Search

Presently, thermal neutron detectors fabricated from boron-10 enriched hexagonal boron nitride (h-10BN) ultrawide bandgap semiconductor grown by metal organic chemical vapor deposition (MOCVD) hold the record high detection efficiency among all solid-state detectors at 59%. To overcome the short comings of MOCVD growth, including inherently low growth rate and unavoidable impurities such as carbon in metal organic source, we demonstrate here the growth of natural hexagonal boron nitride (h-BN) semi-bulk wafers using halide vapor phase epitaxy (HVPE), which is an established technique for producing GaN semi-bulk crystals at a high growth rate. Electrical transport characterization results revealed that these HVPE grown materials possess an electrical resistivity of 1 × 1013 Ω cm, and a charge carrier mobility and lifetime product of 2 × 10−4 cm2/V s. Detectors fabricated from a 100 μm thick h-BN wafer have demonstrated a thermal neutron detection efficiency of 20%, corresponding to a charge collection efficiency of ∼60% at an operating voltage of 500 V. This initial demonstration opens the door for mass producing high efficiency h-BN semiconductor neutron detectors at a reduced cost, which could create unprecedented applications in nuclear energy, national security, nuclear waste monitoring and management, the health care industry, and material sciences.

Thermal neutron detectors in a lateral scheme were fabricated from a 70 μm thick freestanding B-10 enriched hexagonal BN (h-10BN). Two sets of channel peaks corresponding to the neutron capture by 10B occurring in h-10BN comprising turbostratic domains (t-10BN) have been recognized in the nuclear reaction pulsed height spectrum, from which a bandgap of 5.5 eV was directly deduced for t-10BN. Improved device performance over the prior state-of-the-art implies that the transport properties in the lateral plane of t-BN domains are sufficiently good and their presence in h-BN is not a showstopper for the further advancement of h-BN detector technologies.

The detection of fast neutrons is regarded technically challenging because the interaction probability of fast neutron with matter is extremely low. Based on our recent development of hexagonal boron nitride (BN) semiconductor thermal neutron detectors with a record high efficiency of 59%, we report here the feasibility studies of BN detectors for detecting fast neutrons. A BN detector with a detection area of 2.1 cm2 was fabricated from a 90 μm thick BN epilayer. In the presence of a bare Cf-252 source emitting fast neutrons ranging from 1 to 9 MeV, the detection efficiency was estimated to be about 0.1%. The measured mean free path of fast neutron in BN is about 7.6 cm. Together with the capability of BN for thermal neutron detection, the present results indicate that by incorporating BN with a large thickness, BN neutron detectors are expected to possess the unique capability of directly detecting thermal to fast neutrons as well as outstanding features resulting from the ultrawide bandgap of BN. The identification of a single material that is sensitive to both thermal and fast neutrons is valuable for the development of novel neutron detection technologies.

In this paper, an analysis of maritime communications between a ship at sea and a shore station using a ray-based Monte Carlo method has been proposed. A sea surface is modeled by the one-side Pierson-Moskowitz spectrum. A Monte Carlo method based on a ray tracing is employed to predict the scattering from the temporally evolving rough sea surface for each time step. The maximum of double reflections is restricted in the ray tracing for a practical purpose. The total received signal is the superposition of the direct propagating field and the scattered field by the sea surface. The statistical numerical result is finally obtained by averaging the results from many realizations. Numerical results for the link performance evaluation obtained by the proposed method are illustrated. The results show that the misalignment of transmitting and receiving antennas due to the rolling ship has relatively less impact on the communication link performance than the multipath fading due to the scattering by the temporally evolving rough sea surface.