Characterization of the neutron flux on the surface of a liquid water target intended for 18F production

Abstract

18F is an important radioisotope used in nuclear medicine for diagnostic purposes. It is produced most often via 18O (p,n)18F reaction. During its production, the oxygen contained in the H218O molecules placed in the cooled target is irradiated by accelerated protons. The production of each 18F atom is accompanied by anisotropic high-energy neutron emission, which is a problem for ensuring radiation safety. These neutrons are responsible for the high radiation dose in a cyclotron pit during 18F production as well as for activation of cyclotron components or might be responsible even for target material changes due to their high penetration properties. A precise characterization of a neutron field on the surface of the liquid water target, used for 18F production, is presented. The neutron flux characterization is performed both by using large-area activation detectors and stilbene detectors at 1 m from the target in a forward direction. The experiments show significant discrepancies between calculated and measured reaction rates in higher threshold reactions 58Ni(n,x)57Co and 51V (n,α)48Sc, confirming that the current neutron production reaction models are not suitable for the characterization of the secondary neutrons leaking from the target.