Continuous Energy Distribution Research Articles

Continuous distribution of interface states in n-type double-side poly-Si/SiOx passivating contact solar cells

Advanced passivation contact in high-efficiency silicon solar cells plays an important role for the sake of minimizing recombination losses. A stack of heavily doped polycrystalline silicon (poly-Si) and tunnel SiOx contact has attracted much attention, benefitting from its excellent characteristics of carrier selectivity and passivation, and which has been successfully applied in double-side poly-Si/SiOx passivating contact solar cells. Note that characteristics analysis of defects at tunnel SiOx/c-Si (crystalline silicon) interface remains an issue of concern. Herein, we observe capacitance transient spectroscopy arise from both electron and hole traps in the passivating contact structure of poly-Si(p+)/tunnel SiOx/c-Si(n). Subsequently, we propose a skillful procedure of deep-level transient spectroscopy (DLTS) measurement to confirm that the observed electron and hole traps are located at the tunnel SiOx/c-Si interface but not in the bulk of the c-Si substrate. Finally, we show a clear physical picture of continuous energy distribution for interface states.

High-Entropy Materials in Electrocatalysis: Understanding, Design, and Development.

Electrocatalysis is a crucial method for achieving global carbon neutrality, serving as an essential means of energy conversion, and electrocatalyst is crucial in the process of electrocatalysis. Because of the abundant active sites, the multi-component synergistic effect of high-entropy materials has a wide application prospect in the field of electrocatalysis. Moreover, due to the special structure of high-entropy materials, it is possible to obtain almost continuous adsorption energy distribution by regulating the composition, which has attracted extensive attention of researchers. This paper reviews the properties and types of high-entropy materials, including alloys and compounds. The synthesis strategies of high-entropy materials are systematically introduced, and the solid phase synthesis, liquid-phase synthesis, and gas-phase synthesis are classified and summarized. The application of high-entropy materials in electrocatalysis is summarized, and the promotion effect of high-entropy strategy in various catalytic reaction processes is summarized. Finally, the current progress of high-entropy materials, the problems encountered, and the future development direction are reviewed. It is emphasized that the strategy of high flux density functional theory calculation guiding high-entropy catalyst design will be of great significance to electrocatalysis.

The Monte Carlo Thermal-Physics Coupling Method Based on Functional Expansion Tallies

In the Monte Carlo thermal-physical calculations for nuclear reactors, the precise and effective transfer of data between different meshes is a difficult issue of thermal and physical coupling. Converting two separate meshes and transferring the data is an exceptionally difficult and complex task within the conventional nuclear thermal-physics coupling approach. The newly developed Functional Expansion Tallies (FET) method can obtain the continuous distribution of parameters in the solution space. By applying FET method to nuclear thermal-physics coupling, the no-mesh continuous fission energy distribution can be obtained, which is suitable for more complex meshes. Additionally, the computational memory can be minimized by substituting the data from numerous mesh power distribution data points with the coefficients of the function.

Oceanic eddies induce a rapid formation of an internal wave continuum

Oceanic internal waves are a major driver for turbulent mixing in the ocean, which controls the global overturning circulation and the oceanic heat and carbon transport. Internal waves are observed to have a continuous energy distribution across all wave frequencies and scales, commonly known as the internal wave continuum, despite being forced at near-inertial and tidal frequencies at large scales. This internal wave continuum is widely thought to be developed primarily through wave-wave interactions. Here we show, using realistic numerical simulations in the subpolar North Atlantic, that oceanic eddies rapidly distribute large-scale wind-forced near-inertial wave energy across spatio-temporal scales, thereby forming an internal wave continuum within three weeks. As a result, wave energy dissipation patterns are controlled by eddies and are substantially enhanced below the mixed layer. The efficiency of this process potentially explains why a phase lag between high-frequency and near-inertial wave energy was observed in eddy-poor regions but not in eddy-rich regions. Our findings highlight the importance of eddies in forming an internal wave continuum and in controlling upper ocean mixing patterns.

Manifold Path Guiding for Importance Sampling Specular Chains

Complex visual effects such as caustics are often produced by light paths containing multiple consecutive specular vertices (dubbed specular chains) , which pose a challenge to unbiased estimation in Monte Carlo rendering. In this work, we study the light transport behavior within a sub-path that is comprised of a specular chain and two non-specular separators. We show that the specular manifolds formed by all the sub-paths could be exploited to provide coherence among sub-paths. By reconstructing continuous energy distributions from historical and coherent sub-paths, seed chains can be generated in the context of importance sampling and converge to admissible chains through manifold walks. We verify that importance sampling the seed chain in the continuous space reaches the goal of importance sampling the discrete admissible specular chain. Based on these observations and theoretical analyses, a progressive pipeline, manifold path guiding , is designed and implemented to importance sample challenging paths featuring long specular chains. To our best knowledge, this is the first general framework for importance sampling discrete specular chains in regular Monte Carlo rendering. Extensive experiments demonstrate that our method outperforms state-of-the-art unbiased solutions with up to 40 × variance reduction, especially in typical scenes containing long specular chains and complex visibility.

Dark Energy Is the Cosmological Quantum Vacuum Energy of Light Particles—The Axion and the Lightest Neutrino

We uncover the general mechanism and the nature of today’s dark energy (DE). This is only based on well-known quantum physics and cosmology. We show that the observed DE today originates from the cosmological quantum vacuum of light particles, which provides a continuous energy distribution able to reproduce the data. Bosons give positive contributions to the DE, while fermions yield negative contributions. As usual in field theory, ultraviolet divergences are subtracted from the physical quantities. The subtractions respect the symmetries of the theory, and we normalize the physical quantities to be zero for the Minkowski vacuum. The resulting finite contributions to the energy density and the pressure from the quantum vacuum grow as loga(t), where a(t) is the scale factor, while the particle contributions dilute as 1/a3(t), as it must be for massive particles. We find the explicit dark energy equation of state of today to be P=w(z)H: it turns to be slightly w(z)<−1 with w(z) asymptotically reaching the value −1 from below. A scalar particle can produce the observed dark energy through its quantum cosmological vacuum provided that (i) its mass is of the order of 10−3 eV = 1 meV, (ii) it is very weakly coupled, and (iii) it is stable on the time scale of the age of the universe. The axion vacuum thus appears as a natural candidate. The neutrino vacuum (especially the lightest mass eigenstate) can give negative contributions to the dark energy. We find that w(z=0) is slightly below −1 by an amount ranging from (−1.5×10−3) to (−8×10−3) and we predict the axion mass to be in the range between 4 and 5 meV. We find that the universe will expand in the future faster than the de Sitter universe as an exponential in the square of the cosmic time. Dark energy today arises from the quantum vacuum of light particles in FRW cosmological space-time in an analogous way to the Casimir vacuum effect of quantum fields in Minkowski space-time with non-trivial boundary conditions.

Numerical Simulation of Trapped Hole Lateral Migration and Induced Threshold Voltage Retention Loss in a SONOS Flash Memory

We establish a one-dimensional transient simulation to study trapped hole lateral migration in the silicon nitride of a silicon-oxide-nitride-oxide-silicon (SONOS) flash memory cell. In our model, the nitride traps have a continuous energy distribution. Trapped hole emissions to the valence band of the silicon nitride, free hole recapture into the nitride traps, and hole drift-diffusion in the valance band are included in the simulation. Two major trapped hole emission processes, thermally assisted tunneling (ThAT) and the Frenkel-Poole emission are considered. We simulate trapped hole lateral migration in a SONOS cell with program electrons stored at the source side and erase holes at the drain side. Our simulation shows that the ThAT is a dominant transport mechanism for trapped hole lateral migration in retention. We also simulate a threshold voltage (Vt) retention loss due to trapped hole lateral migration at different program levels. The trend of the simulated Vt retention loss is consistent with the measurement result.

Including a distribution of threshold displacement damage energy on the calculation of the damage function and electron's Non Ionizing energy Loss

The Norgett-Robinson-Torrens displacements per atom function is commonly used to estimate the amount of atomic displacement produced by incident energetic particles. At low incident energy, this function is defined as a step function presenting a single threshold displacement damage energy. But materials have different threshold as a function of the crystallographic orientation . Molecular dynamic simulations show that a continuously varying damage energy probability is best suited to represent the threshold damage region. This work proposes a method to introduce in the NRT damage function a continuous damage energy distribution. The impact of a change on the damage function in the threshold region is evaluated on the incident electrons' Non Ionizing energy Loss .

Computerized glow curve deconvolution (CGCD): A comparison using asymptotic vs rational approximation in thermoluminescence kinetic models

In this study an open-source tool GCD Analyzer, based on Microsoft® Excel, for Computerized Glow Curve Deconvolution analysis (CGCD) of thermoluminescence (TL) glow peak has been developed using a more accurate rational approximation. It is capable of deconvolution of glow curves having discrete or continuous trap distribution and can be used for routine as well as emergency radiation dosimetric analysis. This tool has a unique feature of TL glow curve analysis by combining discrete and continuous energy distributions for crystalline, amorphous, and mixed materials. To obtain best values of trap parameters like activation energy (E), frequency factor (s), order of kinetics (b), GCD Analyzer has the capability of analyzing glow curve using selectable individual peak of various physical kinetic models i.e. FOK, SOK, GOK, MOK, and Continuous Traps Distribution (CTD) with subtraction of optional background signal. The residual graph gives a clear visual understanding of the Figure of Merit (F.O.M). A comparison of asymptotic and rational approximation to the built-in second order exponential integral function E2(E/KT) for E/KT < 600 is also presented. The results are verified by deconvolution of test synthetic glow curves with F.O.M up to 0.0005%, experimental glow curve for CTD with F.O.M of 0.9781%, and in the case of GLOWCANIN project glow curves, the F.O.M is comparable to the least values achieved by inter-comparison participants.

Analysis of multipactor in a rectangular waveguide using Spark3D software

Multipactor is a resonant nonlinear electron multiplication effect that may occur in high power microwave devices at very low pressures, such as those operating in particle accelerators and satellite subsystems. In this research, multipactor of a rectangular waveguide was analyzed using the commercially available, numerical simulation software “Spark3D.” The electromagnetic wave in the simulation was a TE10 mode-2.85 GHz wave of varying power, fed into the impedance transformer waveguide. At the lowest threshold, multipactor is generated in the minimum height region in the impedance transformer and nowhere else. More precisely, the multipacting electrons have a continuous energy distribution since the emitted secondary electrons carry a random initial velocity distribution. We observed that there are cases where the impact electron energy decreases despite an increase in power due to growing non-resonance of the microwave field and electron oscillations, resulting in not only two threshold points where secondary emission yield (SEY) = 1 but several more. As a consequence, it was uncovered that when the average SEY in the highest field region is close to or less than one, multipactor may be caused in a lower field region where the SEY is effectively higher than one. The numerical results are compared with data from the experiment. While there is some deviation between the thresholds obtained from Spark3D and the experiment, the results at higher power levels are consistent with the experiment in the view of the SEY for each power level.

Effect of Ge:Si ratio and charging energy on carriers trapping in Y2(Ge,Si)O5:Pr powders observed with thermoluminescence methods

Oxyorthosilicates have long history in luminescent applications e.g. as phosphors, scintillators and in emerging technologies like luminescence thermometry. In this study we report detailed research on the thermoluminescent (TL) properties of Y2(Gex,Si1-x)O5:Pr. Despite regular TL measurements, Tmax-Tstop, initial rise analysis, fading, decay of persistent luminescence, and dose effect are presented and analyzed. The possibility to deliberately manage the electron trap depths adjusting the Ge/Si ratio is explored. Glow curve of each composition consists of two main TL peaks. In the (Ge,Si) solid continuous distribution of trap energies is observed. Strong thermoluminescence is observed from all compositions after charging with X-ray radiation. Efficiency of charging traps with UV-C photons (fitting the 4f→5d1 transition energy) lowers systematically as the Si content increases. A growing coupling of the excited 5d1 level of Pr3+ and the host conduction band with the increase of Ge content is responsible for this effect. A scheme of the TL-related processes is presented using the vacuum referred binding energy (VRBE) approach for all the Y2(Gex,Si1-x)O5:Pr phosphors.

Characterization of the thermoluminescence glow curve of Li2B4O7:Cu,Ag

This work presents the characterization of the thermoluminescence (TL) glow curve of a copper and silver doped lithium tetraborate (Li2B4O7:Cu,Ag) made using three different methods to identify the number of peaks, the trap structure and the kinetics parameters: TM-Tstop, glow curve fitting (GCF) and various heating rates (VHR).The obtained results can be consistently explained assuming four TL glow peaks in the region 50 - 270 °C, originated by a complex trap structure: continuous energy distributions for peaks I, III and IV, and general order kinetics (GOK) discrete trapping level for peak II. Quite similar activation energy values have been obtained by GCF and VHR methods.

Coincidence energy spectra due to the intrinsic radioactivity of LYSO scintillation crystals

BackgroundLutetium oxyorthosilicate or lutetium yttrium oxyorthosilicate (LYSO) scintillation crystals used in most current PET scanner detectors contain 176Lu, which decays by beta emission to excited states of 176Hf accompanied by the emission of prompt gamma rays or internal conversion electrons. This intrinsic radioactivity can be self-detected in singles mode as a constant background signal that has an energy spectrum whose structure has been explained previously. In this work, we studied the energy spectrum due to the intrinsic radioactivity of LYSO scintillation crystals of two opposing detectors working in coincidence mode. The investigation included experimental data, Monte Carlo simulations and an analytical model.ResultsThe structure of the energy spectrum was completely understood and is the result of the self-detection of beta particles from 176Lu in one crystal and the detection of one or more prompt gamma rays detected in coincidence by the opposing crystal. The most probable coincidence detection involves the gamma rays of 202 and 307 keV, which result in two narrow photopeaks, superimposed on a continuous energy distribution due to the beta particle energy deposition. The relative intensities of the gamma ray peaks depend on crystal size and detector separation distance, as is explained by the analytical model and verified through the Monte Carlo simulations and experiments.ConclusionsThe analytical model used in this work accurately explains the general features of the coincidence energy spectrum due to the presence of 176Lu in the scintillation crystals, as observed experimentally and with Monte Carlo simulations. This work will be useful to those research studies aimed at using the intrinsic radioactivity of LYSO crystals for transmission scans and detector calibration in coincidence mode.

Photo- radio- and thermoluminescence of sintered transparent Al2O3:Eu2+ in the range 15–800 K

Photoluminescence, radioluminescence, and thermoluminescence of transparent alumina in the range 15–800 K is presented. Blue 4f65d1→4f7(8S7/2) luminescence of Eu2+ exclusively under ultraviolet excitation at ~280 nm was observed and its thermal stability was evaluated. Excitation with X-rays produced violet broad band peaking around 380 nm as well as infrared luminescence of Cr3+ impurity around 694 nm due to the 2Eg→4A2g transition. The temperature dependence of both these emissions was tested. The infrared luminescence could be effectively excited through broad bands peaking at ~400 and 550 nm. A Well-resolved vibronic structure of the 550 nm excitation band of Cr3+ is discussed. Significant red-shift of the Cr3+ luminescence was observed with increasing temperature. Both Cr3+ ion, as well as intrinsic defect(s) of F-type center, are active in thermoluminescence of that transparent ceramic material. Detailed analysis of thermoluminescence generated by the F-center showed a set of traps in a broad range of temperatures (40–570 °C). Their thermoluminescence is government by first-order kinetics. Continuous distribution of trap energies was proved. A vacuum referred binding energy scheme including centers involved in the luminescence of the system is presented.

Energy window optimization for Y-90 Bremsstrahlung SPECT imaging: A Monte Carlo simulation study

Introduction: In Y-90 imaging, image quality is highly dependent on choosing the energy window and collimator design because, the Y-90 bremsstrahlung photons have a continuous and broad energy distribution. The aim of this work was to optimize the bremsstrahlung energy window setting and collimator to improve both resolution and sensitivity. Materials and methods: We simulate the Siemens Medical System Symbia using the SIMulation of Imaging Nuclear Detectors (SIMIND) Monte Carlo program. The SIMIND was used to generate the Y-90 bremsstrahlung Single Photon Emission Computed Tomography (SPECT) projection of the point source. In order to assess the effect of the energy window on the resolution, six energy windows settings and two collimators denoted Medium Energy (ME) and High Energy (HE) were used. Results: The experimental measurement and simulation results exhibited a similar pattern in point spread functions (PSFs) with energy window. The simulation data show that, the geometric component reaches 73% for 51-120 keV energy window with HE collimator. In addition, the results showed that, the Full Width at Half Maximum (FWHM) and Full Width at Tenth Maximum (FWTM) (FWHM =7 mm + FWTM =35mm) was better in this window relative to other window. Conclusion: The optimal energy window for Y-90 bremsstrahlung SPECT imaging was 51- 120 keV. The obtained optimal energy window and optimal HE collimator has the potential to improve the image resolution and sensitivity of Y-90 SPECT images.

Beta-ray imaging system with γ-ray coincidence for multiple-tracer imaging.

Beta-ray imaging systems are widely used for various biological objects to obtain a two-dimensional (2D) distribution of β-ray emitting radioisotopes. However, a conventional β-ray imaging system is unsuitable for multiple-tracer imaging, because the continuous energy distribution of β-rays complicates distinguishing among different tracers by energy information. Therefore, we developed a new type of β-ray imaging system, which is useful for multiple tracers by detecting coincidence γ-rays with β-rays, and evaluated its imaging performance. Our system is composed of position-sensitive β-ray and γ-ray detectors. The former is a 35×35×1-mm3 Ce-Doped((La, Gd)2 Si2 O7 ) (La-GPS) scintillation detector, which has a 300-µm pitch of pixels. The latter is a 43×43×16-mm3 bismuth germanium oxide (BGO) scintillation detector. Both detectors are mounted on a flexible frame and placed in a user-selectable position. We experimentally evaluated the performance of the β-ray detector and the γ-ray efficiencies of the γ-ray detector with different energies, positions, and distances. We also conducted point sources and phantom measurements with dual isotopes to evaluate the system performance of multiple-tracer imaging. For the β-ray detector, the β-ray detection efficiencies for 45 Ca (245-keV maximum energy) and 90 Sr/90 Y (545 and 2280-keV maximum energy) were 14.3% and 21.9%, respectively. The total γ-ray detection efficiency of the γ-ray detector for all γ-rays from 22 Na (511-keV annihilation γ-rays and a 1275-keV γ-ray) in the center position with a detector distance of 20mm was 17.5%. From a point-source measurement using 22 Na and 90 Sr/90 Y, we successfully extracted the position of a positron-γ emitter 22 Na. Furthermore, for a phantom experiment using 45 Ca and 18 F or 18 F and 22 Na, we successfully extracted the distribution of the second tracer using the annihilation γ-ray or de-excitation γ-ray coincidence. In all the imaging experiments, the event counts of the extracted images were consistent with the counts estimated by the measured γ-ray efficiencies. We successfully demonstrated the feasibility of our β-ray autoradiography system for imaging multiple isotopes. Since our system can identify not only a β-γ emitter but also a positron emitter using the coincidence detection of annihilation γ-rays, it is useful for PET tracers and various new applications that are otherwise impractical.

On the orange-red persistent luminescence of ScPO4:Eu3+

ScPO4:0.1%Eu3+ sintered materials were found to generate orange-red persistent emission following exposure to X-rays. A set of thermoluminescence techniques and isothermal decay measurements were used to study the mechanisms of the carriers’ trapping and detrapping processes. The two main TL peaks at around 90 °C (363 K) and 160 °C (433 K) were found to result from so-called continuous distribution of trap depths. Room temperature fading proved that the persistent luminescence was connected with the strong TL peak at ∼90 °C (363 K). The persistent emission kinetics was characterized by a linear dependence of I−1(t). This was consistent with the continuous distribution of trap energies observed in the material. The main persistent emission components were located around 590, 620, and 720 nm. The most significant contribution was from the first and the third of them, resulting from the 5D0→7F1 and 5D0→7F4 transitions, respectively.

The Study on Interpretation of the Scatter Degradation Factor using an additional Filter in a Medical Imaging System

X-rays used for diagnosis have a continuous energy distribution. However, photons with low energy not only reduce image contrast, but also contribute to the patient’s radiation exposure. Therefore, clinics currently use filters made of aluminum. Such filters are advantageous because they can reduce the exposure of the patient to radiation. However, they may have negative effects on imaging quality, as they lead to increases in the scattered dose. In this study, we investigated the effects of the scattered dose generated by an aluminum filter on medical image quality. We used the relative standard deviation and the scatter degradation factor as evaluation indices, as they can be used to quantitatively express the decrease in the degree of contrast in imaging. We verified that the scattered dose generated by the increase in the thickness of the aluminum filter causes degradation of the quality of medical images.

Linewidth broadening and tunneling of excitons bound to N pairs in dilute GaAs:N

The exciton bound to a pair of nitrogen atoms situated at nearby lattice sites in dilute GaAs:N provides an energetically uniform electronic system, spectrally distinct from pairs with larger or smaller separations, and can even be grown with a uniform pair orientation in the crystal. We use photoluminescence excitation spectroscopy on an ensemble of N pairs to study the narrow continuous energy distribution within two of the individual exchange- and symmetry-split exciton states. Inhomogeneous linewidths of 50–60 μeV vary across the crystal on a mesoscopic scale and can be 30 μeV at microscopic locations indicating that the homogeneous linewidth inferred from previous time-domain measurements is still considerably broadened. While excitation and emission linewidths are similar, results show a small energy shift between them indicative of exciton transfer via phonon-assisted tunneling between spatially separated N pairs. We numerically simulate the tunneling in a spatial network of randomly distributed pairs having a normal distribution of bound exciton energies. Comparing the ensemble excitation-emission energy shift with the measured results shows that the transfer probability is higher than expected from the dilute pair concentration and what is known of the exciton wavefunction spatial extent. Both the broadening and the exciton transfer have implications for the goal of pair-bound excitons as a single- or multi-qubit system.

Energy Window and Contrast Optimization for Single-photon Emission Computed Tomography Bremsstrahlung Imaging with Yttrium-90.

Purpose:In yttrium-90 (Y-90) single-photon emission computed tomography (SPECT) imaging, the choice of the acquisition energy window is not trivial, due to the continuous and broad energy distribution of the bremsstrahlung photons. In this work, we investigate the effects of the energy windows on the image contrast to noise ratio (CNR), in order to select the optimal energy window for Y-90 imaging.Materials and Methods:We used the Monte Carlo SIMIND code to simulate the Jaszczak phantom which consists of the six hot spheres filled with Y-90 and ranging from 9.5 to 31.8 mm in diameter. Siemens Symbia gamma camera fitted with a high-energy collimator was simulated. To evaluate the effect of the energy windows on the image contrast, five narrow and large energy windows were assessed.Results:The optimal energy window obtained for Y-90 bremsstrahlung SPECT imaging was 120–150 keV. Furthermore, the results obtained for CNR indicate that the high detection is only for the three large spheres.Conclusion:The optimization of energy window in Y-90 bremsstrahlung has the potential to improve the image quality.