NEUTRON SOURCE WITH HEAVY WATER MODERATION AND APPLICATIONS TO

THERMAL NEUTRON IMAGING

DESCRIPTION

TECHNICAL FIELD AND PRIOR ART

The invention concerns thermal neutron sources, in particularforthermal neutron imaging applications. It also concerns a thermal neutron imaging system and a thermal neutron imaging method.

Neutron imaging is known, but involves the use of big facilities like particle accelerators.

A portable neutron source is known, which comprises a neutron generator and a moderator 4, usually made of HDPE.

The existing portable neutron sources do not provide a sufficient flux of thermal neutrons to perform thermal neutron imaging . Usually neutrons sources are rated in terms of intensity (the number of neutrons produced by second (s ¹ )) but not in terms of flux (number of neutrons produced by second (s ^_1 ) and by surface unit (s cm‘ ³ )), as required by neutron imaging.

There is therefore a need for a new thermal neutron source or generator for thermal neutron imaging, producing a high flux of thermal neutrons/s/cm ² , both for intensity and resolution concerns.

According to another aspect, the invention concerns imaging techniques in order to image composite materials, for example materials comprising both carbon and ceramic materials, more generally materials which are usually not differentiated by known imaging techniques like X-ray imaging.

There is also a need for a new imaging process, producing sufficient contrast between different materials, in particular materials having similar X-ray attenuation coefficients, which cannot be differentiated by usual imaging techniques, in particular X-ray imaging. For example no device or method is able to provide images of objects or devices comprising hydrogen or an hydrogen -containing compound like for example water, included or embedded in an environment made of ceramics or metal, like a car engine or an aircraft engine. There is also no device nor method for imaging a lubricant inside a piece made of ceramic or metal, for example in order to detect whether a seal or a gasket is tight or watertight enough and/or in order to detect leaks. Usual investigation techniques of a part or of an object or of a piece of material containing such a seal or gasket involve opening or demounting said part or object or piece. It would be very helpful to have a device or a method in order to image such a part or object or piece without having to open or demount it.

SUMMARY OF THE INVENTION

The invention first concerns a neutron generator for thermal neutron imaging, comprising:

- a neutron generator;

- a neutron moderator;

- a neutron reflector at the moderator outlet.

The generator generates essentially fast neutrons, and the moderator slows them down, thus generating thermal neutrons. The moderator has an outlet through which the thermal neutrons escape and travel. Some of them are "off axis" : they are not travelling along the beam axis of the generator and/or towards a target or a sample material to be investigated. The neutron reflector, located downstream from the moderator, is for reflecting at least part of the thermal neutrons towards the beam axis of the device, which neutrons escape from the moderator in a direction not aligned with said beam axis or do not propagate towards a target or a sample material to be investigated.

Both said moderator and said reflector can be made of a same material and can be combined in a same piece.

In an embodiment, said moderator and/or said reflector comprise or can be made of HDPE.

In a more preferred embodiment, said moderator and/or said reflector may comprise, or be made of, heavy water.

The moderator has for example a thickness comprised between 20 mm or 25 mm and 70 mm or 80 mm. A moderator made of heavy water will usually be thinner than a moderator made of HDPE; for example a heavy water moderator can have a thickness of about 40 mm, or comprised between 30 mm and 50 mm, whereas an HDPE moderator can have a thickness of about 60 mm , or comprised between 50 mm and 70 mm.

Preferably the reflector has a thickness of between 2 cm and 10 cm.

A neutron generator according to the invention can further comprise at least one shield, in order to shield the area around the generator, or at least part of it, from gamma rays and/or X rays.

A neutron generator according to the invention can comprise an X-ray filter located at the outlet from said neutron reflector; this filter can have for example a thickness of between 2 mm and 10 mm and can be made of lead or bismuth.

Preferably, a neutron generator according to the invention has an output flux of thermal neutrons between 10 ⁶ s ¹ cm ^-2 and 10 ⁹ s ¹ cm' ² , or between 10 ⁷ s ¹ cm ^-2 and 10 ⁸ s ¹ cm' ² . Such flux ranges allow a rather low exposure time, in particular if the detector comprising microchannel plates is implemented.

The invention also concerns a thermal neutrons imaging system comprising:

- a neutron generator according to the invention, as disclosed above and/or in this application ;

- a detector of thermal neutrons, preferably comprising microchannel plates (MCP); MCPs are preferred because they are particularly efficient in this imaging application.

An imaging system according to the invention, can further comprise a camera imaging an image formed by said detector.

The invention also concerns an imaging process for imaging a sample of material, comprising implementing a thermal neutrons imaging system according to the invention.

The invention also concerns an imaging process for imaging a sample of material, comprising: - positioning said sample between a neutron generator according to the invention as disclosed above and/or in this application and a detector, for example of the type comprising microchannel plates;

- generating thermal neutrons, having for example an output flux of between 10 ⁵ s ¹ cm' ² and 10 ⁹ s ¹ cm' ² ,

- detecting a flux of thermal neutrons travelling through said sample with a neutron detector;

- based on signals provided by said detector, forming an image of said sample with imaging means.

In an imaging process according to the invention, said sample can comprise a mixture or a combination of materials having different thermal neutron attenuation coefficients. For example, it can comprise a mixture or a combination of steel and/or iron and at least one of Boron, Lithium, Carbon, plastic, gold, cobalt, Teflon, aluminum Other combinations of materials are disclosed in this application.

This invention is particularly suited for imaging different combinations of materials, in particular any piece made of ceramic or metal and a fluid comprises in sadi piece, for example water or oil or a lubricant or any liquid . The fluid can be or comprise hydrogen or a hydrogenous material (for example: PTFE, or an organic material, or oil, or grease, or a lubricant, etc). Said piece made of ceramics or metal can be for example:

- a pipe, or an engine such as a car engine or an aircraft engine, or any other part of a vehicle, for example at least part of an aircraft wing or of an aircraft fuselage or of an aerospace device or vehicle;

- at least part of a turbine;

- or at least part of an energy storage material, for example a battery, in order to investigate ions in said material.

BRIEF DESCRIPTION OF THE DRAWINGS

- Figure 1 shows a neutron generator according to the invention;

- Figure 2 shows a neutron generator according to the invention comprising a combined moderator and reflector according to one aspect of the invention; - Figure 3 shows curves giving the relative flux of thermal neutrons depending on the reflector thickness, with respect to the flux for an HDPE moderator (without reflector),

- Figure 4 shows the relative thermal flux obtained with different materials used for both the moderator and the reflector;

- Figure 5 shows an imaging system comprising neutron generator according to the invention, a detector and imaging means;

- figure 6A shows a CPU cooling device and figure 6B shows its thermal neutron image;

- figure 7 shows a thermal neutron image of a pen.

DETAILLED DESCRIPTION OF SPECIFIC EMBODIMENTS

An example of a thermal neutron source or generator nl according to the invention for an imaging device is illustrated on figure 1.

It comprises a neutron generator 2 forgenerating neutrons, most of them being fast neutrons, which are then thermalized by a moderator 4.

More precisely, the neutron generator 2 comprises an ion generator (for example an RFI plasma ion source or a Penning ion source, or any other ion source) including for example a microwave generator; the ion source produces ions from a source of heavy water or from a deuterium- tritium source. The generator further has an accelerating portion in order to accelerate the ions towards a target 22, for example made of titanium or of a titanium alloy. Such a generator produces a neutron beam from a fusion reaction, not from fission which is the reaction taking place in a reactor.

This invention is for performing imaging and the beam of thermal neutrons (which have an energy below 1 eV, for example around 25 meV) produced by the source 1 has a flux which is preferably between 10 ⁶ s ¹ cm' ² or 10 ⁷ s ¹ cm ^-2 and 10 ⁸ s ¹ cm' ² or 10 ⁹ s ¹ cm ^-2 at the outlet of the moderator. In order to collect as many thermal neutrons as possible, a reflector 6 is located at the outlet of the moderator so that thermal neutrons which would not be traveling in the direction of the detector or along direction D (beam axis) could possibly be scattered or reflected back in direction D oriented to the detector. Some high energy neutrons may also escape from the target without being thermalized and can be thermalized by the reflector 6, but the essential effect of the reflector is on thermal neutrons.

The moderator and the reflector can be made of a same material, for example HDPE or heavy water.

The reflector 6 can have for example the shape of a ring, comprising a central hole 61 aligned with the outlet 41 of the moderator. It has an extension, along the beam axis D of the neutrons towards a sample, comprised between 2 or 3 cm and 10 cm or 15 cm, depending on the material.

The neutrons are produced from the target 22, then moderated by the moderator 4, thereby producing essentially thermal neutrons which escape the moderator to enter the reflector 6. A flux of thermal neutrons travels in the direction of the beam axis D and therefore does not interact with the reflector, but part of the thermal neutrons which do not travel in the direction D is reflected or scattered by the reflector 6 and then travels in the direction D. As explained below in connection with figure 3, the flux increase due to the reflector 6 can be significant.

The generator can be housed in a shield 8, 8' for example made of High- density polyethylene (HDPE), possibly with some %, for example 5%, of boron. At least part of the reflector 6 can be positioned in an aperture 81 made in at least part of the shield 8 and/or in an aperture 83 made in part of the moderator.

It is possible to add a further shield 10 for stopping gamma radiations; this further shield can be made of lead. It comprises a hole 101 which is aligned with the hole 61 of the reflector.

A pinhole 12 can be positioned downstream from the reflector, for example between the outlet of the reflector and the further shield 10. It is aligned with the hole 61 of the reflector. The diameter of the pinhole 12 is for example comprised between 1 cm and 5 cm. The pinhole is made in a piece of material which preferably extends over the front face 63 of the reflector (see for example extension 12' of the pinhole on figure 1), so that no neutron can escape from said front face 63: thus, a neutron traveling towards the sample material to be investigated travels through the hole of the pinhole along the beam axis D or along a direction close to it. The pinhole 12 can absorb thermal neutrons which are not directed towards the sample or neutrons which are directed towards the sample but which would not escape from the reflector through the outlet of the hole 61. It is made for example of boron.

A filter 14 can be used for stopping X-rays and can be made of bismuth or lead. It is some mm thick and can be located at the outlet of the device against the further shield 10.

On figure 1, the reflector 6 and the moderator 4 are different pieces. In another embodiment of a source 1' according to the invention, illustrated on figure 2, the reflector 6 is combined with the moderator 4, both being made of a same material.

As already mentioned above, the extension of the reflector along the beam axis D of the neutrons towards a sample is of some cm, comprised between 2 or 3 cm and 10 cm or 15 cm.

Figure 3 shows curves obtained by simulation and giving the relative flux (in s cm‘ ² ) of thermal neutrons measured at the detector inlet according to the reflector thickness for various configurations (moderator and reflector both made of HDPE, moderator and reflector both made of heavy water, moderator made of HDPE and heavy water reflector) , the flux being relative to the flux obtained with an HDPE moderator without reflector. These curves show that the reflector 6 has a significant contribution to the neutron flux and that there is an optimal range for the length I of the reflector (measured along the beam axis of the generator), comprised between 3 cm or 5 cm, and 7 cm or 10 cm. Above 10 or 15 cm, the efficiency of the reflector decreases. The different curves have general shapes which are similar but a combination of a moderator and a reflector both made of heavy water usually has a shorter optimal range (optimal thickness range between 2 cm and 8 cm) than the combination of an HDPE moderator and an HDPE reflector. As can be understood from these curves, a reflector improves the output flux by at least 30% to 70% whereas a heavy water reflector provides a higher rate of improvement, for example between 40 and 80 % . The combination of a moderator and a reflector both made of heavy water is particularly efficient. The invention is particularly well suited for non-destructive imaging of composite parts, comprising for example a combination of a metal and Boron or Lithium or plastic. For example, Stainless steel and iron (Fe) have a thermal neutron attenuation coefficient of about 1 cm ¹ , while the attenuation coefficient of: - Boron is about 99 cm ¹ ;

- plastic (polyethylene, PE) is between 6 and 7 cm' ¹ ;

- gold is about 6.3 cm' ¹ ;

- cobalt is about 3.9 cm' ¹ ;

- Copper (Cu) is about 1 cm ¹ .

Thermal neutron attenuation coefficients of these and other materials are given in the table I below.

Thermal neutron attenuation

Material coefficient (cm ^-1 )

B 98,93846571 Li 33,09715866 Al 0,104507425 Si 0,116753226 Ti 0,591337823 Cr 0,544611463 Mn 1,229496675 Fe 1,204032626 Co 3,921467341 Ni 2,101264972 Cu 1,003239046 Cd 54,77654111 Gd 1509,085309 Au 6,285026849 Pb 0,372041352 Bi 0,258131665

SiO ₂ 0,286906605 AI ₂ O ₃ 0,377131452 Water 5,647912871 PE 6,579583169 Carbon fiber 0,501392978 SiC 0,374603781 Viton 1,169526336 Heavy water 0,651435986 Stainless 1,173421489

Silicone 4,011068222 Teflon 0, 360571269

Kovar 1,901346366

Table I

Carbon fibers have a thermal neutron attenuation coefficient of about 0,5 cm ¹ and teflon, SiC, AI2O3, have coefficients of about 0,4 cm ¹ ; hydrogen or a hydrogenous material embedded in any of these materials can be seen on thermal neutron images.

Aluminum (Al) has a very low thermal neutron attenuation coefficient of about 0,1 cm ¹ which is much less than that of steel or iron or Cu or carbon fibers or plastic or boron or Co, or Au, or Li, allowing a huge contrast between Al and any of these materials by thermal neutron imaging.

The above table shows that a material including at least one of Li, B, Cd and Gd can be particularly interesting for thermal neutron imaging. Li and Cd are for instance found in batteries, and therefore relevant to research toward sustainable technologies. Gd can be dissolved and used as a contrast agent to highlight defects for in ceramic materials; in another application, it can be mixed with or in glue in order to boost the contrast between glue and other material(s): the quality of the assembly of two parts, for example two metal parts, glued with said glue, can be checked by neutron imaging according to the present invention, in particular if Gd is added to the glue.

As already mentioned this invention is particularly adapted to image or visualize materials or compounds like a fluid or a liquid, for example oil or a lubricant or water, or like plastic (PE) or organic materials or a material containing hydrogen or a hydrogenous material, for example rubber, inside ceramic or metallic parts, for example in an engine or in a valve or in a part of an aircraft or of a vehicle (car, truck) or of a machine or in airbags.

This invention is also particularly adapted to image or visualize:

- residual materials, for example organic or ceramic materials, after manufacturing a piece, for example by casting, like a turbine blade;

- the presence or absence or the position of one or more rubber ring(s) inside pieces or parts, like for example airbags or metal or ceramic pieces. More generally, this invention is also particularly adapted to image or visualize samples comprising at least one material having a thermal neutron attenuation coefficient higher than 1 cm ⁴ , for example water or PE or Co or Au, or B, or Ni or Fe in a material having a thermal neutron attenuation coefficient less than 0.5 cm ⁴ or less than 0.2 cm ⁴ , for example aluminum or a semiconductor material, in particular silicon (Si).

Figure 4 shows the thermal flux obtained by simulation for different materials (heavy water (curve I), Polyethylene (curve II), light water (curve III)) used for the moderator 4, depending on the moderator thickness (measured along the D axis). The better efficiency of heavy water is clear and for a smaller moderator thickness (2 to 6 cm, the optimal range being between 5 cm and 7 cm for HDPE and between 6 cm and 8 cm for light water).

A detector for an imaging device according to the invention is most preferably a microchannel plate (MCP) detector. It allows imaging a sample with both a good resolution and a reasonable exposure time (for example between 1 h and 5 mn depending on the flux of thermal neutrons). A MCP is for example disclosed in FR 2925218. A MCP has in particular a very high detection efficiency which makes it very well adapted to a lower flux of neutrons produced by a generator according to the invention compared to a reactor.

Figure 5 shows a neutron generator according to the invention and a microchannel plate (MCP) detector 20, which can be combined with a camera 30 and possibly a lens 40.

A sample 50 is schematically represented between the neutron generator and the detector 20. A beam 52 of thermal neutrons is generated by the source and propagates along the beam axis D to the sample 50.

The distance L between the pinhole 12 or the outlet of the thermal neutron source 1, 1' and the inlet face of the detector can be for example between 0.5 m and 2 m or 3 m. The resolution of the system is limited by the ratio of this distance L over the diameter D of the pinhole and is for example comprised between 20 and 500, for example between 100 and 200. For the same distance L, a smaller diameter D improves the resolution: but the neutron source has a limited flux compared to a reactor, therefore a smaller diameter D limits the neutron flux. When a reactor is implemented for performing thermal neutron imaging, the ratio L/D can be of about 1000 because of the very high intensity or flux generated by the reactor.

The incoming flux of neutrons on the detector is proportional to (1/L): the longer the distance L, the smaller the incoming flux of neutrons. The distance L should therefore not be too long in order to have a sufficient flux on the inlet face of the detector.

As mentioned above a ratio L/D comprised between 20 and 200 is a good compromise for this invention.

Figure 6A shows a CPU cooling block 70 and figure 6B shows the thermal neutron image (the thermal neutron flux being about 10 ⁷ s ¹ cm' ² ) of the same block including water 72 and air bubbles 74 which can be seen on the figure. The cooling block is made of aluminum, which is particularly transparent to neutrons, contrary to the inside cooling fluid which blocks or stops the neutrons.

Figure 7 shows a thermal neutron image of a pen made of metal which is very transparent to neutrons and makes it possible to visualize the presence of ink inside the pen. This shows the feasibility of imaging of fluid or a liquid like water or like a lubricant in an metal piece, like a tube or a duct, for example in an engine.

The invention is particularly well adapted to:

- The imaging of inserts, for example made of plastic or ceramic, in metallic parts;

- or to the detection of gaps or inclusions in a material;

- or to the detection of water infiltration in a material, for example in metal pieces, or in a soil, or in a porous material;

- or to the detection of corrosion in metal pieces, in particular in aluminum pieces, for example in aircrafts parts, for example aircrafts wings or in an aircraft fuselage or in parts of an aerospace device or vehicle;

- orto the detection of ceramic residues in channels made in metal pieces, for example in turbine blades or in aluminum pieces for example in aircrafts parts, for example aircrafts wings or in an aircraft fuselage or in parts of an aerospace device or vehicle;

- or to test the porosity or the internal structure of parts or pieces made by additive manufacturing; - or to identify the location of ions or ions transports in energy storage materials, for example in batteries or in or around electrodes;

- or to identify the transport or the location of fluid(s) in porous material(s), for example in a soil or in stone (archaeology);

- or to identify two-phase liquids in pipe(s), for example in heat pipe(s); - or to identify Carbon or CO? sequestration or bio-sequestration in a material.

When imaging gaps or cracks or ceramic residues or material or porosity in any material, a contrast agent, for example Gd dissolved in a liquid, can be used to enhance the differences between the different parts of the piece or of the sample.