Neutron source term characterization for a GE PETtrace-8 cyclotron target: a comparative study using PHITS 3.33 and MCNP6.3.0

Santos Oliveira R.; Batista da Silva J.; Gustavo da Silva I.; Benavente-Castillo J.A.; de Sousa Lacerda M.A. · Radiation Measurements (2026) · DOI: 10.1016/j.radmeas.2026.107798

Resumen

This study presents a systematic investigation of neutron source-term modeling for a GE PETtrace-8 cyclotron, focusing on proton irradiation of an [18O]H2O target to produce 18F. Monte Carlo simulations were performed with PHITS 3.33 and MCNP6.3.0 to update previously reported neutron source terms and quantify code-dependent discrepancies associated with different nuclear data libraries and reaction models, particularly in the low-energy regime (<20 MeV). Neutron energy and angular distributions were computed using equivalent tally methodologies in both codes, considering ENDF/B-VIII.0, JENDL-5, and TENDL-2021 libraries, as well as CEM03.03 in MCNP and INCL4.6+GEM in PHITS. While the calculated neutron spectra exhibit similar overall shapes, significant differences are observed in angular distributions and absolute fluence values, even when the same evaluated data are used. These discrepancies are mainly attributed to differences in intranuclear cascade and pre-equilibrium models, which strongly affect neutron multiplicities and emission anisotropies in low-energy proton-induced reactions. Among all configurations, TENDL-2021-based simulations show the best agreement with the manufacturer's reference data. The results also indicate that MCNP systematically predicts harder spectra and enhanced backward emission relative to PHITS, underscoring the sensitivity of source-term characterization to the underlying physics models. Overall, the study demonstrates that inter-code discrepancies are governed not only by the choice of nuclear data libraries but also by model implementations, highlighting the need for careful benchmarking and validation. The TENDL-based source term obtained here is proposed as a consistent baseline for preliminary neutron-field characterization and shielding design in GE PETtrace-8 cyclotron facilities operating up to 16.5 MeV. Copyright