Simulation analysis of 35 MeV high-power electron accelerator driven white neutron source target.

The white neutron source driven by an electron accelerator utilizes a pulsed electron beam to bombard a target, producing neutrons through photoneutron reactions. The white neutron source of photoneutron reaction has advantages such as compact structure, low cost, capability of generating ultra-short pulse, and wide applications in the resonance energy region, effectively complementing reactor neutron sources and spallation neutron sources. The development of high-current, high-power electron accelerator-driven white neutron sources is of significant importance for neutron science research and nuclear technology applications.

Theoretical modeling and simulation-based assessment of graded-bandgap AlGaAs/GaAs electron-injection cathode.

The electron emission model of a negative electron affinity graded-bandgap AlGaAs/GaAs electron-injection cathode was developed from two-dimensional continuity equations. The emission current was obtained from a simulation of the model, and the emission current efficiency and emission current per unit length were calculated. Based on the simulation results and preparation conditions, the range of optimum parameters for the cathode structure were determined.

Mie-type GaAs nanopillar array resonators for negative electron affinity photocathodes.

This paper presents modeling results of Mie-type GaAs nanopillar array resonant structures and the design of negative electron affinity photocathodes based on Spicer's three-step model. For direct-bandgap GaAs with high intrinsic absorption coefficient in the 500 ∼ 850 nm spectral range, photoelectrons were found to be highly localized inside the nanopillars near the top and side surfaces where electrons can be efficiently transported and emitted into vacuum, and the light reflectance can be reduced to ∼1% level at resonance wavelengths. Predictions of spectrally resolved photoemission indicate that these nanophotonics resonators, when properly optimized, can increase the photo-electron emission quantum efficiency at resonance wavelengths to levels limited only by the surface-electron escape probability, significantly outperforming traditional flat wafer photocathodes.

Rhodamine B derivative-modified up-conversion nanoparticle probes based on fluorescence resonance energy transfer (FRET) for the solid-based detection of copper ions.

Herein, a novel solid-based up-conversion fluorescence resonance energy transfer (FRET) sensor was developed using rhodamine B hydrazide, which provided a selective fluorescence response and suitable affinity towards Cu ions over other biologically relevant metal ions because the Cu ion could promote the hydrolysis of α-amino acid esters of rhodamine B hydrazide and yield the Cu·α-amino acid chelate. This solid-based detection system is more convenient for the detection of Cu based on color change and emission spectra instead of the complicated and tedious measurements than other up-conversion sensors and up-conversion luminescent nanoparticles used as an excitation source; moreover, the proposed system shows high selectivity, minimum photo-damage to living organisms, and high chemical stability.

Comparison of photoemission characteristics between square and circular wire array GaAs photocathodes.

Two types of negative electron affinity gallium arsenide (GaAs) wire array photocathodes were fabricated by reactive ion etching and inductively coupled plasma etching of bulk GaAs material. High density GaAs wire arrays with high periodicity and good morphology were verified using scanning electron microscopy, and photoluminescence spectra confirmed the wire arrays had good crystalline quality. Reflection spectra showed that circular GaAs wire arrays had superior light trapping compared with square ones.

Dynamics of graded-composition and graded-doping semiconductor nanowires under local carrier modulation.

Scanning photocurrent microscopy is a powerful tool for investigating charge transfer and internal fields, which strongly influence carrier statics and dynamics in semiconductor nanowires. We performed comprehensive numerical modeling of the carrier dynamics of graded-composition and graded-doping AlGaAs nanowires to achieve a greater understanding of these nanowires. The simulation results indicated that the built-in electric field changes the shape of the scanning photocurrent microscopy profiles, which helped us to judge the dopant level, Al composition range and doping type of the material.

Negative electron affinity GaAs wire-array photocathodes.

Negative electron affinity GaAs wire-array photocathodes have been fabricated by reactive ion etching and inductively coupled plasma etching of bulk GaAs material followed by Cs-O activation. Scanning electron microscope has revealed that the thus obtained high-density GaAs wire arrays had high periodicity, large height, and good morphology. Photoluminescence spectra indicated the wire arrays were of good crystalline quality and free from any obvious damage.

Effects of graded band-gap structures on spectral response of AlGaAs/GaAs photocathodes.

The effects of AlGaAs/GaAs layer thickness and Al composition range on the spectral response and integral sensitivities of reflection-mode graded band-gap AlGaAs/GaAs photocathodes have been investigated and simulated. The experimental results demonstrate that the spectral response over the wavelength region of interest for graded band-gap photocathodes is greater than that for uniform band-gap cathodes, and the increase in long-wavelength response is more pronounced. These results can be attributed to the built-in electric field in the graded band-gap AlGaAs layer.

Resolution characteristics of graded doping and graded composition transmission-mode AlGaAs/GaAs photocathodes.

The resolution model of a graded doping and graded composition transmission-mode AlGaAs/GaAs photocathode is solved numerically from the two-dimensional continuity equations. According to the model, we calculate the theoretical modulation transfer function (MTF) of different graded doping and graded composition structures. The simulation results show that both graded composition and graded doping structures can increase the resolution of the photocathode.

Energy distributions of electrons emitted from reflection-mode Cs-covered GaAs photocathodes.

By calculating the energy distributions of electrons reaching the photocathode surface and solving the Schrödinger equation for an electron tunneling through the surface potential barrier, we have obtained an equation to calculate the energy distributions of electrons emitted from reflection-mode Cs-covered GaAs photocathodes based on a two-minima diffusion model. According to the equation, we studied the effects of incident photon energies, diffusion lengths, and surface potential barrier on the electron energy distributions. The equation was also used to fit the measured electron energy distribution curves and the cathode performance parameters were obtained from the fitting.