Development of Dry Distillation Equipment and Separation Process for Production of 211At

Abstract

With a half-life of 7.2 h, the decay of 211At produces α particles that have a limited range in tissues (55-80 μm), equivalent to approximately 6-8 cell widths. Its LET value of 98.84 keV/μm is very close to the optimal LET value (100 keV/μm) for effective internal radiation therapy, exhibiting strong cytotoxicity. Consequently, 211At is considered the most suitable α emitting isotope for targeted internal radiation therapy. It is also used as an alternative internal radiation source for α irradiation studies involving extractants and similar substances, replacing long-lived nuclides such as 238/239Pu and 241Am. However, due to the limitations in 211At yield, the maximum irradiation dose is difficult to increase, imposing constraints on irradiation dosage. Hence, it becomes crucial to enhance the radiochemical separation yield of 211At, whether for tumor-targeted therapy or α irradiation investigations. The production of 211At is typically accomplished using a cyclotron through the 209Bi(α,2n)211At reaction. And the radiochemical separation techniques employed for purification 211At primarily involve wet and dry distillation methods. Nonetheless, the current dry distillation methods have certain drawbacks, including unstable yield and the requirement for physically stripping irradiated bismuth from the target substrate, which poses a risk of nuclide escape. To solve these issues, a dry distillation process for production of 211At with an integrated semi-automatic dry distillation separation equipment was designed and optimized in this paper. The main influencing factors on dry distillation separation were determined using iodine as a simulation for astatine due to their similar chemical character, and feasible steps for the 211At dry distillation separation process were improved. It is found that once the furnace temperature is capable of vaporizing the target product, the key factors influencing the dry distillation process revolve around carrier gas velocity, holding time, and vacuum conditions. The results of 211At dry distillation separation show that when the α beam intensity of accelerator is 20 μA, α energy is 28.5 MeV, and the Bi target is bombarded for 4 h, the 211At yield of separated by the equipment for single run can reach more than 17.87 mCi (6.61×108 Bq). The nuclear purity of the prepared 211At is more than 99.9%, and free of impurities such as Bi and Cu. The integrated semi-automatic equipment has a recovery rate of over 98% for 211At in a separation time of 45-60 min.

Keywords

• 211at
• dry distillation separation
• process
• iodine for simulated process