Design of target systems for the high-yield production of Tc-99m with a cyclotron

Objectives Several groups have reported success in the production of technetium-99m (Tc-99m) through proton irradiation of molybdenum-100 (Mo-100) via the 100Mo(p,2n)99mTc reaction. Most published methods have used natural or enriched Mo foils or melted/sintered pellets for target manufacture. The purpose of this study was to experiment with a variety of target designs to maximize production yields for routine, large-scale Tc-99m production.

Methods Thermal modeling was performed to understand heat flow through Mo-coated targets for Tc-99m production. A series of target holders were then designed and some tested for the ACSI TR19, GE PETtrace and TCC CP42 cyclotrons. In addition, we have developed a method for routine coating of Mo-100 targets by electrophoretic deposition on a tantalum backing, followed by high temperature sintering. Automated target transfer and dissolution systems were implemented on the TR19 as well as shielded and unshielded GE PETtrace cyclotrons.

Results Thick, uniform coatings of Mo-100 were consistently achieved, and could be readily dissolved for Tc-99m extraction. We have performed several irradiations at up to 60 µA proton beam current. Targets facing the beam at a 90° angle required thicker coatings and had much less beam current tolerance due to poor heat transfer across the Mo layer, in part due to the presence of voids in the Mo coating (Fig. 1). It was found that heat transfer through the Mo layer itself is the limiting design aspect, with thin and/or compact Mo coatings being essential to optimize production yields. Higher current irradiations are currently underway using an improved target design.

Conclusions We have designed target systems for the direct production of Tc-99m applicable to a number of different commercial cyclotrons. A robust method for target backing preparation was developed, but several challenges remain to overcome limited heat dissipation across the Mo layer.

Research Support NSERC, CIHR and Natural Resources Canada