WO2016107932A1

WO2016107932A1 - Method for producing a neutron reflector and neutron reflector produced by such a method

Info

Publication number: WO2016107932A1
Application number: PCT/EP2016/050030
Authority: WO
Inventors: Valeri NESVIJEVSKI; Alexei BOSAK
Original assignee: Nesvijevski Valeri; Bosak Alexei
Priority date: 2014-12-29
Filing date: 2016-01-04
Publication date: 2016-07-07
Also published as: FR3031228B1; FR3031228A1

Abstract

The invention relates to a method for producing a neutron reflector, comprising: producing a nanodiamond powder, said nanodiamonds having a surface saturated with fluorine, and forming a wall (11, 12) of the reflector encapsulating a layer (11c, 12c) of said nanodiamonds in an envelope (11a, 11b; 12a, 12b) made of a material that does not absorb the neutrons. The invention also relates to a neutron reflector comprising a wall (11, 12) formed from a layer (11c, 12c) of nanodiamond powder in an envelope (11a, 11b; 12a, 12b) that does not absorb the neutrons, said nanodiamonds having a surface saturated with fluorine.

Description

METHOD FOR MANUFACTURING A NEUTRON REFLECTOR

AND NEUTRON REFLECTOR OBTAINED BY SUCH A METHOD

FIELD OF THE INVENTION

The present invention relates to a method of manufacturing a neutron reflector and a neutron reflector obtained by such a method.

BACKGROUND OF THE INVENTION

Different materials are adapted to reflect neutrons according to the speed of said neutrons.

For neutrons with a velocity between 10 and 160 m / s, publications highlighted the reflective properties of nanodiamonds [1] - [3].

Such nanodiamonds have a size of between a few nanometers and a few tens of nanometers.

These nanodiamonds are typically, but not exclusively, obtained by detonation of a mixture of explosives and graphite in a low oxygen atmosphere.

These nanodiamonds have a core consisting of carbon atoms and a surface envelope comprising various impurities, including hydrogen atoms.

To form a neutron reflector, a layer of nanodiamonds is trapped in a holding envelope.

However, the hydrogen atoms, present in the surface envelope and / or in the water present on the surface of the nanodiamonds, are attributed to a loss of effectiveness of the reflection. Indeed, the hydrogen atoms cause an inelastic neutron scattering ("up-scattering" in the English terminology) so that said neutrons pass through the layer of nanodiamonds instead of reflecting on the particles [1]. Hydrogen atoms are also responsible for nuclear neutron absorption.

One solution would be to cool the nanodiamonds to a very low temperature so as to minimize the phenomenon of inelastic neutron scattering on the hydrogen atoms.

However, such a solution makes the implementation of the neutron reflector more complex. Moreover, it does not make it possible to remedy the phenomenon of nuclear absorption.

Another solution would be to replace hydrogen with deuterium, which does not interfere with the reflective properties of nanodiamonds. This would involve making nanodiamonds from explosives or other precursors containing deuterium. However, such a manufacturing process does not currently exist on an industrial scale and would prove very expensive.

BRIEF DESCRIPTION OF THE INVENTION

An object of the invention is to improve the reflection of neutrons in a wide range of speed. Another object of the invention is to design a method of manufacturing a neutron reflector which is easy to implement industrially and is inexpensive.

According to the invention, there is provided a method of manufacturing a neutron reflector comprising:

obtaining a powder of nanodiamonds, said nanodiamonds having a surface saturated with fluorine,

the formation of a wall of the reflector by encapsulating a layer of said nanodiamonds in an envelope made of a material that does not absorb neutrons.

According to one embodiment, said nanodiamonds are obtained by detonation of a mixture of explosives and graphite and substitution, on the surface of said nanodiamonds, of at least a portion of the hydrogen atoms by fluorine atoms.

Alternatively, said nanodiamonds are obtained by a laser process and then substitution, on the surface of said nanodiamonds, of at least a portion of the hydrogen atoms by fluorine atoms.

Advantageously, the substitution of the hydrogen atoms by the fluorine atoms is obtained by the reaction of the nanodiamonds with fluorine gas.

Said reaction is advantageously carried out at a temperature of between 300 and 600 ° C.

In particularly advantageous manner, the encapsulation of nanodiamonds in the envelope is implemented at a temperature between 400 and 500 ° C and under a vacuum between 10 ^"4 and ^{10" -3} mbar or under an inert gas

Another object relates to a neutron reflector obtained by the method described above. Said reflector comprises a wall formed of a nanodiamond powder layer in a casing made of a material that does not absorb neutrons, said nanodiamonds having a fluorine saturated surface.

The envelope is advantageously made of a material consisting of atoms chosen from the following elements: O, C, F, Be, D, Zr and Al.

Preferably, the envelope comprises a sheet and / or a plate of polytetrafluoroethylene (PTFE), aluminum or beryllium.

Advantageously, on the inner face of the reflector, intended to be exposed to neutrons, the envelope has a thickness between 5 and 30 μηι. Moreover, the thickness of the nanodiamond layer is typically between 1 and 1000 mm.

The reflector further advantageously comprises a structure for holding the envelope containing the nanodiamonds.

The invention also relates to a neutron guide comprising a reflector as described above.

The invention also relates to a neutron trap comprising a reflector as described above.

The invention also relates to an assembly comprising a neutron source and a reflector as described above surrounding said source.

Finally, the invention relates to a method for reflecting neutrons having a speed of between 160 and 700 m / s, characterized in that it comprises the provision of a device comprising a reflector as described above and the reflection of said neutrons on the fluorine saturated surface of the nanodiamonds of said reflector.

BRIEF DESCRIPTION OF THE DRAWINGS

Other characteristics and advantages of the invention will emerge from the detailed description which follows, with reference to the appended drawings in which:

FIG. 1 is a block diagram of an installation comprising a neutron trap,

FIG. 2 is a sectional view of the wall of the neutron trap,

FIG. 3 is a sectional view of the neutron trap cover, FIG. 4 is a block diagram of an assembly of a neutron source and a neutron reflector,

FIG. 5 is a curve illustrating the reflection of neutrons as a function of their speed expressed in m / s.

DETAILED DESCRIPTION OF THE INVENTION

The invention applies to the reflection of neutrons in a speed range of between 10 and 700 m / s.

Indeed, as shown in FIG. 5, the known devices make it possible to reflect the neutrons either in a speed range of less than 160 m / s (curve a, obtained with a coating of DLC (acronym for the Anglo-Saxon term "Diamond- Like Carbon ")) or in a speed range greater than 700 m / s (curve b, obtained with a graphite wall). Curve c illustrates the reflection obtained with a supermirror. The curve d shows the reflection obtained with a reflector according to the invention and shows that the invention makes it possible to cover a speed range for which no reflection could previously be obtained (namely the range between 160 and 700 m / s). . The nanodiamonds used in the present invention are generally obtained, conventionally, by detonation of a mixture of explosives and graphite in a low oxygen atmosphere, or by a laser process. Reference may be made to Articles [4, 5] which respectively describe each of these methods.

Said nanodiamonds are in the form of a powder whose size is between a few nanometers and a few tens of nanometers.

In FIG. 5, the points e represent the neutron reflection obtained with a reflector consisting of a 3 cm layer of such nanodiamonds in a non-neutron absorbing envelope. As can be seen, even if the reflection is improved with respect to the devices corresponding to the curves a and c, it is limited to neutrons having a speed lower than 160 m / s.

According to the invention, said nanodiamonds resulting directly from the abovementioned manufacturing processes are not used, but a fluorination process is applied to them.

For this purpose, the powder is exposed to fluorine gas at high temperature (typically between 300 and 600 ° C.) at a pressure of approximately 1 bar.

The document [6] describes an example of implementation of such a method of fluorination of nanodiamonds. It will be noted that in this document the fluorination process is an intermediate step of a method of functionalization of the nanodiamonds, the fluorine atoms being intended to be subsequently substituted by functional groups. On the contrary, the present invention uses nanodiamonds resulting preferably directly from the fluorination.

The fluorination process makes it possible to substitute the hydrogen atoms with fluorine atoms on the surface without affecting the core composition of the nanodiamonds.

On the other hand, the fluorine atoms do not deteriorate the reflective properties of the nanodiamonds. On the other hand, since the optical potential of fluorine is slightly positive while that of hydrogen is slightly negative, the reflectivity of the fluorinated nanodiamonds is slightly improved.

On the other hand, fluorination makes it possible to eliminate the hydrophilic groups on the surface of nanodiamonds. This therefore decreases the presence of water on the surface of nanodiamonds.

Another advantage of fluorination is that it reduces the concentration of metallic impurities present on the surface of nanodiamonds. However, these impurities are harmful insofar as, when they are irradiated by neutrons, they become moderately radioactive and therefore require special reprocessing.

At the end of the fluorination step, the surface of the nanodiamonds is saturated with fluorine. The estimation of the fluorine content can be carried out for example by EDX spectroscopy (acronym for the English term "Energy Dispersive X-ray"). In addition, nanodiamonds have a surface hydrogen atom content that is at least 50 times lower than the initial content.

The estimation of the hydrogen content can be carried out according to the method described in [7].

Fluorination does not remove the hydrogen atoms in the core of nanodiamonds, but insofar as these atoms are present in small amounts, they do not interfere with reflective properties.

Finally, the fluorinated nanodiamonds have a low residual content of metal impurities, so that even if these residual impurities are irradiated, the resulting radioactivity is low enough not to require specific reprocessing.

In order to manufacture a neutron reflector, the fluorinated nanodiamonds described above are used so as to form a layer contained in an envelope and this layer is put in place in a structure which makes it possible to maintain the layer in the desired position.

The thickness of the nanodiamond layer is as great as possible to improve the reflectivity, the layer to be thicker as the neutron velocity is large.

Advantageously, depending on the speed of the neutrons, said thickness is between 1 and 1000 mm.

The casing has sufficient mechanical rigidity to give the reflector the desired shape. The envelope is made of a material that does not absorb neutrons, that is to say a material containing only atoms selected from the following elements: O, C, F, Be, D, Zr and Al. For example, the envelope may be made of polytetrafluoroethylene (PTFE), aluminum or beryllium. The thickness of the envelope is preferably the finest possible while having a suitable mechanical stability. Typically, the envelope has a thickness of between 5 and 30 μηι.

The conditioning of the nanodiamonds in the envelope is carried out under particular temperature and pressure conditions to preserve the reflection properties of the layer. In particular, the presence of water, which would absorb the neutrons, should be avoided. Thus, the introduction of the nanodiamonds in the envelope is advantageously carried out at a temperature of the order of 400 to 500 ° C, under vacuum (of the order of 10 ^"4 to 10 ^{" 3} mbar) or presence of an inert gas (or gas mixture).

One application of the invention relates to a neutron trap comprising a reflector as described above.

Figure 1 is a block diagram of an installation comprising such a neutron trap. A similar installation, in which nanodiamonds are not fluorinated, is described in [2]. The trap is typically in the form of a cavity delimited by a wall provided with an opening 10 through which neutrons are introduced into the trap. The opening has a surface of a few cm ² . For example, the trap has a cylindrical shape and the opening is arranged in the circumferential wall of the trap.

The walls of the trap are formed of the neutron reflector described above. Thus, as illustrated in FIG. 2, which has a cross-sectional view of the wall 11, each wall comprises a layer 1 1 c of fluorinated nanodiamonds contained in an envelope 11a, 11b and held in a structure defining the shape of the trap.

On the outer face of the trap, the envelope January 1 comprises for example an aluminum plate having a thickness of about 1 mm, this plate essentially fulfilling a mechanical support function.

On the inside of the trap, the envelope 1 1b comprises for example an aluminum foil having a thickness of the order of 5 to 30 μηη.

The thickness of the layer 1 1 c of fluorinated nanodiamonds is between 1 and 1000 cm depending on the speed of the neutrons.

The trap 1 further comprises a lid 12, also comprising a reflector as described above. As illustrated in FIG. 3, the cover 12 comprises a layer 12c of fluorinated nanodiamonds contained in an envelope 12a, 12b defining the shape of the trap.

On the outer face of the cover, the envelope 12a comprises for example an aluminum plate having a thickness of the order of 1 mm.

On the inside of the cover, the envelope 12b comprises for example an aluminum foil having a thickness of the order of 5 to 30 μηη.

The thickness of the layer 12c of fluorinated nanodiamonds is between 1 and 1000 cm depending on the speed of the neutrons.

The cover 12 has an aluminum window 120 through which the neutrons can exit the trap 1.

Returning to Figure 1, the trap 1 is arranged in a vacuum chamber 2. The enclosure comprises, facing the window 10, a quartz window 20 through which the neutrons can enter the chamber 2.

A neutron source (not shown) emits a beam 3 of neutrons having a diameter of about 1 cm towards the window 20 and the opening 10.

In the path of the beam 3, the installation comprises a speed selector 4 and a valve 5. These devices are known as such and will not be described in detail.

The neutrons introduced into the trap 1 undergo multiple reflections by the nanodiamonds contained in the walls of the trap and the lid. The residual content in Hydrogen atoms on the surface of nanodiamonds provide a much higher reflectivity than known traps.

Opposite the window 120 of the trap cover, the installation comprises a detector 6 for counting the number of neutrons.

Another application of the invention relates to a neutron guide comprising a reflector as described above.

Another application of the invention, illustrated schematically in FIG. 4, concerns an assembly formed of a source 7 of neutrons surrounded by a reflector 8 as described above. The wall 1 1 of the reflector being similar to that of the trap 1 described with reference to Figure 1, it is not described again here. The reflector 8 comprises a window 1 10 for example made of aluminum to allow the output of neutrons. Naturally, the source and the associated trap can take different forms depending on the speed of the neutrons emitted. REFERENCES

[1] The reflection of very cold neutrons from diamond powder nanoparticles, V.V.

Nesvizhevsky et al, Nuclear Instruments and Methods in Physics Research

A595 (2008) 631-636

[2] Storage of very cold neutrons in a trap with nano-structured walls, E.V.

Lychagin et al., Physics Letters B 679 (2009) 186-190

[3] Application of Diamond Nanoparticles in Low-Energy Neutron Physics, V.V.

Nesvizhevsky et al, Materials 2010, 3, 1768-1781

[4] Diamonds in detonation, N. Roy Greiner et al, Nature, Vol. 333, June 1988, pp. 440-442

[5] Structure of Nanodiamonds Prepared by Laser Synthesis, M. V. Baidakova et al, Physics of the Solid State, 2013, Vol. 55, No. 8, pp. 1747-1753

[6] US 2005/0158549

[7] Study of Bound Hydrogen in Powders of Diamond Nanoparticles, A.R. Krylov et al, ISSN 1063-7745, Crystallography Reports, 201 1, Vol. 56, No. 7, pp. 102-

Claims

1. A method of manufacturing a neutron reflector comprising:

the formation of a wall (1 1, 12) of the reflector by encapsulating a layer (11c, 12c) of said nanodiamonds in an envelope (11a, 11b, 12a, 12b) made of an absorbent material not the neutrons.

2. The method of claim 1, wherein the nanodiamonds are obtained by detonation of a mixture of explosives and graphite and substitution on the surface of said nanodiamonds, at least a portion of the hydrogen atoms by atoms of fluorine.

3. The method of claim 1, wherein the nanodiamonds are obtained by a laser process and substitution on the surface of said nanodiamonds, at least a portion of the hydrogen atoms by fluorine atoms.

4. Method according to one of claims 2 or 3, wherein the substitution of hydrogen atoms by the fluorine atoms is obtained by the reaction of nanodiamonds with fluorine gas.

5. The process according to claim 4, wherein the reaction is carried out at a temperature between 300 and 600 ° C.

6. Method according to one of claims 1 to 5, wherein the encapsulation of nanodiamonds in the envelope is implemented at a temperature between 400 and 500 ° C and under a vacuum between 10 ^"4 and 10 ^{" 3} mbar or under an inert gas.

7. Neutron reflector comprising a wall (1 1, 12) formed of a layer

(1 1 c, 12c) nanodiamond powder in a casing (11a, 11b; 12a, 12b) of a non-neutron absorbing material, said nanodiamonds having a saturated fluorine surface.

8. Reflector according to claim 7, wherein the envelope is made of a material consisting of atoms selected from the following elements: O, C, F, Be, D, Zr and Al.

9. Reflector according to claim 8, wherein the envelope comprises a sheet and / or a plate of polytetrafluoroethylene (PTFE), aluminum or beryllium.

10. Reflector according to one of claims 7 to 9, wherein, on the inner face of the reflector, the envelope has a thickness between 5 and 30 μηι.

1 1. Reflector according to one of claims 7 to 10, wherein the thickness of the layer (1 1 c, 12c) of nanodiamonds is between 1 and 1000 mm.

12. Reflector according to one of claims 7 to 11, further comprising a structure for holding the envelope containing the nanodiamonds.

13. A neutron guide comprising a reflector according to one of claims 7 to

10.

14. Neutron trap comprising a reflector according to one of claims 7 to

10.

15. An assembly comprising a source (7) of neutrons and a reflector (8) according to one of claims 7 to 10 surrounding said source.

16. A method for reflecting neutrons having a speed of between 160 and 700 m / s, characterized in that it comprises the provision of a device comprising a reflector according to one of claims 7 to 12 and the reflection of said neutrons on the saturated fluorine surface of the nanodiamonds of said reflector.