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Neutrons from an atmospheric nuclear explosion can be detected by sensors in orbit. Current tools for characterizing the neutron energy spectrum assume a known source and use forward transport to recreate the detector response. In realistic scenarios the true source is unknown, making this an inefficient, iterative approach. In contrast, the adjoint approach directly solves for the source spectrum, enabling source reconstruction. The time–energy fluence at the satellite and adjoint transport equation allow a Monte Carlo method to characterize the neutron source's energy spectrum directly in a new model: the Space to High-Altitude Region Adjoint (SAHARA) model. A new adjoint source event estimator was developed in SAHARA to find feasible solutions to the neutron transport problem given the constraints of the adjoint environment. This work explores SAHARA's development and performance for mono-energetic and continuous neutron energy sources. In general, the identified spectra were shifted towards energies approximately 5% lower than the true source spectra, but SAHARA was able to capture the correct spectral shapes. Continuous energy sources, including real-world sources Fat Man and Little Boy, resulted in identifiable spectra that could have been produced by the same distribution as the true sources as demonstrated by two-dimensional (2D) Kolmogorov–Smirnov tests. [ABSTRACT FROM AUTHOR]

Low temperature neutron irradiated [Al.sub.0.27][Ga.sub.0.73]N/GaN heterostructures reveal a complex temperature dependent displacement damage formation process. This process results in differences in drain currents at low (80 K) versus high (294 K) temperatures. Irradiation increases the gate and drain currents at 80 K, and decreases the drain current at room temperature. These effects saturate at ~ 3 x [10.sup.10] n/[cm.sup.2] indicating complexing with a native impurity. After a room temperature anneal, the effect on the gate current persists and the drain current partially recovers. A two-step persistent interface trap formation model is presented that explains these results. This model is further supported by CV measurements at 80 K and 294 K after annealing. Index Terms--Gallium nitride, heterostructure field effect transistor (HFET), high electron mobility transistor (HEMT), neutron radiation effects.

The effects of low energy (0.45 MeV) electron radiation on the gate and drain currents of [Al.sub.0.27][Ga.sub.0.73]N/GaN HFETs are investigated using IV and CV measurements. Following irradiation, the gate and drain currents increase at low temperatures and reach a saturation level. The gate leakage currents do not fully account for the drain current increase. Following a room temperature anneal, the gate and drain currents return to pre-irradiation levels. These results are explained by the buildup of positive charge in the AlGaN layer at low temperature and traps formed via a complexing precursor in the AlGaN layer near the interface. The positive charge increases the carrier concentration in the 2DEG and hence the drain current. The traps act as trap-assisted-tunneling centers that increase the gate leakage current. Index Terms--Electron irradiation, gallium nitride, heterojunction field effect transistors.

Growth of Li2B4O7 by the hydrothermal method was attempted in 10−6 MLiOH, by both spontaneous nucleation and transport growth. Li2B4O7 grew out of the first spontaneous nucleation reaction, while the seed dissolved and γ-LiBO2 formed during the transport growth reactions. A species thermodynamic stability study showed that γ-LiBO2 formed exclusively from 350°C to 550°C, and Li3B5O8(OH)2 was observed at 250°C; Li2B4O7 was not observed. A solubility study on Li2B4O7 did not achieve equilibrium conditions, but did demonstrate that γ-LiBO2 is significantly more thermodynamically stable than Li2B4O7 under the tested conditions. Formation of Li2B4O7 hydrothermally was shown to result from kinetic growth. [ABSTRACT FROM AUTHOR]