescence imaging device according to one of the claims 6 to 9, further comprising
- a collimator device (240) being arranged between the support device (210) and the X-ray detector device (230) to suppress a fluorescence signal background from multiple Compton scattering of the X-ray photons inside the object.
Method of generating an X-ray beam, comprising the steps of:
- generating an electron beam (4) by flowing a working gas (3) via a first gas supply conduit (13) along a flow direction through an acceleration section (14) of a gas channel (12) and irradiating a first pulsed laser beam (2) into the working gas (3), so that a plasma with an accelerating plasma field is created in the working gas (3) and the electron beam (4) is provided by electrons accelerated in the plasma field, and

- generating the X-ray beam (1) by Thomson scattering of the electron beam (4) and a second pulsed laser beam (5),
characterized in that
- the gas channel (12) further includes a beam forming section (15) being arranged downstream from the acceleration section (14), wherein

- a pressure gradient is created in the beam forming section (15) so that the working gas flowing along the flow direction is subjected to a gas density reduction, wherein

- the beam forming section (15) is configured for creating the pressure gradient so that the transverse emittance of the electron beam is preserved in the beam forming section (15) due to an adiabatic variation of the transverse focusing forces in the plasma channel and the divergence of the electron beam (4) is reduced in the beam forming section (15) relative to the acceleration section (14).
Method according to claim 11, wherein
- the beam forming section (15) comprises a portion of the gas channel (12) between a second gas supply conduit (16) and an exit end (17) of the gas channel (12), and

- the pressure gradient is adjusted by supplying working gas to the second gas supply conduit (15).
Method according to one of the claims 11 to 12, including at least one of the features
- the beam forming section (15) has a channel length (L) between the second gas supply conduit (16) and the exit end (17) of at least 0,5 mm, in particular at least 1 mm,

- the beam forming section (15) has a channel length (L) between the second gas supply conduit (16) and the exit end (17) of at most 5 cm, in particular at most 3 cm,

- the working gas comprises hydrogen, and

- the second pulsed laser beam (5) has a pulse duration longer than the pulse duration of the first pulsed laser beam (2).
Method of X-ray fluorescence imaging of an object (6) to be imaged, comprising the steps of:
- positioning the object (6) at a support device (210),

- generating an X-ray beam (1) with a method according to one of the claims 11 to 13, and directing the X-ray beam (1) to the object (6),

- scanning the X-ray beam (1) over a region of interest of the object (6), and

- detecting X-ray fluorescence emission (7) created in the object (6).
Method according to claim 14, wherein
- the step of scanning the X-ray beam (1) includes stepwise changing an orientation of the X-ray beam (1) relative to the support device (210).
Method according to one of the claims 14 to 15, wherein
- the X-ray beam (1) has an energy of at least 150 keV, in particular at least 200 keV, particularly preferred at least 250 keV, and up to 400 keV.
Method according to one of the claims 14 to 16, further comprising the step of
- subjecting the X-ray fluorescence emission (7) to a beam collimation.
Method of X-ray fluorescence imaging of an object (6) to be imaged, comprising the steps of:
- positioning the object (6) at a support device (210),

- generating an X-ray beam (1) and directing the X-ray beam (1) to the object (6),

- scanning the X-ray beam (1) over a region of interest of the object (6), and

- detecting X-ray fluorescence emission (7) created in the object (6),
characterized in that
- the X-ray beam (1) has an energy of at least at least 150 keV, in particular at least 200 keV, particularly preferred at least 250 keV, and up to 400 keV, in order to suppress the fluorescence signal background from multiple Compton scattering events of the X-ray photons inside the object.