EP0338619B1 - High-flux neutron source with long life target - Google Patents

Description

The invention relates to a high flux neutron generator with a target struck by a beam of isotope ions of hydrogen, said target being constituted by a structure comprising a metal layer with a high absorption coefficient with respect to the hydrogen, produced on a support layer made of a metal with a high coefficient of heat conductivity and a low degree of volatilization.

Such generators, such as that described in patent FR-A 2 438 953 , are used for example in techniques for examining matter by fast, thermal, epithermal or cold neutrons.

Neutrons are generated by reactions between nuclei of the heavy isotopes of hydrogen: deuterium and tritium. These reactions occur because a target, containing deuterium and tritium, is subjected to the bombardment of a beam of deuterium ions and tritium ions accelerated under a high potential difference. The deuterium ions and the tritium ions are formed in an ion source in which a gaseous mixture of deuterium and tritium is ionized. The collision between a deuterium nucleus and a tritium nucleus provides a neutron with an energy of 14 MeV, and a particle -α with an energy of 3.6 MeV.

In order to obtain the maximum reaction yield, it is in particular advisable to have the highest possible density of target nuclei. A commonly used means for making such targets with isotopes of hydrogen consists in fixing the nuclei in the crystal lattice of a hydrurable material.

Among these materials, titanium is often used because of its lower stopping power, which results in better neutron yield. However, these materials have the drawback of insufficient mechanical strength when the hydrogen concentration is high and the material in "thick layer" (disintegration phenomenon causing the dispersion of metal particles, which is detrimental to the voltage withstand of the devices ion beam acceleration).

Consequently, these materials must be used in thin layers on a support or a substrate which must have a low absorption and diffusion coefficient with respect to hydrogen, a good thermal conductivity allowing the dissipation of the dissipated energy. and good resistance to corrosion with respect to the coolant. A copper support, for example, partially meets these criteria but has a high sputtering coefficient. A target with good mechanical strength is difficult to achieve with this support because the coefficient of linear expansion of titanium is very different from that of copper. Furthermore, in the case of a beam with a non-uniform energy density, the lifetime of the target would be very limited because after erosion of the titanium layer at the places of high density of the ion beam, the copper of the support would quickly be pulverized on the surrounding titanium surface and would considerably slow down the energy of the ions and consequently the yield of neutrons; this would lead simultaneously to the piercing of the support layer.

One way of avoiding this phenomenon is to form an intermediate layer of a material such as molybdenum, more resistant to ionic erosion and less permeable to hydrogen, between the support layer and the hydrogen-absorbing surface metal layer. to its isotopes. Thus the concentration of hydrogen ions in the surface layer increases rapidly until a state of equilibrium is established in which the quantity of hydrogen which penetrates into said surface layer is equal to that which leaves it by diffusion. We then reach the maximum concentration of atoms of tritium in the thin layer of titanium and therefore the highest neutron yield.

In the aforementioned French patent No. 2,438,953 , the development of a second intermediate layer of a material such as vanadium, the coefficient of linear expansion of which is situated between those of the support layer and of the first intermediate layer, allows better adhesion of surfaces in contact.

The successive improvements made to the development of the target in the abovementioned embodiments were aimed at extending its lifetime by slowing down the process of erosion of the substrate under the effect of ion bombardment, it being understood moreover that the beam is made up of an equimolecular deuterium-tritium mixture so that the ions extracted from the source and implanted in the target after acceleration do not lead to a depletion of the target nuclei in favor of the nuclei of the beam.

At this stage, the ion implantation of the beam takes place in layers of the support materials, the stopping power of which, much higher than in the active layer, causes the neutron emission to drop considerably, leading to the end of the operational life of the tube .

The object of the invention is to provide a neutron generator with a target which is struck by a beam of hydrogen ions, the lifetime of this target subject to the influence of an ion beam bombardment. high intensity being longer than the lifetime of known neutron generator targets.

According to the invention, the neutron generator of the kind mentioned in the preamble is remarkable in that the activated layer with high absorption coefficient consists of a stack of 2 to 5 identical metallic layers isolated from each other by a diffusion barrier, the thickness of said layers with a high absorption coefficient being equal for example to the penetration depth of the deuterium ions which strike the target.

In this way, the deep layers are hydrated only in stages as the diffusion barriers are pierced under the effect of erosion due to bombardment. The lifespan of known targets comprising a single layer as active layer can thus be multiplied by the number of metallic layers superimposed in the target of the invention.

In addition, limiting the diffusion of tritium to the thickness of a layer makes it possible not to dilute the concentration of the target nuclei beyond the beam penetration zone, which has the double advantage of accelerating the impregnating the target and improving the neutron yield.

Another advantage consists in the reduction of the total amount of deuterium-tritium mixture necessary for the functioning of the tube, especially marked with regard to the amount of tritium which is known to decompose progressively into He3, which correlatively increases the pressure residual in the tube.

The metal of the highly hydrogen permeable layers belongs to the group comprising titanium, zirconium, scandium, yttrium and lanthanides, while the metal forming the support layer belongs to the group comprising molybdenum, tungsten, tantalum, chromium and niobium.

Diffusion barriers can be developed by chemical means such as nitriding in reactive plasma, deposition of passivated layer by oxidation or by physical means such as deposition of an appropriate metallic layer, ion implantation, etc.

The following description with reference to the appended drawing, all given by way of example, will make it clear how the invention can be implemented.

Figure 1a is a schematic longitudinal section of a neutron generator equipped with the target according to the invention.

FIG. 1b represents on a larger scale a part of the target of the generator represented by FIG. 1a.

In the neutron generator represented in FIG. 1a, an envelope 1 contains a gaseous mixture in equal proportions of deuterium and tritium under a pressure of the order of a few tenths of Pascals (thousandths of a millimeter of mercury). This gas mixture is supplied via a pressure regulator 2. The gas pressure is controlled using an ionization pressure gauge 3. The mixture of deuterium and tritium is ionized in the ion source 4 and an ion beam is extracted therefrom by the acceleration electrode 5 secured to the casing 1 and cooled at 6 by a circulation of water. With respect to this electrode 5, the anode 7 is brought to a positive very high voltage potential (+ THT).

The ion source 4 of the Penning type further comprises two cathodes 8 and 9 brought to the same negative potential of the order of 5 kV relative to the anode 7 and a permanent magnet 10 creating an axial magnetic field and the magnetic circuit is closed by the ferromagnetic socket 11 which envelops the ion source 4. The positive high voltage + THT is applied to the source by the cable 12, the end of which is surrounded by the insulating sleeves 13 and 14.

The ion beam passes through the suppressor electrode 15 and strikes the target 16 cooled at 17 by a circulation of water. Part of this target is shown on a larger scale in Figure 1b.

The target 16 consists of a molybdenum substrate 18 forming the support layer on which a titanium layer 19 is formed. According to the invention, a first hydrogen diffusion barrier 20 is successively produced, followed by a titanium layer. 21, then in the same way, the diffusion barriers 22, 24 and 26 alternating with the titanium layers 23, 25 and 27 respectively, which have the same thickness.

The choice of the thickness of said layers is related to the penetration depth of the deuterium ions coming to strike the titanium target to generate there by collision with the implanted tritium ions, a neutron emission of 14 MeV. This avoids the depletion of the surface concentration of the target in tritium nuclei which would result from their diffusion towards the inside of a thicker layer.

The regeneration of the tritium target nuclei is suitably ensured if the deuterium-tritium mixture inside the neutron tube of FIG. 1a is found in equal amounts.

Due to the non-uniform distribution in density of the ion beam indicated in 28, there is implantation of tritium in greater quantity in the target area struck by the central part of the beam and correspondingly an increasingly accentuated erosion of the first layer of titanium 27, as one approaches this central part as indicated in 29. The drilling of layer 27 therefore occurs in this same central part followed by erosion and then drilling of the diffusion barrier 26. The titanium layer 25 already partially impregnated with the ions having crossed the eroded zones of the layer 27 and of the barrier 26 then begins to be directly impregnated by the beam while the barrier 24 fulfills its protective role to limit the diffusion of hydrogen ions and therefore maintain their concentration at roughly the same level as it had in the immediately upper layer.

So step by step, after each piercing of a diffusion barrier, we will impregnate the underlying titanium layer while preventing the next barrier by diffusion penetration of tritium ions to the lower layers. As a result, the concentration rate of hydrogen ions in the successive titanium layers and consequently the level of neutron emission are kept substantially constant as these successive layers are eroded.

We limited ourselves to achieving a target with five active layers allowing a gain close to five over the lifetime, the multiplication of a number of layers greater than the above value being liable to pose more complex problems. to master.

• 1 - On dépose une couche de titane sur un substrat en molybdène constituant l'anode du dispositif de pulvérisation dont la cathode est une cible en titane. Cette cible est bombardée par les ions d'un gaz neutre et lourd tel que l'argon à coefficient de pulvérisation élevé. Les atomes d'argon ionisés sont ensuite projetés sur le substrat jusqu'à ce que l'épaisseur désirée soit atteinte.
• 2 - On évacue l'argon que l'on remplace par de l'azote, gaz moins lourd et non neutre à partir duquel il va se déposer une couche de nitrure de titane qui constitue la barrière de diffusion par dessus la couche précédente de titane.
• 3 - On évacue l'azote pour le remplacer par de l'argon afin de déposer une autre couche de titane.
• 4 - Les étapes 2 et 3 sont renouvelées autant de fois qu'on le désire en introduisant alternativement de l'argon et de l'azote dans le bâti de pulvérisation sans interrompre nécessairement le processus de dépôt.

The target of the invention can be produced according to a sputtering method comprising the following steps:

• 1 - A layer of titanium is deposited on a molybdenum substrate constituting the anode of the sputtering device whose cathode is a titanium target. This target is bombarded by ions of a neutral and heavy gas such as argon with a high spray coefficient. The ionized argon atoms are then projected onto the substrate until the desired thickness is reached.
• 2 - The argon is removed and replaced with nitrogen, a lighter, non-neutral gas from which a layer of titanium nitride is deposited which forms the diffusion barrier over the previous layer of titanium .
• 3 - The nitrogen is removed to replace it with argon in order to deposit another layer of titanium.
• 4 - Steps 2 and 3 are repeated as many times as desired by alternately introducing argon and nitrogen into the spray frame without necessarily interrupting the deposition process.

This method of obtaining diffusion barriers by nitriding in reactive plasma is not limiting. It obviously does not exclude the use of barriers obtained by any other chemical process such as oxidation, or physical process such as the deposition of intermediate metallic layers or barriers produced by ion implantation.

Claims (4)

1. A neutron generator, comprising a target which is struck by a hydrogen isotope ion beam and which is formed by a structure comprising an active layer which is formed by a metallic layer having a high hydrogen absorption coefficient which is deposited on a carrier layer which is made of a metal having a high thermal conductivity coefficient and a low degree of volatilization, characterized in that said layer having a high hydrogen absorption coefficient is formed by a stack of from 2 to 5 identical layers which are isolated from one another by a diffusion barrier.
2. A neutron generator as claimed in Claim 1, characterized in that the metal of said layers having a high hydrogen absorption coefficient belongs to the group comprising titanium, zirconium, scandium, yttrium and the lanthanides, whereas the metal forming said carrier layer belongs to the group comprising molybdenum, tungsten, tantalum, chromium and niobium.
3. A neutron generator as claimed the Claim 1 or 2, characterized in that said diffusion barriers are formed by titanium nitride layers.
4. A target intended for a neutron generator as claimed in any one of the preceding Claims.

Images