Identification of activation products and radiation dose estimates in Havar® foils for a GE PETtrace800 medical cyclotron via high resolution gamma ray spectroscopy and MCNPX simulation

Abstract

During production of F-18 in a medical cyclotron, high energy protons and secondary neutrons hit the highly reactive components of the target assembly resulting to activation products. In determining the presence of activation products and quantifying their respective activities, high resolution gamma spectroscopy using high purity germanium (HPGe) was used. From the results of the spectroscopy analysis, the doses were estimated using two methods: calculation method and MCNPX simulations. For the gamma spectroscopy analysis, the sample was prepared using aqua regia and was sent to PNRI. The analysis was done using GEM Series HPGe Coaxial Detector System. The results of the gamma spectroscopy showed the presence of 56Co, 57Co, 58Co, and 54Mn after a cooling period of eighty (80) days. For the MCNP simulations, the Havar® window foil was simulated as an isotropic disc source and a soft - tissue mimicking box detector was used and positioned at 100 cm from the center of the simulated window foil. *F8 tally was used to estimate the effective dose arising from a single activation product homogeneously distributed in the window foil and was done under the assumption that no other radiation sources were present in the simulation. Using calculation method, the effective dose estimates of 56Co, 57Co, 58Co, and 54Mn were found to be of minimal values during replacement of 18O vial at one meter. Moreover, it was found out that the highest contributors of dose during replacement of an 18O vial and maintenance of cyclotron are 56Co and 58Co. After a period of one year, the guiding activation products become 57Co and 54Mn due to their long half - lives. Furthermore, the annual dose estimates of these activation products were found to not exceed the annual dose limits for the whole body and for the extremities. As for disposal and storage, a period of four years will allow the Havar® window foil to reach the exemption levels set by PNRI.