JP2023154459A - Target material for production of molybdenum-99 and method for producing the same - Google Patents

Abstract

To develop a shape-controlled target material, by paying attention to the demand for a method that enables filtering of a water-dispersed and neutron-irradiated target and extracting of high-concentration Mo-99 in a solution.SOLUTION: An elongated MoO3 whisker material with a large difference between its long and short diameters is used as a target, which is irradiated with neutrons and then dispersed in water. The target is separated from the solution through a filter, extracting Mo-99 in the solution at a high concentration.SELECTED DRAWING: Figure 3

Description

The present invention relates to a raw material for producing Mo-99 (molybdenum-99), which is a medical radioactive isotope.

Mo-99 is an isotope of Mo, which decays to produce the medical radioactive isotope Tc-99m (technetium-99m). Until now, Mo-99 has been produced using the nuclear fission method, which extracts fission products from spent nuclear fuel in nuclear reactors using highly enriched U-235, but this method cannot be implemented in Japan due to nuclear security concerns. There wasn't. As an alternative, nuclear reaction methods have been investigated in which Mo-99 is produced by neutron irradiation of the Mo-98 isotope in a research nuclear reactor.

JP2020-051916 (Patent application 2018-182149)

Hatsukawa, Grant-in-Aid for Scientific Research (C), 25420913 Ilyin et al. , Physics Proc. , 72 (2015) 548 Chu et al. , Inter. J. Appl. Ceram. Technol. , 18 (2021) 889 Chu et al. , Jpn. J. Appl. Phys. , 61 (2022) SB1018

At this time, after neutron irradiation to Mo oxide sintered body or Mo metal fine particle target enriched with Mo-98 isotope, Mo-99 is extracted after dissolving the entire amount with NaOH aqueous solution, and unreacted Mo-98 is reused. A method to do so is being considered. On the other hand, this melting and reuse process had to be carried out using a hot cell and a manipulator, which posed problems in workability and safety. Furthermore, the NaOH aqueous solution contains a large amount of unreacted Mo-98, and it is extremely difficult to isotopically separate only Mo-99 from this. One method currently being developed is to use a strong acid solution as an extraction medium and adsorb it onto an ion exchange resin, but the acid is harmful to the human body and must be removed by neutralization or precipitation before formulation. Ta.

In response to this, a technique was conceived in which a porous _MoO3 target is irradiated with neutrons in a nuclear reactor, and the hot atom effect is used to extract Mo-99 ions whose valence has changed due to water flow, and this technique is partially realized. It has been found from experimental results that this is the case (Patent Document 1). This result created the possibility of extracting Mo-99 simply by contacting water with the MoO ₃ target after neutron irradiation.

Hot atoms are a phenomenon in which nuclides produced by recoil of gamma rays generated after a nuclear reaction have high kinetic energy and become ions of different valences. This is suitable for isotope extraction because the generated nuclide flies out of the target or is selectively dissolved in a solution insoluble in the target, especially water. On the other hand, in the case of the nuclear reaction of Mo-98(n,γ)Mo-99, the kinetic energy obtained by Mo-99 is only 190 eV, and although the valence may change, it can only penetrate 3 nm through the Mo-O solid. Can not. Therefore, it was predicted that by simply irradiating a target containing Mo with neutrons, most of the generated Mo-99 would not be able to escape from the target and be extracted into the solution. Neutron irradiation from an accelerator using Mo-100 (n, 2n) Mo-99, which generates high γ-ray energy, is used to diffuse Mo-99 hot atoms in a solid and bring it to the target surface for extraction. (Non-Patent Document 1) or metal Mo nanoparticles (Non-Patent Document 2) that have a smaller target diameter and a larger surface area for diffusion. The former had limited irradiation facilities, and the latter required centrifugation of metallic Mo nanoparticles for Mo-99 extraction. Therefore, in order to put the water extraction method using hot atoms into practical use, there was a problem that it was not possible to extract Mo-99 with the high throughput required for practical use with a simple Mo-containing nanoparticle target. .

Instead of these nanoparticles, a method using a porous MoO ₃ target was devised (Patent Document 1). On the other hand, in the case of porous MoO3, there was a reciprocity in that if the holes were made larger to allow water to pass through, the target mass would decrease and the amount of Mo-99 produced would decrease. Furthermore, destruction caused by water osmotic pressure was a problem. Here, the whiskers, which have a small particle size in one direction and a large particle size in the other direction, can shorten the diffusion length of Mo-99 hot atoms out of the target, and because they are larger than nanoparticles, they can be separated from water by filtration. is possible. Therefore, if molybdenum oxide whiskers could be produced, they would be suitable for the Mo-99 hot atom water extraction method and would be a target material that would solve the problems of the nanoparticles and porous MoO ₃ described above.

Among molybdenum oxides used as targets, synthesis examples of α-MoO ₃ , which is stable at high temperatures, and β-MoO ₃ whiskers, which can be synthesized at low temperatures, are known (Non-Patent Document 3); There was no report on whether 99 could be extracted. In particular, whiskers grow along specific crystal planes, and it was unclear whether these crystal planes were suitable for extraction. Furthermore, when the major axis is longer than the minor axis, the surface area becomes smaller than that of ultrafine particles with the same minor axis, so it was not obvious whether Mo-99 could be extracted with water using whiskers compared to isotropic particles.

In addition, in the water extraction method, if hydration occurs in which water molecules enter the crystal, the rate at which Mo-99, which is a hot atom, is extracted out of the target through the diffusion of hydrated molecules will increase. is expected. α-MoO ₃ , which is widely used among MoO ₃ , is not hydrated, but β-MoO ₃ , which is stable at low temperatures, is known to be hydrated (Non-patent Document 4), and water that has entered the crystal is known to hydrate. We thought that the molecules would easily diffuse Mo-99 hot atoms out of the target. Therefore, β-MoO ₃ whiskers were considered to be a more suitable target for the water extraction method using Mo-99 hot atoms. On the other hand, due to hydration, β-MoO ₃ changes into a different crystal structure and stabilizes, so it was unclear whether water molecules hydrated at room temperature, where water extraction is performed, have sufficient diffusion ability. In addition, before Mo-99 hot atoms are extracted with water, they give and receive electrons from water and the surroundings, becoming ions with different valences, and may become insoluble in water, causing β-MoO ₃ whiskers to become Mo It was not obvious whether it could be used as a target for -99 production.

The present invention relates to a material in which a neutron irradiation target is dispersed in water to facilitate separation, and then a radioactive isotope can be extracted into the solution for the production of Mo-99. Focusing on the need for a method to extract high-concentration Mo-99 from a solution that can be filtered with a filter for neutron-irradiated targets dispersed in water, we developed a target material with controlled shape and phase. It is a technical challenge to do so.

More specifically, the target material according to the first aspect of the present invention is a target material for producing Mo-99 and Tc-99m by neutron irradiation, is composed of a crystalline molybdenum oxide phase, and has a short elliptical shape. It is characterized by consisting of solid particles with a diameter of 2-400 nm and a major axis of 70-2,000,000 nm.

The method for producing a target material according to the second aspect of the present invention uses an elongated MoO ₃ whisker material whose major axis and minor axis are significantly different as a target material, disperses it in water after neutron irradiation, separates it from the solution with a filter, and Mo- 99 is extracted into a solution at a high concentration.

According to the present invention, a neutron-irradiated target dispersed in water can be filtered with a filter, and a high concentration of Mo-99 can be easily extracted from the solution.

X-ray diffraction pattern of β-MoO ₃ whiskers prepared by PWD method. A lattice image of β-MoO ₃ whiskers synthesized by the PWD method. (a) is a transmission electron microscope bright field image of β-MoO ₃ whiskers prepared by in-gas evaporation method, and (b) is a transmission electron microscope lattice of β-MoO ₃ whiskers prepared by in-gas evaporation method. image. γ-ray spectrum from water in which a β- _MoO3 target irradiated with neutrons is dispersed and separated.

Mo-99 was extracted by preparing elongated molybdenum oxide whiskers with greatly different major and minor axes, irradiating them with neutrons as targets, and then bringing them into contact with water. Hereinafter, embodiments of the present invention will be described based on the drawings and tables.

A thin Mo metal wire with a diameter of 0.5 mm and a length of 25 mm was heated in the atmosphere at 550° C. for 10 hours and at 600° C. for 5 hours to produce a thin wire having a Mo/α-MoO ₃ core-shell structure. This was evaporated using a pulsed fine wire discharge (PWD) device. The oxygen partial pressure was fixed at 25 kPa as the atmospheric gas, and argon was added to this to make the total pressure 25, 50, and 100 kPa, and the chamber was filled. The core-shell thin wire was connected to this through an electrode. A 30 μF capacitor charged at 6 kV was connected to the electrode, and the core-shell thin wire was heated and evaporated with a large current pulse. The whiskers grown in the gas phase were collected using a filter, and the phase was observed using X-ray diffraction and the particle size was observed using a transmission electron microscope.

Figure 1 shows the X-ray diffraction pattern of the sample. FIG. 1 is an X-ray diffraction pattern of β-MoO ₃ whiskers produced by the PWD method (pulsed thin wire discharge method) in Example 1. The samples prepared in 25 kPa oxygen, 50 kPa oxygen + argon, and 100 kPa oxygen + argon are shown. In all samples, the main phase was β-MoO ₃ , but α-MoO ₃ and Mo were also found as impurities.

The results of this transmission electron microscope observation are shown in FIG. FIG. 2 is a lattice image of β-MoO3 whiskers synthesized by the PWD method in Example 1. From this spacing and their angles, it was concluded that this was β-MoO3. Furthermore, as a result of shape measurement of 300 whiskers, the average, minimum, and maximum values of the major axis and minor axis are shown in Table 1. Although almost no whiskers were found when fabricated at 100 kPa, it was found that increasing the pressure increases the density of vapor and the frequency of collisions, resulting in an increase in both the major and minor axes. As a result, β-MoO ₃ whiskers with a major axis of 216-427 nm and a minor axis of 23-47 nm were produced.
(Table 1)

Table 1

β-MoO ₃ whiskers were prepared by the in-gas evaporation method. α-MoO ₃ particles were placed in a boat maintained at 800, 850, 900, and 1000° C. while oxygen gas was flowing in a tube furnace to generate MoO ₃ vapor. This was transported to a low-temperature part using oxygen gas and after quenching, whiskers were grown in the gas phase. This was collected using a filter. The phase was identified by X-ray diffraction, and as in Example 1, it was found to be almost single-phase β-MoO ₃ . There were fewer whiskers when evaporated at 800 and 850°C, but the amount of whiskers increased when evaporated at 900 and 1000°C.

FIG. 3 shows a transmission electron micrograph of β-MoO ₃ whiskers produced by evaporation at 1000°C. FIG. 3 is a transmission electron microscope image of β-MoO ₃ whiskers produced by the in-gas evaporation method in Example 2, in which (a) shows a bright field image and (b) shows a lattice image. From the bright field image in (a), many whiskers with a major axis of approximately 300 nm and a minor axis of 10 nm were visible. Table 2 shows the geometric mean diameters of the major and minor axes. Furthermore, from the interplanar spacing and angle of the lattice image in (b), it was found that this was a β-MoO ₃ whisker.
(Table 2)

Table 2

The β-MoO ₃ whiskers produced in Example 2 were irradiated with neutrons, dispersed in water, and then centrifuged, and the resulting solution was measured for radioactivity. Two types of β-MoO ₃ whiskers (whiskers 1 and 2), each weighing approximately 0.33 g, were irradiated for 20 minutes with a thermal neutron flux of 3×10 13 n/cm 2 s. After 4 days, this was dispersed in water at a ratio of 1 g/50 ml and allowed to stand at room temperature for 20 hours. This was separated using a centrifuge and then filtered through filter paper to obtain a solution. Radioactivity was measured by analyzing gamma rays from the whiskers and solution using a germanium detector and comparing them with standard samples. In order to correct the decay of Mo-99 due to the time between whisker and solution measurements, we calculated the radioactivity at the time of whisker measurement using the decay constant from the radioactivity of the solution, and calculated the proportion of Mo-99 from the whisker. This was used as an indicator of whether it was extracted into the solution.

FIG. 4 shows the γ-ray spectrum of a solution obtained by centrifugation after dispersing whisker 1. That is, FIG. 4 is a γ-ray spectrum from water in which the neutron-irradiated β-MoO ₃ target was dispersed and separated in Example 3. Radioactivity was calculated from the peak intensity of Mo-99. Table 3 shows the radioactivity of the neutron-irradiated whiskers and the solution obtained by dispersing them in water and centrifuging them. For whiskers 1 and 2, 61.5 and 71.4% of Mo-99 were extracted into the solution. These results revealed that by using whiskers as targets, it was possible to extract a considerable amount of Mo-99 as a solution without heating, just by dispersing it in water, centrifuging it, and filtering it. A high extraction rate can be obtained not only with β-MoO ₃ but also with α-MoO ₃ and MoO ₃ hydrate.
(Table 3)

（比較例１） Table 3

(Comparative example 1)

The results of using commercially available isotropic α-MoO ₃ particles, which are not whiskers, as a target and dispersing and separating them in water after irradiation with neutrons are described in the literature (Misaki Seki, Nagaoka University of Technology master's thesis, 2018). ing. According to this, isotropic α-MoO ₃ particles with a particle size of 700 nm irradiated for 7 hours with a peak energy of 0.025 eV and a neutron flux of 2 x 10 ¹³ n/cm ² s exhibit Mo-99 radioactivity of 177.1 MBq. Since 29.84 MBq of radioactivity was detected from the solution, only 16.6% of Mo-99 could be extracted from the target into the solution. In comparison, in Table 1 for Example 4, the extracted ratios were 61.5 and 71.4%, which was a significant increase in the extraction ratio than in the comparative example.

none.

Claims (3)

A solid target material for producing Mo-99 and Tc-99m by neutron irradiation, consisting of a crystalline molybdenum oxide phase, with a short axis of ellipse approximation of 2-400 nm and a long axis of 70-2,000,000 nm. Target material consisting of particles. Target material according to claim 1, characterized in that the crystalline molybdenum oxide phase is β-MoO ₃ . A method for producing a target material, in which a long and narrow MoO ₃ whisker material with greatly different major and minor axes is used as a target material, and after neutron irradiation, it is dispersed in water, separated from the solution by a filter, and Mo-99 is extracted into the solution at a high concentration.