Energy-Dependent Neutron Emission in Medical Cyclotrons: Differences Between 18F and 11C and Implications for Radiation Protection

This study investigates neutron radiation sources in medical cyclotrons used for PET isotope production, focusing on differences between 18F and 11C. Neutron and gamma dose rates were measured in the bunker and operator control room during routine production with an 11 MeV Eclipse cyclotron. 18F production generated approximately 2.5 times higher neutron levels in the bunker than 11C. Shielding performance also varied: the same wall reduced neutron fluxes by factors of kF = 14,000 for 18F and kC = 86,000 for 11C, while gamma shielding was similar for both isotopes (kγ ≈ 28,000). However, the neutron shielding factor calculated from the data for 18F should be taken as kF ≥ 1.4 × 104, because several neutron readings reached the upper limit of the detector range, which indicates a partial underestimation of the dose in the bunker. Consequently, neutron levels in the control room during 18F production were about 15-fold higher than during 11C production. These differences result from distinct neutron generation mechanisms. The 18O(p,n)18F reaction produces primary neutrons with a Maxwellian spectrum (~2.5 MeV), while 11C neutrons arise solely from secondary interactions in structural materials. The findings emphasize the need for composite shielding adapted to isotope-specific spectra. Annual dose estimates (260 18F and 52 11C productions) showed neutron exposure (3.78 mSv/year, 57%) exceeded gamma exposure (2.82 mSv/year, 43%). The total dose of 6.6 mSv/year is ~33% of regulatory limits, supporting compliance but underscoring the need for dedicated neutron dosimetry.