Improvements in the production of a low cost targetry for direct cyclotron production of 99mTc

Introduction The established methods for the production of 99Mo, based on fission in nuclear reactors, continue to present problems as a result of the plant’s aging and the significant investments needed for maintenance or for their renewal. Much research work is thus in progress on the study of alternative methods for the production of 99mTc in quantities and with the degree of purity required for the clinical use. Between them, the cyclotron production of 99mTc via the 100Mo(p,2n)99mTc reaction has turned out as the most attractive alternative. One critical aspect regarding the production of 99mTc with cyclotron is the need for a robust and reliable target production process. Several techniques have been indicated as extremely promising such has plasma spray and laser cladding; however these methods require specialized instrumentation and complex operations to be performed handling activated materials in order to recover irradiated Mo. In this work we report the development of the work done at the University of Bologna, as a part of a wider INFN project, as regards the methods of preparation of solid targets suitable for the production of 99mTc irradiating a target of 100Mo, employing a cyclotron for biomedical use, normally operated for the production of PET radionuclides. Material and Methods Irradiations were performed with a 16.5 MeV GE PETtrace cyclotron equipped with a solid target station previously developed by our group (1). In initial tests, a stack of 1–3 metallic foils, 100 μm thick, of natMo were irradiated with protons in the 15.9→9.8 MeV energy range. Foils were then dissolved in a HNO3-HCl solution and samples were analyzed with high resolution gamma-ray spectrometry (Canberra, including a HPGe detector with a 30% relative efficiency and a resolution of 1.8 keV at 1332 keV) using Genie2000 software; the measurement campaign lasted several weeks to take into account the different half-lives of the produced radionuclides. Results were extrapolated to a highly enriched 100Mo target and compared to Monte Carlo simulations previously performed with FLUKA and TALYS codes (2). In order to investigate a method of preparation of the target that would make easier the recovery of the enriched material and recycling for the preparation of a new target, it was subsequently studied the preparation of pellets of Molybdenum trioxide. MoO3 powder (Sigma Aldrich, 99.9% trace metals basis, particle size < 150 μm) was used to prepare pellets using a 10 ton press. Pellets obtained in this way were then sintered on a Platinum support using a CARBOLITE furnace under a controlled atmosphere; the temperature was ramped according to a controlled and reproducible temperature cycle. Sintered pellets were subjected to visual inspection, mechanical tests of resistance to loading and downloading in the cyclotron target station, thermal tests and then irradiated at increasing current. The irradiated targets were again visually inspected then weighed, dissolved and subjected to gamma-ray spectrometry analysis. Results and Conclusion The experimental saturation yield for 99mTc calculated on the basis of the gamma-ray analysis of irradiated metal foils, gave an extrapolated yield of 1.115 ± 0.015 GBq/μA for a 100 μm thick 100Mo enriched target, in accordance with the value of 1.107 ± 0.002 GBq/μA obtained in Monte Carlo simulations. On these bases, an irradiation of 3 h at 50 μA is expected to produce 16.3 ± 0.2 GBq of 99mTc; considering the use of an efficient purification system, a radionuclidic purity > 99.9 % 2 h after the EndOfBombardment and a specific activity comparable with the actual standards are expected as achievable. Experiments on sintering pellets are still on going at the time of writing this report; initial results showed that addition of proper aggregating materials allows for suitable pellets preparation. The sintering process allows to obtain pellets having sufficient mechanical strength to withstand loading and downloading operations. Initial irradiation tests with beam current up to 25 μA were performed successfully with no changes in mass and mechanical properties of the pellet. These encouraging results suggest that sintered pellets may be a relatively inexpensive and easy solution to prepare 100Mo targets for the cyclotron production of 99mTc. Further experimental tests at higher beam current will be performed in order to assess the maximum current achievable with no damage of the target. At the same time, a prototype automated module based on standard industrial components is in testing phase as regards performance in the separation and purification processes