Calibration experiment and the neutronics analyses on the LHD neutron flux monitors for the deuterium plasma experiment

Abstract Prior to the deuterium experiments of the Large Helical Device (LHD), an in situ neutron calibration experiment was carried out by using an 800 MBq 252 Cf neutron source in November 2016, where a neutron flux monitor, a neutron activation system and a vertical neutron camera are calibrated. This paper concentrates on the in situ calibration for the neutron flux monitor. The neutron flux monitor based on 235 U fission chambers and 10 B and 3 He proportional counters are positioned at three locations outside the LHD cryostat. The counts are measured during continuous rotation of the 252 Cf neutron source on the typical magnetic axis of the plasma by a toy train, where the neutron source can be regarded as a ring neutron source. Detection efficiencies for the 252 Cf ring source are derived by (total counts)/(total emitted neutron number) in the continuous rotation of the neutron source. Detection efficiencies for the real plasma neutron source are obtained from the measured detection efficiencies with the correction factor taking account of the deference between the 252 Cf ring source and the real plasma neutron source evaluated by MCNP calculations with the three-dimensional model of the LHD. Finally, the uncertainty of the total neutron emission rate measurement is discussed.