Detection of neutrons, at high total efficiency, with greater resolution in kinetic energy, time and/or real-space position, is fundamental to the advance of subfields within nuclear medicine, high-energy physics, non-proliferation of special nuclear materials, astrophysics, structural biology and chemistry, magnetism and nuclear energy. Clever indirect-conversion geometries, interaction/transport calculations and modern processing methods for silicon and gallium arsenide allow for the realization of moderate- to high-efficiency neutron detectors as a result of low defect concentrations, tuned reaction product ranges, enhanced effective omnidirectional cross sections and reduced electron–hole pair recombination from more physically abrupt and electronically engineered interfaces. Conversely, semiconductors with high neutron cross sections and unique transduction mechanisms capable of achieving very high total efficiency are gaining greater recognition despite the relative immaturity of their growth, lithographic processing and electronic structure understanding. This review focuses on advances and challenges in charged-particle-based device geometries, materials and associated mechanisms for direct and indirect transduction of thermal to fast neutrons within the context of application. Calorimetry- and radioluminescence-based intermediate processes in the solid state are not included.

https://iopscience.iop.org/article/10.1088/0953-8984/22/44/443201/pdf

The physics of solid-state neutron detector materials and geometries

Optical and electrical properties of hydrided palladium thin films studied by an inversion approach from transmittance measurements

Thin films of rare-earth (RE)–oxygen–hydrogen compounds prepared by reactive magnetron sputtering show a unique color-neutral photochromic effect at ambient conditions. While their optical properti...

Oxyhydride Nature of Rare-Earth-Based Photochromic Thin Films

Gadolinium nanoparticles, deposited via an inert gas evaporation method, show improved stability towards oxidation and it is therefore possible to carry out an ex-situ investigation on "bare" Gd nanoparticles, i.e., in the absence of a protective Pd layer, for the first time. A size-induced structural transformation from hexagonal close packing to the higher-symmetry face-centered cubic structure is observed. The important observation of hydrogen-Gd-nanoparticle interaction at room temperature and atmospheric pressure, without a Pd catalytic layer, makes Gd nanoparticles a potential candidate for hydrogen-sensing, switching, and storage applications.

Stability and Hydrogenation of “Bare” Gadolinium Nanoparticles

Oxyhydride compounds of GdHO and GdHO:Tb3+ were successfully prepared by solid state reaction of GdH3, Gd2O3 and Gd2O3:Tb3+ under a H2 atmosphere. XRD analysis suggests that GdHO has a cubic fluorite structure (space group Fmm, lattice parameter a = 5.38450(4) A for GdHO and 5.38523(6) A for GdHO:Tb), which is different from the tetragonal fluorite-like superstructure for previously reported LnHO (Ln = La, Ce, Pr and Nd) (space group P4/nmm) materials. The prepared GdHO:Tb3+ shows photoluminescence bands in the range between 470 and 650 nm, which are attributed to Tb3+:5D4 → 7FJ transitions. In the photoluminescence excitation spectrum, a broad 4f–5d band of Tb3+ was clearly observed at around 310 nm, which is one of the lowest 5d energy positions compared with other Tb3+-doped compounds. This shift may be caused by partially coordinated hydride ions with strong covalency around Tb3+, which cause a large nephelauxetic effect and crystal field splitting. The lifetime of Tb3+:5D4 is almost unchanged from 83 to 500 K and decreases above 500 K. The thermal luminescence quenching of the 5D4 → 7FJ transition is caused by a thermally activated crossover to the ground state through the 5d energy state. The band gap energy of GdHO (5.39 eV) was smaller than that of cubic and monoclinic Gd2O3 (5.93 and 6.10 eV, respectively). Based on the constructed vacuum referred binding energy diagram and DFT calculation, the top of the valence band (VB) energy of GdHO is much higher than that of cubic and monoclinic Gd2O3 due to the higher energy level of the hydride 1s orbital than that of the oxide 2p orbital.

Preparation, electronic structure of gadolinium oxyhydride and low-energy 5d excitation band for green luminescence of doped Tb3+ ions

We have measured the optical transmittance of thin films of gadolinium trihydride GdH3 over a broad spectral range, 0.05–6 eV. The analysis indicates that GdH3 is a wide-gap semiconductor with an energy gap of 2.55 eV. The fundamental absorption and impurity-related transitions dominate the spectra in the high-energy region, 1–6 eV. Hydrogen vibration modes dominate the spectra in the low-energy region, 0.05–0.2 eV. A strong free-carrier absorption effect is observed. In addition, the frequency-dependent refractive index, extinction coefficient, dielectric function, and optical conductivity are determined from the transmission spectra. This work also evaluates the plasma frequency associated with free carriers.

M. W. Lee

Papers

Determination of the optical constants of the γ-phase GdH3 thin films