Report on metal targets and associated dissolution strategy

The production and dissolution of W-186 metal targets were investigated. A W-186 target undergoes a double-neutron capture to produce W-188, a beta-emitting isotope used in nuclear medicine. Its daughter nuclide, Re-188, decays and is accompanied by a 155 keV predominant energy γ-emission, which could be used for γ-cameras, for imaging, biodistribution, or absorbed radiation dose studies. Additionally, its high beta emissions can penetrate and destroy abnormal tissue or cancer for therapy.
Both oxide and metallic W-186 have been reported in the literature. Oxide targets are preferred for their ability to dissolve and retrieve the W-186 product after irradiation. However, the ampoules of tungsten oxide targets have been reported to break, limiting the possibility of upscaling. Additionally, the lower W-density in the oxide form is unfavorable, given the limited availability in high-flux positions in research reactors. Although it is unclear why these ampoules break, a working hypothesis presumes it is due to the heat generated, the low thermal conductivity, and the thermal expansion of the material during irradiation. Therefore, W-metal targets are investigated to improve target performance during irradiation and increase production per irradiation cycle.
In this report, a feasibility study of W-metal targets was performed by making and dissolving W-metal target samples. Various metal targets were made by pressing, sintering, and cutting W-powder. The die and punch had a cylindrical cavity with a diameter of 9.8 mm. The applied force varied from sample to sample, from 4 to 6 tons in a hydraulic press. The samples were then sintered in a Hyptec 5 atmosphere at 1750°C for 2 and 24 hours. Following the production and characterization of these targets, they were then dissolved to study the time and conditions needed to extract medical isotopes.
To dissolve these targets, a single-step hydrogen peroxide method was used. The reaction was heated in a water bath up to 45°C and confirmed by weighing the residual W-mass. The reaction was observed to occur in three phases. Phase one—the initial dissolution phase—occurred over the first 12 minutes, with small hydrogen bubble formation. In phase two, a vigorous reaction was present between 12 and 16 minutes, where the target fell apart into a powder. After 60 minutes, it was found that around 91% of the initial target was dissolved. In phase three, the dissolution of the residual W-mass proceeded slowly until the reaction was stopped at 90 minutes. At 90 minutes, the residual W-mass was found to be between 1-2% of the initial target mass.