Research On Neutron Detection Technology Based On 4H - SiC

Physics experiments carried on Chinese Fast Burst Reactor II (CFBR II) demand neutron detectors with compact size, fast response and high radiation resistance. Novel neutron detectors which can be applied in harsh radiation circumstances are highly expected.4H-Silicon carbide (4H-SiC) has been recently proposed in alternative to silicon in semiconductor-based radiation detectors. The higher band gap energy and greater displacement threshold energy of4H-SiC should theoretically lead to neutron detectors made of4H-SiC capable of operating at elevated temperatures and in high radiation fields. However, studies on neutron detectors made of4H-SiC have just been prompted for the last decade, properties of4H-SiC neutron detectors need to be systematically addressed. In this dissertation, the design, fabrication, electrical characterization and application in diverse neutron fields of4H-SiC neutron detector have been investigated. The relationship between detector structure and detector properties has been investigated. The4H-SiC epitaxial layers are grown onto4H-SiC substrate wafers by vapor-phase epitaxy.4H-SiC Schottky diodes are then fabricated by planar technology. The Schottky barrier height (φb) is determined to be1.66eV from IV characteristic. In addition, the ideality factor (η) of the4H-SiC Schottky diode is obtained as1.07, indicating that the current is dominated by thermionic current. When applying a reverse bias of700V, the leakage current is21nA.The charge collection efficiency (CCE) of the4H-SiC Schottky diode is measured through a standard procedure which normalizes the pulse height with respect to the response obtained in the same experimental condition of a Si Schottky diode. CCE for3.5MeV alpha particles is48.7%at zero bias, and is as high as99.4%at150V. In order to simulate the working conditions of neutron detector, the4H-SiC Schottky diode is irradiated in a mixed neutron/y field of a critical assembly in Nuclear Physics and Chemistry Institute, Mianyang (China). CCE decreases by increasing the neutron fluence. CCE of the4H-SiC diode can reach88.6%even after an irradiation at8.26×1014n/cm2. Both (μτ)e and (μτ)h decrease by increasing the neutron irradiation fluence, resulting in the degradation of CCE. The decrease of μτ values can be ascribed to the creation of defects during neutron irradiation.(μτ)e is found to be (1.3±0.2)×10-8cm2/V and (μτ)hto be (0.8±0.1)×10-8cm2/V for the diode irradiated at8.26×1014n/cm2.226Ra source is utilized to investigate the response of4H-SiC detector to ionizing particles. The energy resolution for4.8-7.7MeV alpha particles is in the range of0.61%-0.9%. And the energy linearity is as high as0.99999. Both the energy resolution and energy linearity are comparable with those of Si-based detector system.A4H-SiC sandwich neutron spectrometer prototype is fabricated and tested. The relationship between spectrometer structure and spectrometer properties is studied by Monte-Carlo simulation. The6LiF (90%enriched in6Li) converter layer is deposited on the front surface of the Schottky diode by magnetron sputtering.6LiF films are characterized by scanning electron microscopy.6LiF layers with a thickness ranging from108nm to2.5μm are obtained by adjusting the time and power of sputtering. The estimated energy resolution for thermal neutrons is in the interval (3.4%,4.8%). The preliminary experimental results have confirmed the feasibility of SiC sandwich neutron spectrometer.In order to enhance the intrinsic neutron detecting efficiency,4H-SiC neutron detector is optimized by Monte-Carlo simulation. As a result, the theoretical intrinsic neutron detecting efficiency of4H-SiC neutron detector for thermal neutrons can exceed5%.10B4C converter layer is fabricated by magnetron sputtering, and successfully applied to detect slow neutrons. The4H-SiC neutron detector is able to detect neutrons at zero bias, with sacrificing20%of its intrinsic neutron detecting efficiency. Proton recoil techniques have been used to detect mono-energy fast neutrons ranging from1.5to4.99MeV. The intrinsic neutron detecting efficiency of4H-SiC neutron detector is in the interval (0.01%,0.2%), and increases by increasing incident neutron energy, which is in good agreement with Monte-Carlo simulation results.4H-SiC neutron detector systems working in current mode have been successfully developed to measure the neutron waveform of CFBR Ⅱ. The parameters deduced from results of the4H-SiC neutron detector system are in good agreement with those obtained from plastic scintillator detectors. Radiation resistance of4H-SiC neutron detector is evaluated in pulsed neutron field. The4H-SiC neutron detector system has been demonstrated to be well suited for neutron waveform measurement for their fast response and excellent radiation resistance.It is concluded that a working4H-SiC neutron detector can be utilized in diverse applicions, including neutron waveform measurement in CFBR II, ex-core neutron flux monitoring in nuclear reactors, fission neutron spectrum measurement. With the4H-SiC neutron detector developed in this work, the demand for compact size, fast response and high radiation resistance neutron detectors can be satisfied.