Evaluation of Uranium Dioxide for Solid-State Direct-Conversion Neutron Detection

A solid-state direct-conversion neutron detector is a device that utilizes neutron moderating materials that function as the active signal detecting semiconductor. Uranium oxides are an example of such materials that meet these criteria and offer the compelling property of the possible increased energy deposition from neutron-induced fission. Fission fragments released from neutron interactions can deposit up to an additional 165MeV of energy within the detector resulting in a theoretical increase in electrical carriers and an improvement to the overall detection efficiency. This dissertation evaluates the usage of uranium dioxide (UO2) as the candidate material for such a detector. High electrical resistivity is desirable for these detectors to overcome the inherent background noise from α- particle self-irradiation. UO2 has a low electrical resistivity for this application, and methods of increasing this property are discussed.

Electrical resistivity has been measured on single-crystal samples. Each sample was ion-implanted as Los Alamos National Laboratory (LANL) with various dopant materials. Electrical resistivity was measured to be between 102–103 Ω-cm which is below the threshold of 108 Ω-cm required for direct-conversion detectors. Pulse height spectra were collected during the irradiation of these UO2 single crystals with a neutron flux produced by a 0.59μg 252Cf source. A positive response was observed, but energy calibration was not possible from the α-decay of urania. In addition to evaluating doped single crystals, thin films were produced using the chemical solution deposition method. However, thin films are shown to be ineffective for this application due to the neutron mean free path exceeding that of the thin film volume.

Cerium dioxide (CeO2) has been used as a non-radioactive structural simulant of UO2 to facilitate additional studies on structural changes caused by dopant impurities. These CeO2 samples were doped with La2O3, Gd2O3 and TiO2 at concentrations of 0.1, 5 and 10 mol% and sintered at 1400°C for 10 hours in air. X-ray diffraction determined that the lattice parameters showed minor expansion based on these dopant concentrations. It is proposed that further research is required utilizing doped and undoped high-quality samples of single-crystal UO2 to expand its application towards solid-state detectors.