(450e) Fabrication of Molybdenum-100 Coatings for the Production of Commercial Quantities of Technetium-99m Via the Mo-100(p,2n) Reaction On Low Energy Cyclotrons

Alternative production methods of the most common nuclear medicine tracer, technetium-99m, are currently being explored by many centers in reaction to the uncertainty of molybdenum-99 availability from reactor based fission of uranium-235. While the accelerator production of technetium-99m via the 100Mo(p,2n) reaction has been known for a long time[i] the practicality of producing an adequate supply of the radionuclide with acceptable purity and acceptable economics has yet to be demonstrated. The fabrication of robust thick coatings of molybdenum-100 is a prerequisite to being able to accomplish this. Low energy proton cyclotrons (16-19 MeV) are increasingly available as more health care facilities adopt positron emission tomography and these facilities are potentially a viable source of technetium-99m. However these facilities rarely have the infrastructure to irradiate solid target materials at the high currents required to produce adequate supplies of a radionuclide for local regional use. We are presently implementing the methodology and infrastructure on such low energy proton cyclotrons in order to determine the feasibility to provide urban areas with an independent supply of technetium-99m.

The irradiation energy, irradiation duration, and target purity affect the radionuclidic distribution. Molybdenum-100 is presently commercially available as a metal at 97 to 99.5% isotopic enrichment. The remaining molybdenum isotopes (92 through 99) produce other radio-technetiums via ^92-100Mo(p,x) reactions that are chemically indistinguishable from Tc-99m. These radionuclides have production cross-sections and half-lives that potentially lead to unacceptable radiation doses to patients[ii]. The production of these other radio-technetiums, and the ratio of metastable technetium-99 to ground state technetium-99, are dependent on the irradiation energy and irradiation time. Increasing the irradiation energy will both increase the total amount of technetium-99m produced and increase the radio-contaminants. This effort aims to quantify these contaminants for low energy cyclotron production.

Our efforts are directed toward producing a robust solid target infrastructure that can dissipate up to 5.7 kW of beam power. Targets able to dissipate 4 kW have been constructed by others using a thermal spray method with natural enrichment molybdenum.[iii] Targets of sufficiently enriched molybdenum-100 that can be irradiated at high currents and are economically viable have not been demonstrated. We are investigating three general routes of target fabrication onto a chemically inert target backing; electroplating, sintering, and ion beam sputtering of molybdenum. The evaluation criteria for each method include the adhesion to the backing, efficiency of production, purity, and ease of recycling the molybdenum-100 target material.

At the present time thin electroplated molybdenum-100 targets have been fabricated on rhodium backing and irradiated. In order to maximize the purity of the deposit and simplify the recycling of the molybdenum electroplating from an aqueous solution is being pursued. The production of thick layers of molybdenum from aqueous solution is contraindicated by the reducing nature of molybdenum which causes a build up of hydrogen on the deposited surface. Sintering of molybdenum is a known process for the formation of predictable thick layers but the adhesion to a suitable backing remains to be resolved. Initial tests are being conducted by UHV Technologies Inc. (Ft. Worth, TX) to ion beam sputter natural molybdenum. The results of these three methods to produce an economically viable and robust molybdenum-100 target amenable to recycling of the enriched molybdenum will be presented.