Thermal neutron measurement by single crystal CVD diamond detector applied with the pulse shape discrimination during deuterium plasma experiment in LHD

Neutron-induced signal on the single crystal chemical vapor deposition diamond-based neutral particle analyzer

A diamond-based neutral particle analyzer (DNPA) array composed of single-crystal chemical vapor deposition (sCVD) diamond detectors was installed on the Large Helical Device (LHD) for measuring the helically trapped energetic particles. In high neutron flux experiments, the unwanted neutron-induced pulse counting rate should be estimated using the neutron diagnostics because a diamond detector is sensitive to neutrons as well as energetic neutral particles. In order to evaluate the quantitative neutron-induced pulse counting rate on the DNPA, the response functions of the sCVD diamond detector for mono-energetic neutrons were obtained using accelerator-based D-D and D-⁷Li neutron sources in Fast Neutron Laboratory (FNL). As a result of the neutron flux estimation by the Monte Carlo N-Particle code at the NPA position in the LHD and the response function obtained in the FNL experiment, the counting rate of the neutron-induced signal was predicted to be 1.1 kcps for the source neutron emission rate of S_n = 1 × 10¹⁵ n/s. In the LHD experiment, the neutron-induced signals were observed by closing the gate valve during the plasma discharges. It is found that the counting rates of the neutron-induced signals proportional to S_n reached 1.1 kcps at S_n = 1 × 10¹⁵ n/s. As a result of the quantitative estimation of the neutron-induced signals on the DNPA using other neutron measurements, it has become possible to accurately measure energetic neutral particles in the high neutron flux experiment.