Method for producing a metallic thorium target for the production of radionuclides

Abstract

The invention relates to the field of nuclear technology, and more particularly to technology for producing radioactive isotopes for medicinal purposes, especially actinium-225, as well as other radionuclides, formed during the charged-particle (proton) irradiation of thorium in an accelerator. A key issue for achieving a high yield of radionuclides during proton irradiation of a target is that of manufacturing the target from sufficiently thick metallic thorium enclosed in a hermetic metallic niobium casing which is resistant at high temperature to chemical and radiation damage. Effective water cooling during irradiation is achieved by assuring good contact of a niobium foil (proton beam entrance and exit windows) with the thorium, as well as with other niobium parts of the casing. For this purpose, during production of the target, diffusion welding is carried out at a temperature of 1250-1430°С and a pressure of 30-150 kg / cm2 on the surface area of the target, wherein a hollow channel is formed in advance in the niobium body of the target to compensate for greater expansion of the thorium during welding. The target may be additionally coated from the outside with a protective layer of from 40 to 100 μm of another metal resistant to water during irradiation.

Description

[0001] Method for manufacturing a target from metallic thorium for producing radionuclides

[0002] Field of technology

[0003] The invention relates to the field of nuclear technology, namely, the technology of producing radioactive isotopes for medical purposes. The invention concerns the production of agnium-225 by irradiating thorium ( 232 Th) by accelerated protons or other particles. Actinium-225 and its daughter product, bismuth-213, are particularly promising for cancer therapy. By irradiating a metallic thorium target, other radionuclides can also be produced, including radium-223, protactinium-230 with uranium-230, and others.

[0004] Manufacturing a reliable target is a central issue, determining the productivity, safety, and reliability of the production process. Achieving high productivity requires sufficiently massive targets irradiated with high-intensity beams of charged particles. At the same time, the targets are subjected to significant thermal, chemical, and radiation loads, and the destruction of a highly radioactive target during irradiation is unacceptable.

[0005] Metallic thorium is a better target material than thorium compounds (such as thorium dioxide), as the compounds contain less of the working material from which the target radionuclide is formed and have poorer thermal conductivity than metallic thorium. During irradiation in an accelerator or nuclear reactor, the target is typically cooled by a stream of water. Metallic thorium is a chemically active material. The cladding material must not only be resistant to thorium at the high temperatures generated in the target during high-intensity irradiation but also must not react with the cooling water, which produces active ions, radicals, and hydrogen peroxide during irradiation. This active water corrodes many materials that are otherwise stable in water, such as aluminum, molybdenum, tungsten, and graphite.Metallic niobium is one of the best materials for the shell, since the eutectic formation temperature with thorium is high and is about 1435 °C, and niobium does not react with active water during irradiation.

[0006] Prior art

[0007] A known method for producing a metallic thorium target for producing radioactive isotopes [S. Mirzadeh et al., Method for the simultaneous preparation of Radon-211, Xenon-123, Astatine-211, Iodine-125, and Iodine-123. Patent US 4,664,869, 1987], involves wrapping a thorium-232 sample in aluminum foil. A disadvantage of this target is that it is not designed for high-current irradiation; it uses a non-hermetic shell, which can lead to target destruction during irradiation and the release of resulting radionuclides into the coolant circuit.

[0008] In another well-known method [JR Griswold et al. Appl. Radiat. Isot, 2016, v. 118, 366-374], 0.6-0.7 mm thick thorium disks are hermetically sealed using electron beam welding into a stainless steel (Inconel) shell. This approach does not achieve good contact between the target shell and the metallic thorium, so only thin targets are used. To improve performance, several successively arranged targets with water slits for cooling are used, which reduces performance because some of the proton energy is lost in these water slits.

[0009] The closest technical solution for target production is the method of producing a target from metallic thorium [B.L. Zhuikov et al. Method for producing actinium-225 and radium isotopes and a target for its implementation (variants). Russian Federation Patent No. 2373589, September 23, 2008, BI No. 32, 2009], where a massive monolith of metallic thorium in the form of a disk 7 mm thick is welded by diffusion welding in a vacuum to input windows made of hot-rolled molybdenum foil 100 μm thick, and then coated from the outside with electrolytic nickel (nickel thickness 60 μm). In this case, the temperature of diffusion welding is about 900 °C, and the specific pressure is 280 kg / cm 2The diffusion-welded part is further sealed by welding it along the target's contour with 0.5 mm-thick niobium rings using electron beam welding. A disadvantage of this method is that, although molybdenum has high thermal conductivity, it is not the best material for the target shell, as it is difficult to machine, brittle, and reacts with cooling water during irradiation. A nickel coating does not always provide reliable protection under high-intensity irradiation. Thorium targets in a nickel-coated graphite shell, a target in a stainless steel shell, and a target in a niobium shell, manufactured using diffusion welding, as well as electron beam or laser welding along the target's contour, are also described. However, the manufacturing method for the latter is not specified, and the manufacturing conditions are critical for the characteristics and quality of the target.

[0010] Disclosure of invention

[0011] The technical result of this invention is the production of a thorium target that is hermetically sealed, provides heat dissipation and is thus reliable when irradiated in a charged particle (proton) accelerator with a high beam intensity and, at the same time, is sufficiently massive and thick to ensure a high yield of actinium-225 and other radionuclides formed in this target.

[0012] The technical result of the invention is achieved by the manufacturing method utilizing diffusion welding of metallic thorium to the plane of the niobium target shell foil (the target's beam entrance and exit windows), which ensures good contact between the shell and the metallic thorium in the beam zone and effective target cooling during irradiation. A disk of metallic thorium is placed tightly inside a massive niobium ring. Diffusion welding of the niobium foil to the plane of the thorium disk and niobium ring is performed in the temperature range of 1250 to 1430 °C, with the best results achieved in the range of 1350 to 1400 °C. Moreover, the pressure on the target surface area is 30-150 kg / cm. 2 , and the best results are achieved at pressures from 50 to 100 kg / cm 2. The diffusion welding time at the specified temperature ranges from 20 to 120 min, with 30-60 min being optimal. At temperatures lower than 1250 °C, diffusion is insufficient to ensure a strong connection between thorium and niobium. The best results are observed at temperatures above 1350 °C, and at 1435 °C and above, a eutectic reaction between niobium and thorium may occur with the formation of a liquid phase, as follows from the phase diagram of the Th-Nb system [Phase diagrams of binary metallic systems: Handbook in 3 vols. V2 / Under the general editorship of N.P. Lyakishev., Moscow: "Mashinostroenie", 1997. - 1024 p.], which may lead to the destruction of the shell. And due to fluctuations and uneven temperature distribution, it is best to maintain it below 1400 °C.

[0013] An experiment using metallographic analysis of thin sections using optical and electron microscopes showed that at the specified temperatures, complete connection occurs at a pressure of 30-150 kg / cm 2, and at higher pressures at a given temperature, symptoms of part flattening appear. The most reliable and reproducible results are observed at pressures from 50 to 100 kg / cm 2 .

[0014] The experiment also showed that with a time of less than 20 minutes, a sufficiently strong connection is not formed in this part, while an exposure of 30-60 minutes is sufficient for the connection in the optimal temperature range at optimal pressure, and with an exposure of more than 120 minutes, practically no new changes occur.

[0015] At these temperatures, thorium has a higher coefficient of thermal linear expansion than niobium (at 1400 °C - approximately 14 10' 6 hail 1 and 10 10' 6 hail 1(respectively). After conducting the first experiments on diffusion welding of the target, a defect—a swelling—was sometimes formed on the foil, and various defects were sometimes detected during metallographic analysis. This indicated that, with the dense packing of the target material in the capsule and with the higher coefficient of thermal expansion of thorium, the target "swells" during heating. Therefore, a V-shaped groove was subsequently machined from the inside of the massive niobium ring of the target body. A V-shaped groove is convenient from a technological standpoint, but other groove shapes are also possible. The calculated difference in the volumetric expansion of thorium and niobium upon heating to operating temperatures is approximately 2% of the volume of thorium, but taking into account the unevenness of expansion on one hand and the elasticity of the materials on the other, a groove of up to 6% of the volume of thorium in the target, but not less than 1%, was produced. As a result, swelling and defects were not observed in this case.

[0016] As previously established, the optimal thickness of the niobium foil—the entrance and exit "windows" of the target's proton beam—is typically between 70 and 150 µm. Thinner foils, when used in real-world applications, can develop defects after irradiation with an intense proton beam, compromising the target's hermetic seal. Excessively thick foils reduce heat transfer and lead to proton energy loss, which reduces the yield of the resulting radionuclides. However, an experiment showed that after diffusion welding, waves up to 50 µm in height appear on the niobium foil windows outside the target. This apparently occurs because, during diffusion welding at high temperatures and under high-pressure conditions, the ring is fixed, while the niobium foil, along with the thorium, expands more, causing waves to form after cooling.Therefore, we used niobium foils with an initial thickness of 200 µm or more, and after diffusion bonding, we ground off at least 70 µm of the target's surface to remove the resulting waves and achieve the specified window thickness. Furthermore, grinding removes traces of the spacer used during diffusion bonding (usually tungsten or aluminum oxide). During target irradiation, the temperature is much lower than during target fabrication, preventing waves from forming on the target's surface.

[0017] To reduce the likelihood of thorium penetration through foil defects and radionuclides entering the cooling water, an additional coating of the target shell with another material is used on the outside. A nickel coating applied by diffusion welding, electrochemically, or by decomposition of nickel carbonyl (Ni(CO)4) in a dynamic vacuum at a temperature of at least 400°C has been used. Here, nickel does not come into contact with thorium, with which it can interact. Like niobium, nickel does not interact with reactive water during irradiation. Precious metals and some other metals that do not interact with reactive cooling water or with niobium can also be used, at the temperatures reached on the target shell during irradiation with charged particles.

[0018] It is important to note that diffusion bonding of thorium to niobium can, in principle, complicate the subsequent opening of the target and its chemical processing after irradiation to extract actinium-225 and other radionuclides. This problem is successfully solved by selectively dissolving niobium and thorium [S.V. Ermolaev et al. Method for producing actinium-225. Russian Federation Patent No. 2725414, 12.12.2019], as well as other metals (if an additional metal coating is used).

[0019] Brief description of the drawings

[0020] The essence of the claimed method for producing a thorium target in a niobium shell is explained by the attached drawings.

[0021] Fig. 1 shows an example of a target design, where:

[0022] 1 - monolithic disk of metallic thorium;

[0023] 2 - ring-shaped niobium target shell body;

[0024] 3 - niobium foil welded to the thorium disk and to the niobium ring of the target shell body;

[0025] 4 - groove for expansion of thorium during heating;

[0026] 5 - a strengthening niobium ring with a thickness of 300 µm, welded to the niobium body using electron beam or laser welding;

[0027] 6 - electron beam welding seam;

[0028] 7 - direction of the proton beam when irradiating the target. Fig. 2 shows an example of a diffusion welding diagram, where:

[0029] 1 - monolithic disk of metallic thorium;

[0030] 2 - ring-shaped niobium target shell body;

[0031] 3 - niobium foil welded to the thorium disk and to the niobium ring of the target shell body;

[0032] 8 - spacer made of tungsten or aluminum oxide;

[0033] 9 - punches made of metallic molybdenum.

[0034] Fig. 3 shows the results of a metallographic study (using an electron microscope) of diffusion welding carried out at different temperatures in the welding chamber (see Example 1): (a) - 1200 °C, (b) - 1250 °C, (c) - 1350 °C.

[0035] Fig. 4 shows the results of metallographic analysis of the target after diffusion welding at a pressure on the target of 30 kg / cm 2 Welding results in incomplete bonding of the niobium foil with the niobium ring of the housing (optical microscope), and partial penetration of the main target material (thorium) into the gap between the foil and the niobium ring (electron microscope) occurs.

[0036] Fig. 5 shows the results of a metallographic examination of a section using an optical microscope at a maximum pressure of 150 kg / cm 2 , close to excess: a complete connection of the target parts is achieved, but some deformation of the niobium ring is observed (the formation of a bulge on the right).

[0037] Fig. 6 shows the target disks after diffusion welding (temperature 1350 °C, pressure on the target 100 kg / cm 2 ): (g) - without a groove, for the expansion of thorium during heating, which results in a defect - swelling, and the surface of the target is also covered with small waves; (d) - with a V-shaped groove for the expansion of thorium during heating, made on the inner side of the niobium ring of the target body, and after welding the surface of the disk on the foul side is ground off.

[0038] Fig. 7 shows the finished targets:

[0039] (e) - two targets before irradiation, (g) - target and aluminum holder before irradiation in the accelerator, (z) - target in an aluminum holder after irradiation in the accelerator proton beam.

[0040] Implementation of the invention

[0041] Example 1.

[0042] A disk of molten thorium metal, 4 mm thick and 28 mm in diameter, is inserted into a niobium ring of the same thickness with an inner diameter of 28 mm and an outer diameter of 38 mm. The ring containing thorium (target body) is then covered with 100 μm thick niobium metal foil on both sides, covered with 100 μm thick tungsten metal spacers, and placed in a diffusion welding chamber between two 38 mm diameter molybdenum metal cylinders (Fig. 2). The tungsten spacers are needed to prevent interaction between the molybdenum cylinders and the niobium foil. The chamber is evacuated to a residual pressure of 5 10' 3 Pa. The installation press creates a pressure of 50 kg / cm on the target being welded. 2The chamber is heated using a resistance furnace with graphite heaters. Over the course of 30 minutes, the temperature is raised to 1200°C and maintained at that level for 30 minutes. After this, the chamber is cooled for 2 hours, the welded target is cut, and a metallographic analysis of the section is performed (Fig. 3(a)). As can be seen, the welding did not result in the niobium foil (top) bonding to the niobium housing ring.

[0043] Welding is performed in a similar manner, but the chamber temperature is maintained at 1250°C. As can be seen in Fig. 3(b), welding is achieved, but not completely: some of the thorium enters the gap between the niobium ring and the foil.

[0044] Welding is performed in a similar manner, but the chamber temperature is maintained at 1350°C. As can be seen in Fig. 3(c), complete welding of both niobium foils to the niobium ring of the target body is achieved, as well as complete bonding of niobium and thorium.

[0045] Example 2.

[0046] Welding is carried out in a similar manner as in Example 1, the temperature in the chamber is maintained at 1350°C, but at a pressure of 30 kg / cm 2 . As can be seen (Fig. 4), when the pressure is insufficient during diffusion welding, the niobium foil is not completely welded to the niobium ring (optical microscope), and some amount of the target material - thorium (electron microscope) - gets into the diffusion welding zone, which is due to the fact that its coefficient of linear thermal expansion is higher than that of the target body material.

[0047] In Fig. 5 it is evident that although a complete connection of the niobium and thorium parts is achieved, a change in the shape of the niobium ring is observed (the formation of an end bulge on the right), which indicates the application of a limiting and close to excess pressure of 150 kg / cm to the sample. 2 , leading to some deformation of the structure.

[0048] Example 3.

[0049] The target is welded in a similar manner with the temperature in the chamber maintained at 1350°C and a pressure of 100 kg / cm 2, using a spacer (between the molybdenum rod and the niobium foil) made of aluminum oxide a few microns thick, which is cheaper and more convenient than tungsten foil. In this case, a suspension of aluminum oxide in alcohol is applied to the surface and dried, repeating this procedure several times - until a continuous layer of aluminum oxide is achieved. This achieves a good connection between the niobium foil and the niobium ring of the target body, as well as the main part of the thorium with niobium. However, this causes a defect - a swelling (Fig. 6(g)) to form on part of the foil, which is caused by the fact that its coefficient of linear thermal expansion is higher than that of the target body material. And the surface of the foil is covered with small waves. Welding is carried out in a similar manner, but first a V-shaped groove is machined inside the niobium ring, the volume of which is 5% of the thorium volume (the thickness of the thorium disk is 3 mm).No swelling occurs, and grinding the disk surface to a depth of 70 µm produces a high-quality disk without visible defects, waves, or traces of the aluminum oxide spacer (Figs. 6(d)). An additional ring is then welded to the disk around the perimeter on both sides using electron beam welding (Fig. 1), resulting in high-quality targets (Figs. 7(e) and 7(g).

[0050] The target, placed in a metal holder, is irradiated in an accelerator with a proton beam at a current of 60 μA and an energy of 134 MeV at the target's input surface. After irradiation, no inhomogeneities or defects appear on the target (Fig. 7(z)), indicating good target quality and effective cooling.

Claims

Invention formula 1. A method for producing a target from metallic thorium, comprising producing a target body from a monolith of metallic thorium, enclosing it in a sealed shell consisting of a ring of metallic niobium and a foil of metallic niobium welded to thorium using diffusion welding and additionally sealed using laser or electron beam welding for subsequent irradiation in an ion accelerator in order to obtain radionuclides while cooling the target with water, characterized in that before welding, a V-shaped hollow groove is machined in the inner part of the niobium ring of the target body to expand the thorium when heated during welding, and diffusion welding of the foil of metallic niobium to metallic thorium is carried out at a temperature of 1250 to 1430 °C, at a pressure on the surface area of ​​the target of 30-150 kG / cm 2 .

2. The method according to paragraph 1, characterized in that diffusion welding is preferably carried out at a sample temperature of 1350 to 1400°C.

3. The method according to paragraph 1, characterized in that diffusion welding is carried out preferably at a specific pressure on the target surface area from 50 to 100 kg / cm 2 .

4. The method according to paragraph 1, characterized in that the time for performing diffusion welding is 20-120 minutes.

5. The method according to claim 1, characterized in that the time for carrying out diffusion welding is preferably 30-60 minutes.

6. The method according to paragraph 1, characterized in that the volume of the groove is 1-6% of the volume of thorium.

7. The method according to paragraph 1, characterized in that the thickness of the niobium metal foil before diffusion welding is at least 200 µm, and after When carrying out diffusion welding, at least 70 µm of the thickness of the welded niobium foil is ground off from the outside of the target.

8. The method according to claim 1, characterized in that an additional protective layer of 40 to 100 µm made of another metal that is resistant to water during irradiation is applied to the shell foil made of metallic niobium from the outside of the target.

9. The method according to claim 7, characterized in that the metal applied to the shell from the outside of the target is nickel, and the nickel is applied by diffusion welding or electrochemically or by decomposition of nickel carbonyl (Ni(CO)4) in a dynamic vacuum at a temperature of at least 400 °C.

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