Study on the gamma-rays and neutrons energy response optimization of scintillating fiber detector on EAST with Geant4

The new Sci-Fi detector inside the magnetic shielding tube has been designed and assembled, and will be used in the next round of fusion experiments on EAST to provide D-T neutron yield with time resolution. It is calibrated using the accumulated yield of neutrons with energies above 4 MeV provided by the existing neutron activation system, which requires the detector to accurately discriminate fast neutrons, especially those above 4 MeV. In this study, the pulse height spectra of ideal signals produced when neutrons and gamma rays of different energies enter the complete detector model (including the magnetic shielding tube) were obtained by Geant4 simulation. Low-energy neutrons below 10-5 MeV are one of the main interference sources, because they can generate numerous secondary particles in components of the detector system such as the magnetic shielding tube, resulting in high-amplitude output signals. Targeted design of the thermal neutron shield contained a 1-mm Cd layer outside the magnetic shielding tube and a 5-mm inner Pb layer, which can suppress the height of the output signals caused by thermal neutrons to an ideal level. The adverse effects on the measurement of neutrons above 4 MeV and shielding effect on gamma rays were evaluated as relatively acceptable. In addition, whatever the thermal neutron shield was used and not used, studies of the energy response showed that in both cases the detector had relatively flat sensitivity curves in the fast neutron region, and the intrinsic detection efficiencies of about 40 %. For gamma rays with not quite high energy (< 8 MeV), the IDEs of the detector were only about 20 %; while for gamma rays with energies below 1 MeV, the response curve cut off earlier in the low-energy region, which was beneficial to avoid counting saturation and signal accumulation.