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

Highlights: • Measurement of neutron leakage spectra in forward direction. • Measurement of reaction rates in various positions on IBA target. • Calculation of neutron field in all measured positions. • Discrepancies between calculation and experiment. 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.