Modeling of a cyclotron target for the production of11C with Geant4

Background: In medical cyclotron facilities,11C is produced according to the14N(p,α)11C reaction and widely employed in studies of prostate and brain cancers by Positron Emission Tomography. It is known from literature that the11C-target assembly shows a reduction in efficiency during time, meaning a decrease of activity produced at the end of bombardment. This effect might depend on aspects which are still not completely known. Objective: Possible causes of the loss of performance of the11C-target assembly were addressed by Monte Carlo simulations. Methods: Geant4 was used to model the11C-target assembly of a GE PETtrace cyclotron. The physical and transport parameters to be used in the energy range of medical applications were extracted from literature data and11C routine productions. The Monte Carlo assessment of11C saturation yield was performed varying several parameters such as the proton energy and the angle of the target assembly with respect to the proton beam. Results: The estimated11C saturation yield is in agreement with IAEA data at the energy of interest, while it is about 35% greater than the experimental value. A more comprehensive modeling of the target system, including thermodynamic effect, is required. The energy absorbed in the inner layer of the target chamber was up to 46.5 J/mm2 under typical irradiation conditions. Conclusion: This study shows that Geant4 is potentially a useful tool to design and optimize targetry for PET radionuclide productions. Tests to choose the Geant4 physics libraries should be performed before using this tool with different energies and materials.