Field of the invention

According to a first aspect, the invention relates to the field of apparatus for generating a high energy electron beam. More particularly, the invention relates to a multiple energy single electron beam generator, i.e. a generator which is adapted to deliver a single beam of electrons at multiple energies.

According to a second aspect, the invention relates to an apparatus for generating a multiple energy X-ray beam and comprising a multiple-energy single electron beam generator according to the first aspect.

According to a third aspect, the invention relates to an X-ray inspection system to examine the contents of an object and comprising a multiple energy X-ray beam generator according to the second aspect.

Description of prior art

Apparatus for generating a high energy electron beam, for example an electron beam of more than 1 MeV, are well known in the art and are useful for various applications, such as for example for non-destructive security inspection of the contents of containers at ports, airports, railway stations, etc . For the said security applications, it is known to use an apparatus generating high energy X- rays by bombarding a target plate with a high-energy electron beam, to irradiate a container with these X-rays, and to measure and visualize absorption and/or reflection of these X-rays by such container in order to visualize its contents without opening or destructing it, in order to determine whether or not it would contain hazardous and/or illegal material for instance.

In order to be able to inspect large containers and/or containers having thick or dense walls, such as steel-walled containers for instance, high energy X-rays are needed, typically in excess of 1 MeV. It is also known that, in order to be able to better discriminate content materials having different atomic numbers, it is advantageous to irradiate the container with X-rays having two or more distinct energies. Several known solutions have been proposed in this respect.

Ogorov et al. disclose for instance a dual-energy linear electron accelerator producing a single pulsed beam of electrons at two distinct energies and operating in an interlaced mode, wherein each even pulse delivers electrons at 8 MeV and each uneven pulse delivers electrons at 4 MeV ("Radioscopic discrimination of materials in 1 -10 MeV range for customs applications" ; EPAC 2002, Paris, pp.2807-2809). When the electron beam hits a target plate, pulsed X-rays, following the same interlaced pattern at respectively and approximately the same two energies, are produced by Bremsstahlung effect and are directed towards a test object, the contents of which is to be analyzed.

A disadvantage of such known apparatus is that they make use of a linear accelerator (sometimes also called "Linac") for generating the dual-energy electron beam and that a linear accelerator has a limited power, which limits the depth of penetration of the X-ray beam into the container or object to inspect. Another disadvantage is that a linear accelerator has a limited duty cycle, which may cause problems with some types of X-ray detectors which need longer pulse widths than those which can be delivered by a Linac. A known alternative system makes use of two distinct Linacs, producing respectively two separate electron beams at respectively two distinct energies. The two electron beams are directed towards respectively two distinct target plates for generating respectively two X-ray beams. The two target plates are arranged at a distance from each other along a direction of movement of the object to be inspected with respect to the X-ray sources and emit respectively two distinct X-ray beams at two distinct locations of the moving object to be inspected. Two detectors detect the radiation transmitted by the object. By time- reconstruction of the two detector's data, a dual image of a given location of the object at respectively the two energies can be obtained.

Such an alternative solution is disclosed in another embodiment (Fig. 2) of patent publication number WO2010/059249.

These apparatus also make use of linear accelerators and are therefore subject to the same disadvantages as highlighted hereinabove.

A further disadvantage of such alternative apparatus is that the two target plates must be sufficiently spaced apart in order that the two X-ray beams do not to interfere, which increases the space occupied by the system. A further disadvantage is that, still due to the spacing apart of the two X-ray beams, there will be a time lag between an acquisition of an image of a given location on the object at one beam energy and an acquisition of an image of the same given location on the object at the other beam energy, which slows down the inspection process and which furthermore requires to memorize data and to time-reconstruct data if one wants to have a single image of the object at the two beam energies.

Summary of the invention

It is an object of the invention to provide a multiple-energy electron beam generator which addresses the problems of the state of the art generators.

The invention is defined by the independent claims. The dependent claims define advantageous embodiments.

According to the invention, there is provided a multiple energy electron beam generator comprising a generator system configured to deliver a first beam of electrons at a first energy along a first axis at a first output, and at least a second beam of electrons at a second energy along a second axis at a second output, the second energy being different from the first energy and the second axis being different from the first axis. The multiple energy electron beam generator further comprises:

- a controller configured for repeatedly alternating the delivery of the first and second beams of electrons to the first and second outputs respectively, and

- a beam redirection magnet positioned to receive the first and the second beams of electrons according to said first axis and to said second axis respectively and configured to redirect the first and the second beams of electrons towards along a third axis. Compared to the known generators using a single linear accelerator for delivering an interlaced pulsed electron beam at two distinct energies, an electron beam generator according to the invention presents the advantage that it can show improved performance in terms of power. A further advantage is that one can make use of other types of electron accelerators, such as fixed energy recirculating-type electron accelerators (such as a Rhodotron for instance), which results in improved performance in terms of power and/or duty cycle.

Compared to the known generators using two linear accelerators for delivering two distinct electron beams at two distinct energies and directed to two spaced- apart targets, respectively, an electron beam generator according to the invention presents the further advantages that it does not (or at least to a much lesser extent) result in the above-mentioned time lag between two images and that it does not (or at least to a much lesser extent) need time reconstruction of data to obtain a single image at the two energies.

Preferably, the beam redirection magnet comprises a permanent magnet.

Alternatively, the beam redirection magnet comprises an electromagnet and the multiple energy electron beam generator is preferably configured to excite the electromagnet with a constant excitation current.

These two alternatives present the advantage that the magnetic field which is used to redirect the first and the second beams of electrons along the third axis, is kept constant. In other words, this magnetic field is not modified in direction and/or magnitude, which makes the beam redirection magnet much faster and/or simpler and/or cheaper.

Preferably, the generator system comprises at least one Rhodotron to generate the first beam of electrons and/or the at least one second beam of electrons.

Short description of the drawings

These and further aspects of the invention will be explained in greater detail by way of example and with reference to the accompanying drawings in which:

Fig.1 schematically shows an exemplary multiple-energy electron beam

generator according to the invention;

Fig.2 shows a first electron beam current as output by the first electron

beam generator in function of time, and a second electron beam current as output by the second electron beam generator in function of time ;

Fig.3 shows an electron beam current as output by an multiple energy

electron beam generator according to the invention, in function of time; Fig.4 schematically shows a more detailed view of the beam redirection

magnet of the generator of Fig.1 ;

Fig.5 schematically shows a perspective view of the beam redirection

magnet (30) of Fig.4;

Fig.6 schematically shows another exemplary multiple-energy electron

beam generator according to the invention;

Fig.7 schematically shows yet another exemplary multiple-energy electron beam generator according to the invention;

Fig.8 schematically shows an exemplary inspection system according to the invention;

Fig.9 conceptually shows an exemplary quadruple-energy electron beam generator according to the invention. The drawings of the figures are neither drawn to scale nor proportioned.

Generally, identical components are denoted by the same reference numerals in the figures. Detailed description of embodiments of the invention

Fig.1 schematically shows an exemplary multiple-energy electron beam generator (1 ) according to the invention.

It comprises a generator system (2) including, in this example, a first electron beam generator (10) and a second electron beam generator (20).

The first electron beam generator (10) comprises a first electron source (1 1 ), such as an electron gun for example, coupled to a first electron accelerator (12). The first electron accelerator (12) is adapted to accelerate electrons from the first electron source (1 1 ) to a first energy (E1 ) and to deliver a first beam of electrons (13) at said first energy along a first axis (A1 ) at a first output (14) of the generator system (2).

The second electron beam generator (20) comprises a second electron source (21 ), such as an electron gun for example, coupled to a second electron accelerator (22). The second electron accelerator (22) is adapted to accelerate electrons from the second electron source (21 ) to a second energy (E2), different from the first energy (E1 ) and to deliver a second beam of electrons (23) at said second energy along a second axis (A2), different from the first axis (A1 ), at a second output (24) of the generator system (2). On Fig.1 , the first axis (A1 ) and the second axis (A2) are shown to be intersecting, but they may also be parallel to each other.

The first and second electron accelerators (12, 22) may be any kind of electron accelerator, such as a linear or a re-circulating type electron accelerator, of which many are known in the art.

Preferably, the first and second electron beam generators (10, 20) are

Rhodotrons, more preferably pulsed Rhodotrons such as for example the one disclosed in patent publication number WO2014/184306. Preferably, the first electron beam generator (10) is adapted to deliver electrons at a first energy (E1 ) which is higher than 1 MeV, more preferably in the range of 1 MeV to 20 MeV, even more preferably in the range of 1 MeV to 7 MeV, such as 6 MeV for example.

Preferably, the second electron beam generator (20) is adapted to deliver electrons at a second energy (E2) which is higher than 1 MeV, more preferably in the range of 1 MeV to 20 MeV, even more preferably in the range of 7 MeV to 15 MeV, such as 9 MeV for example.

One can for example have that E1 = 6 MeV and E2 = 9 MeV.

Preferably, the first accelerator (12) is adapted or configurable to deliver a first beam of electrons (13) at an average beam power equal or larger than 5 kW, more preferably equal or larger than 10 kW.

Preferably, the second accelerator (22) is adapted or configurable to deliver a second beam of electrons (23) at an average beam power equal or larger than 5 kW, more preferably equal or larger than 10 kW.

The multiple energy electron beam generator (1 ) further comprises a controller (40) configured for repeatedly alternating the delivery of the first and second beams of electrons (13, 23) to the first and second outputs (14, 24) respectively, preferably at an alternating frequency which is in the range of 1 Hz to 10 KHz, preferably in the range of 50 Hz to 500 Hz.

To this end, the controller (40) may for example comprise two conventional and synchronized signal generators acting on gating or triggering of respectively the first and second electron sources (1 1 , 21 ) in an alternating and repetitive manner. A single signal generator, such as a digital pulse generator for example, with a non-inverting output and a synchronized and complementary inverting output may alternatively be used to that end, the non-inverting output triggering or gating the first electron source (1 1 ) and the inverting output triggering or gating the second electron source (21 ), or vice-versa.

Hence, while the first electron beam generator (10) delivers electrons at the first output (14), the second electron beam generator (20) does not deliver electrons at the second output (24), and vice versa, and this in a repetitive manner. Accordingly, Fig.2 shows a first electron beam current (11 ) as delivered at the first output(14) by the first electron beam generator (10) in function of time, and a second electron beam current (I2) as delivered at the second output (24) by the second electron beam generator (20) in function of time. In the example of Fig.2, both the first and second electron beam generators are pulsed

generators, but they may as well be continuous wave (CW) generators or a mix of pulsed and CW generators. On the first time-diagram of Fig.2, one can see that the first electron beam generator (10) delivers a first pulsed beam of electrons (13) having ON and OFF states, wherein electrons are delivered at a first energy (E1 ) during an ON state, and wherein substantially no electrons are delivered during an OFF state. The first duration of the ON state (also known as the pulse duration) is denoted T1 1 . The first repetition frequency (also known as the repetition rate) is denoted f1 = 1 /T1 .

On the second time-diagram of Fig.2, one can see that the second electron beam generator (20) delivers a second pulsed beam of electrons (23) having ON and OFF states, wherein electrons are delivered at a second energy (E2) - which is different from the first energy (E1 ) - during an ON state, and wherein substantially no electrons are delivered during an OFF state.

The second duration of the ON state (also known as the pulse duration) is denoted T21 . The second repetition frequency (also known as the repetition rate) is denoted f2 = 1 /T2.

As one can see from Fig.2, the electron pulses output by the first and the second electron beam generators (10, 20) are time-interlaced. In other words, when the first pulsed electron beam generator (10) outputs electrons, the second pulsed electron beam generator (20) does not output electrons and vice versa. It will be obvious that many other interlacing patterns can be used, such as for example a sequence of two ON pulses delivered by the first electron beam generator (10) while the second electron beam generator (20) is in an OFF state, followed by a sequence of two ON pulses delivered by the second electron beam generator (20) while the first electron beam generator (10) is in an OFF state, repeatedly.

Preferably, 1 Hz<f1 <10KHz and/or 1 Hz<f2<10KHz. For security inspection applications, one more preferably has that 50Hz<f1 <500Hz and/or

50Hz<f2<500Hz.

Preferably f 1 =f2, so that T1 =T2.

In case of a repeated alternation of one pulse output by the first electron beam generator (10) and one pulse output by the second electron beam generator (20), one should of course have that (T1 1 + T21 ) < T1 in order that the pulses do not overlap. As said before, many other interlacing patterns are possible.

It is to be noted that a microstructure may exist in the course of each pulse duration T1 1 and T21 (microstructure not shown on Fig.2), in the sense that electron accelerators often deliver bunches of electrons at an electron bunch frequency f _eb which is much larger than the repetition frequency f1 or f2 of the output pulses.

In a Rhodotron, one may for example have that f _e b > 100MHz (with a duty cycle of 10% for instance, which means that a bunch of 1 ns is output every 10 ns in case f _eb = 100MHz), while f 1 <1 OKHz and/or f2<1 OKHz.

Preferably, the duty cycle T1 1 /T1 is larger than 2%, preferably larger than 5%, more preferably larger than 10%, even more preferably larger than 20%.

Preferably, the duty cycle T21 /T2 is larger than 2%, preferably larger than 5%, more preferably larger than 10%, even more preferably larger than 20%.

For the first accelerator (12), one may for example use a pulsed Rhodotron which is adapted to deliver an average beam power of 15 KW at a beam energy of 6 MeV and at a duty cycle of 50%. For the second accelerator (22), one may for example use a pulsed Rhodotron which is adapted to deliver an average beam power of 22,5 KW at a beam energy of 9 MeV and at a duty cycle of 50%.

Many other combinations are of course possible.

In contrast, current Linacs can only deliver 1 KW average beam power at a duty cycle comprised between 0,1 % and 1 % and for similar beam energies, at least for security inspection applications. It is to be noted that instead of having two different electron accelerators, the generator system (2) may alternatively comprise a single electron accelerator adapted to deliver both the first and second beams of electrons (13, 23) at respectively the first and second outputs (14, 24), at respectively the first and second energies (E1 , E2) and according respectively to the first and second axis (A1 , A2).

The multiple energy electron beam generator (1 ) further comprises a beam redirection magnet (30) positioned to receive the first and the second beams of electrons (13, 23) according to said first axis (A1 ) and said second axis (A2) respectively, and configured to redirect the first and the second beams of electrons (13, 23) along a third axis (A3).

In other words, the beam redirection magnet (30) is a magnet which is adapted to bring the first and the second beams of electrons (13, 23) spatially close together, so as to substantially form a single electron beam, said single electron beam presenting a beam current in function of time as shown on Fig. 3 for example.

One can say that a single electron beam is formed when the first and second beams of electrons (13, 23) are brought to co-linearity, or to close parallelism (distance between the first and second redirected beams is smaller than 50mm, preferably smaller than 10mm, more preferably smaller than 5mm), or to close proximity with a small angle (a < 20°, preferably a < 10°, more preferably a < 5°) between the two beams of electrons. In the latter case, an average beam size - at a focal point (101 ) of the first and the second beam of electrons (13, 23) - is preferably smaller than 50 mm, more preferably smaller than 10 mm, even more preferably smaller than 5 mm.

The beam redirection magnet may have various structures. Concrete examples will be given hereafter. In these examples, the first and second electron beam generators (10, 20) are preferably positioned in such a way that the first axis (A1 ), the second axis (A2) and the third axis (A3) are all in the same plane, but other orientations are of course possible. Fig.4 schematically shows a more detailed top view of an exemplary beam redirection magnet (30).

The beam redirection magnet (30) may be a permanent magnet or an electromagnet, preferably in the form of a dipole. If an electromagnet is used, it is preferably excited with a constant excitation current, so that its magnetic field remains substantially constant when the magnet is in nominal operation.

In the example of Fig.4, the pole faces of the beam redirection magnet (30) are parallel to each other and to the plane of the figure, and its magnetic field (B) is oriented perpendicularly to the plane of the figure. The first and second beam of electrons (13, 23) travel between the pole faces and parallel to the plane of the figure.

In this example, the first energy (E1 ) of the first beam of electrons (13) is higher than second energy (E2) of the second beam of electrons (23), so that a first bending angle (β1 = angle between axes A1 and A3) imposed by the magnetic field (B) on the first beam of electrons (13) will be smaller than a second bending angle (β2 = angle between axes A2 and A3) imposed by the magnetic field (B) on the second beam of electrons (23).

Fig.5 schematically shows a perspective view of the beam redirection magnet (30) of Fig.4, so as to better see the trajectories of the first and second electron beams.

Such electron beam bending dipoles (30) are per se well known in the art of particle accelerators and will therefore not be described in more detail here. Many other configurations are possible for the beam redirection magnet (30). Fig.6 schematically shows for instance another exemplary multiple-energy electron beam generator (1 ) according to the invention. It is similar to the multiple-energy electron beam generator shown in previous figures, except that in this case the beam redirection magnet (30) solely acts on the first beam of electrons (13) and that the second accelerator (22) is positioned such that the second axis (A2) is substantially the same as the third axis (A3). The beam redirection magnet (30) is thus configured to bend (redirect) only the first beam of electrons (13) along the third axis (A3).

In this case, the second energy (E2) of the second beam of electrons (23) is preferably greater than the first energy (E1 ) of the first beam of electrons (13). This presents the advantage that only the lowest energy electron beam needs to be bent (redirected), thereby reducing the power needed for the beam redirection magnet (30).

Instead of using one single beam redirection magnet (30) as shown

schematically on Fig.6, one may use a plurality of magnets arranged in sequence and placed in the path of the first electron beam. In this latter case, the respective bending angles of such a sequence of magnets preferably decrease along the path of the first beam of electrons. One can for example use a first magnet having a first bending angle, followed by a second magnet having a second bending angle smaller than the first bending angle.

The beam redirection magnet (30) may comprise a plurality of beam redirection sub-magnets instead of a single magnet as shown in previous figures.

Fig.7 schematically shows yet another exemplary multiple-energy electron beam generator (1 ) according to the invention and comprising a plurality of beam redirection sub-magnets. It is similar to the multiple-energy electron beam generator shown figures 1 to 5, except that it comprises three beam redirection sub-magnets (31 a, 31 b, 31 c): a first beam redirection sub-magnet (31 a) is placed in the path of the first beam of electrons (13) when coming from the first output (14) of the generator system (2), and generates a first constant magnetic field (B1 ) to redirect the first beam of electrons towards a third beam redirection sub-magnet (31 c) and according to a first intermediate axis (A1 '), a second beam redirection sub-magnet (31 b) is placed in the path of the second beam of electrons (23) when coming from the second output (24) of the generator system (2), and generates a second constant magnetic field (B2) to redirect the second beam of electrons towards the third beam redirection sub-magnet (31 c) and according to a second

intermediate axis (Α2'), and

- a third beam redirection sub-magnet (31 c) is placed in the path of the first and second beams of electrons when coming respectively from the first and the second beam redirection sub-magnets (31 a, 31 b) and generates a third constant magnetic field (B3) to redirect the first and the second beams of electrons along the third axis (A3).

Preferably, the first and second beam redirection sub-magnets (31 a,31 b) are positioned and configured in such a way that an angle between the first and second intermediate axes (A1 ', A2') is smaller than an angle between the first and the second axes (A1 , A2).

Although schematically shown as a single magnet on Fig.7, the first beam redirection sub-magnet (31 a) may comprise a plurality of beam redirection sub- magnets arranged in sequence in the path of the first beam of electrons and gradually bending the first beam of electrons to the first intermediate axis (A1 '). The same holds for the second beam redirection sub-magnet (31 b), by analogy. The beam redirection sub-magnets (31 a, 31 b, 31 c) may be permanent magnets or electromagnets. The invention also provides a multiple energy X-ray beam generator comprising a multiple energy electron beam generator (1 ) as described hereinabove and a conversion target (100) adapted to produce X-rays (50) when impacted by a beam of electrons, wherein said conversion target (100) is positioned across the third axis (A3) and downstream of the beam redirection magnet (30). The conversion target (100) may for example be a metallic plate, such as a plate of tungsten for instance. The invention also provides an inspection system adapted to examine contents of an object (200), said inspection system comprising a multiple energy X-ray beam generator as described hereinabove and an X-ray detector (300) positioned to detect radiation from interaction between the X-rays (50) produced by the conversion target (100) and the object (200).

Fig.8 schematically shows an exemplary inspection system according to the invention. It comprises a multiple energy electron beam generator (1 ) according to the invention and a conversion target (100) positioned across the third axis (A3) and downstream of the beam redirection magnet (30), so as to emit X-rays (50) towards the object (200) to be inspected when impacted by the multiple energy electron beam which is output by the beam redirection magnet (30). It further comprises an X-ray detector (300) positioned to detect radiation from interaction between the X-rays (50) and the object (200).

The invention also bears on a generalisation of the exemplary dual-energy electron beam generators (1 ) disclosed hereinabove to a multiple-energy electron beam generator which is adapted to deliver a single electron beam whose interlaced pulses have more than two different energies.

It will be obvious for a skilled person that such a multiple-energy electron beam generator can for example be realized by using a generator system (2) comprising more than two electron beam generators, each generator being adapted to deliver a beam of electrons at a different energy, at a different output and according to a different axes, and by adapting the controller (40) so that it synchronizes all accelerators in such a way that when anyone of them outputs electrons, all the others do not, and by adapting the beam redirection magnet (30) so that it is positioned to receive the beams of electrons from each accelerator according to their respective axes and so that it re-directs the multiple beams of electrons along the third axis (A3). Fig. 9 conceptually shows an exemplary embodiment wherein the generator system (2) comprises four electron beam generators (10, 20, 10a, 20a) delivering respectively four electron beams (13, 23, 13a, 23a) at respectively four different energies and according to respectively four different directions, and comprising three beam redirection magnets (30a, 30b, 30c) as described hereinabove. Obviously, one may use four accelerators to deliver the four electron beams or a single accelerator adapted to deliver the four electron beams.

Alternatively, the generator system (2) may comprise one electron beam generator adapted to deliver multiple beams of electrons alternatively at different outputs and according to different axes, each beam of electrons having a different energy. Many other combinations are of course possible.

The invention may also be described as follows:

A multiple energy single electron beam generator (1 ) comprising a generator system (2) adapted to deliver a first beam of electrons (13) at a first energy (E1 ) along a first axis (A1 ) at a first output (14) and to deliver a second beam of electrons (23) at a second energy (E2) along a second axis (A2) at a second output (24), the second energy being different from the first energy and the second axis being different from the first axis, a controller (40) configured to pilot the generator system (2) so that the first and second beams of electrons are delivered alternatively at respectively the first and second outputs, and a beam redirection magnet (30) positioned to receive the first and the second beams of electrons (13, 23) according to said first axis (A1 ) and second axis (A2) respectively and configured to redirect said first and second beams of electrons (13, 23) along a third axis (A3).

The present invention has been described in terms of specific embodiments, which are illustrative of the invention and not to be construed as limiting. More generally, it will be appreciated by persons skilled in the art that the present invention is not limited by what has been particularly shown and/or described hereinabove.

Reference numerals in the claims do not limit their protective scope. Use of the verbs "to comprise", "to include", "to be composed of", or any other variant, as well as their respective conjugations, does not exclude the presence of elements other than those stated.

Use of the article "a", "an" or "the" preceding an element does not exclude the presence of a plurality of such elements.