Measurement of neutron spectrum using activation method in deuterium plasma experiment at LHD

•In the Large Helical Device (LHD) of the National Institute for Fusion Science, the experiments using deuterium plasma were performed from March 2017.•The evaluation of the neutron flux in the torus hall as well as the LHD body is very important for designing the decommissioning of the LHD in the future, and the radiation shielding for workers and components around the LHD.•In this study, we performed the activation experiments using multiple activation foils and experimentally obtained the neutron spectrum in the vacuum vessel of the LHD for the first time. In the Large Helical Device (LHD) of the National Institute for Fusion Science, the experiments using deuterium plasma were performed from March 2017. Neutrons with the energy of 2.45 MeV are generated by d (d, n) 3He reactions. The evaluation of the neutron flux in the torus hall as well as the LHD vessel is very important for designing the decommissioning of the LHD in the future, and the radiation shielding for workers and components around LHD. In this study, we performed the activation experiments using multiple activation foils to obtain a neutron spectrum in the vacuum vessel of the LHD. The reaction rate of each activation foil was generally consistent with the calculation result by MCNP6. As a result of the unfolding with SAND-II code, we experimentally obtained the neutron spectrum in the vacuum vessel of the LHD for the first time.