Low Energy Region Research Articles

Processes at lithium-hydride/deuteride surfaces upon low energy impact of H/D

Sputtering, reflection, and retention processes at amorphous and crystalline lithium hydride surfaces due to impact of low energy (1–100 eV) hydrogen and deuterium atoms over the range of 0o −85o angle of incidence at 300 K surface temperature were investigated by atomistic computational methods. Classical molecular dynamics simulations were performed with improved reactive bond-order force field (ReaxFF) potentials that include long-range polarization effects. In addition to probabilities of surface processes, the energy and angular spectra of ejected particles were obtained. Comparison of these results with those previously obtained on pristine lithium surfaces indicates the importance of saturation of the Li surface and near-surface region with hydrogen. We show that such saturation, which is typical in both laboratory and fusion device experiments with lithium coating of the plasma-facing surfaces, significantly changes the surface processes with hydrogen irradiation in the understudied low-energy region of impact energies.

The electronic and optical properties of multi-layer Bi2O2X (X = S, Se, Te) by first-principles calculations

Two-dimensional (2D) Bi2O2X (X = S, Se, Te) have been reported to be promising optoelectronic materials, which have attracted immense attention from researchers. Adjustable optoelectronic materials are very important for the design and application of the device, but how to design its electronic and optical properties is rarely studied. In this research, we have systematically investigated the electronic and optical properties of 2D Bi2O2X (X = S, Se, Te), especially the response based on the number of layers and the strength of strain. We have found the lattice constants and the band gaps of the H-Bi2O2X and Z-Bi2O2X decrease as the number of layers increases, and finally approach the values of the bulk phase. Moreover, the adjustment of the number of layers can change the range of H-Bi2O2S band gap very much, and can even be adjusted from blue light energy to infrared light energy. As the layer numbers increase, the peaks of the imaginary part of the dielectric function and the absorption spectrum of H-Bi2O2S shift to low-energy region (redshift). Furthermore, the band gaps of H-Bi2O2X (X = S, Se, Te), Z-Bi2O2S, and Z-Bi2O2Se increase first and decrease with the increasing compress strain, and that decrease linearly with the increasing tensile strain. Surprisingly, with the increase of tensile strain, the band gap of Z-Bi2O2Te increases first and then decreases instead of the linear decrease like other materials, which may be due to the heavier Te atoms. Our HSE06 calculation results show that the Bi2O2X is a promising tunable photodetector with a broadband photoresponse in the visible light range.

Updated constraints on sterile neutrino mixing in the OPERA experiment using a new νe identification method

Abstract This paper describes a new νe identification method specifically designed to improve the low-energy ($\lt {30}\, {\rm GeV}$) νe identification efficiency attained by enlarging the emulsion film scanning volume with the next-generation emulsion readout system. A relative increase of 25–70% in the νe low-energy region is expected, leading to improvements in the OPERA sensitivity to neutrino oscillations in the framework of the 3 + 1 model. The method is applied to a subset of data where the detection efficiency increase is expected to be more relevant, and one additional νe candidate is found. The analysis combined with the ντ appearance results improves the upper limit on sin 22θμe to 0.016 at 90% C.L. in the MiniBooNE allowed region $\Delta m^2_{41} \sim {0.3}\, {\rm eV}^{2}$.

Investigation and Analysis of Thermoelectrically Cooled CZT Performance

Cadmium zinc telluride (CZT) spectrometers have been considered for objectives and missions with variable ambient temperatures. Spectrometer-grade crystals of various sizes have been studied under conditions as low as −40°C for 2 × 2 × 2 and 5 × 5 × 2-mm3 crystals, and −10°C for 5 × 5 × 5-mm3 crystals for resolution improvement spanning 5.9-, 59.5-, and 122-keV photo peak energies. It is unclear from previously published data if cooling the spectrometer-grade crystals beyond −10°C results in increased resolution improvement or if the effect occurs with higher-energy photo peaks and trends among multiple crystals from the same manufacturer. Therefore, we acquired two CZT crystals from Kromek and cooled them in an insulated box to −25°C. Our measurements were performed every 5°C, and tested with 241Am or 241Am/152Eu mixed sources. The 241Am peaks were compared for both crystals, and the higher-energy resolution changes were explored using the mixed source. Overall, at 59.5 keV, both crystals yielded 3% to 4% resolution improvement for the cooling cycle and 6% improvement during the warming cycle. Resolution performance varied between the two tested crystals, and each had a different temperature where we observed optimum resolution. The 121.8-keV peak resolution improved by 1.2% for the cooling cycle and 3.6% for the warming cycle. There were no discernable resolution increases or changes for the two higher-energy peaks, 224.7 and 334.3 keV, respectively. Slight cooling of the CZT crystals can increase resolution performance by 4% in the lower-energy region.

Energy measurement of 241Am-[formula omitted]Be neutrons by tagged neutron time-of-flight

Neutron energy spectrum of a 241Am-9Be source was measured in the energy range of 0.3 to ∼ 6.5 MeV using γ-ray tagged neutron time-of-flight method. BC501A type organic liquid scintillator detector was used at a flight path of 175 cm to detect neutrons with good energy resolution. The de-excitation γ-ray emitted in coincidence with neutrons was detected using a fast BaF2 detector. The measured data has been compared with the ISO 8529-2 standard neutron reference spectrum and found good agreement in the overlapping energy region. Present measurement applied efficiency correction to the data and extended up to 0.3 MeV in the lower energy region compared to earlier reported measurement using similar technique.

Design and performance study of a proportional counter for low-energy X-rays

A cylindrical proportional counter based on a noble gas was designed to accurately measure low-energy X-ray radiation. The detection capability of the proportional counter with different gases and geometries was calculated through the Monte Carlo method. The optimal filling gas and applicable pipe diameter were determined according to the results of the Monte Carlo simulation. Then, energy calibration was performed at the monoenergetic X-ray radiation facility (MXRF) based on Bragg diffraction, and the energy calibration results were verified using the radionuclides 241Am and 129I. The relative deviation of the theoretical and measured values of the energy does not exceed 1.4%, and the energy resolution of the designed proportional counter is approximately 10%. It was proved that the linearity of the channel and energy of the proportional counter can be used for energy spectrum analysis, and the proportional counter has a good radionuclide recognition capability. Finally, the detection efficiency of the designed proportional counter was simulated and verified at multiple energy points. A detection efficiency calculation model was established through MCNP5, and the detection efficiency of the proportional counter was measured at the MXRF. The results of the experiment and simulation showed that the proportional counter could reach a highest detection efficiency of 92% in the low-energy region. The average deviation between the experiment and simulation was 1.63% at 10–34 keV and 3.90% at 34–75 keV. The proportional counter can be used for measurement and application of X-rays in the low-energy region.

Elastic and inelastic low-energy electron scattering from pyridine.

A comprehensive investigation of elastic and inelastic electron scattering from molecular pyridine is reported using the ab initio R-matrix method with the static exchange plus polarization and close-coupling approximations for incident energies up to 10eV. The two well-known low-lying 1 2B1 and 1 2A2 shape resonances as well as a 2 2B1 mixed-character resonance compare well with the theoretical and experimental results. We also detect five core-excited resonances (1 2A1, 1 2B2, 3 2B1, 2 2A2, and 4 2B1), which lie above the first electronic excitation threshold. The total elastic cross sections and momentum transfer cross sections agree reasonably with previous reference data. Comparisons of the differential elastic cross sections of pyridine with those measured for benzene, pyrazine, and pyrimidine show remarkable agreement at scattering angles above 40° but behave differently for forward scattering below 40° below 6eV, due to the dominant effect of the permanent dipole moment on the differential cross section in the low energy region with narrow scattering angles. Inelastic electronic excitation cross sections are presented, showing the influence of core-excited resonances below the ionization threshold for the first time.

Microscopic observation of two-level systems in a metallic glass model.

The low-temperature quasi-universal behavior of amorphous solids has been attributed to the existence of spatially localized tunneling defects found in the low-energy regions of the potential energy landscape. Computational models of glasses can be studied to elucidate the microscopic nature of these defects. Recent simulation work has demonstrated the means of generating stable glassy configurations for models that mimic metallic glasses using the swap Monte Carlo algorithm. Building on these studies, we present an extensive exploration of the glassy metabasins of the potential energy landscape of a variant of the most widely used model of metallic glasses. We carefully identify tunneling defects and reveal their depletion with increased glass stability. The density of tunneling defects near the experimental glass transition temperature appears to be in good agreement with experimental measurements.

Non-radiative charge transfer process of proton impcating B atom

Electron transfer in heavy particle collisions, involving complicated electron correlation mechanisms, greatly affects the charge state balance in plasma, and is also one of important sources for X-ray radiation. Electron transfer cross section and rate coefficient are important atomic parameters required for the development of nuclear fusion plasma in the national defense industry. We systematically investigate the electron transfer process of proton impacting boron atom in an energy range of <inline-formula><tex-math id="M4">\begin{document}${10}^{-3}-{10}^{3}\;{\rm{e}}{\rm{V}}/{\rm{u}}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="16-20230470_M4.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="16-20230470_M4.png"/></alternatives></inline-formula> based on a fully quantum non-radiative molecular orbital close-coupling method. A total of 15 channels of electron transfer, excitation, and elastic scattering are obtained by using the multi-reference configuration interaction method, and the corresponding molecular energy of each channel is in good agreement with the experimental results. The phenomenon of avoiding crossing between the adiabatic potential energy curves of molecular states is obvious, which constituents the main pathway of electron transfer. The calculation shows that the electron transfer of the 2s orbital is dominated in the process of proton impacting boron atom, while the electron transfer of the 2p orbital contributes a little. In the low energy region, there are obvious quantum resonances in the electron transfer cross section, which originate from the coupling between high energy channel and low energy channel. In addition, we calculate the electron transfer rates of proton-impact boron atom at different temperatures, which can provide important atomic parameter which support for simulating and diagnosing complex plasma environments.

Enhanced optical absorption in two-dimensional Ruddlesden-Popper (C6H5CH2NH3)2PbI4 perovskites via biaxial strain and surface doping.

Two-dimensional Ruddlesden-Popper (2D RP) perovskites can form layered protective materials using long organic cations as "barrier" caps, which is expected to solve the problem of instability of perovskites in the working environment. In this work, we systematically studied the 2D Ruddlesden-Popper (C6H5CH2NH3)2PbI4 hybrid perovskites using density functional theory. The results reveal that the 2D (C6H5CH2NH3)2PbI4 perovskites are semiconductors with band gaps of 2.22 eV. The optical absorption peak of the 2D (C6H5CH2NH3)2PbI4 perovskite structure is located at 532 nm in the visible region. Interestingly, the optical absorption spectrum of the 2D (C6H5CH2NH3)2PbI4 perovskite structure enhanced under suitable strains. The highest optical absorption peak appears in 2D (C6H5CH2NH3)2PbI4 under a -2% strain, and its theoretical photoelectric conversion efficiency is 28.5%. More interestingly, the replacement of surface I atoms with Br is another ways to enhance the optical absorption spectrum of the 2D (C6H5CH2NH3)2PbI4 perovskite structure. The optical absorption peak blue-shifts to the high energy region, which has higher solar energy flux density than the low energy region. The good stability, tuneable band gap and excellent theoretical photoelectric conversion efficiency of the 2D (C6H5CH2NH3)2PbI4 perovskite structure make it a promising candidate for novel 2D hybrid perovskite based photoelectronic devices and solar cells.

Measurement of the 241Am(n,γ) cross section at the n_TOF facility at CERN

The neutron capture cross section of 241Am is an important quantity for nuclear energy production and fuel cycle scenarios. Several measurements have been performed in recent years with the aim to reduce existing uncertainties in evaluated data. Two previous measurements, performed at the 185 m flight-path station EAR1 of the neutron time-of-flight facility n_TOF at CERN, have permitted to substantially extend the resolved resonance region, but suffered in the near-thermal energy range from the unfavorable signal-to-background ratio resulting from the combination of the high radioactivity of 241Am and the rather low thermal neutron flux. The here presented 241Am(n,γ) measurement, performed with C6D6 liquid scintillator gamma detectors at the 20 m flight-path station EAR2 of the n_TOF facility, took advantage of the much higher neutron flux. The current status of the analysis of the data, focussed on the low-energy region, will be described here.

Cross-Section Measurements of the 11B(p,α)2α Reaction near the First Resonant Energy

In preparation for an experiment with a laser-generated intense proton beam at the Laser Fusion Research Center at Mianyang to investigate the 11B(p,α)2α reaction, we performed a measurement at very low proton energy between 140 keV and 172 keV using the high-voltage platform at the Institute of Modern Physics, Lanzhou. The aim of the experiment was to test the ability to use CR-39 track detectors for cross-section measurements and to remeasure the cross-section of this reaction close to the first resonance using the thick target approach. We obtained the cross-section σ = 45.6 ± 12.5 mb near 156 keV. Our result confirms the feasibility of CR-39 type track detector for nuclear reaction measurement also in low-energy regions.

Spin-polarized transport properties in diluted-magnetic-semiconductor/semiconductor superlattices under light-field assisted

Based on the single electron effective mass approximation theory and the transfer-matrix method, the spin polarized transport properties of electrons in a diluted-magnetic-semiconductor/semiconductor superlattice are studied. The influence of a light-field and a magnetic-field on spin polarized transport and the tunneling time in the superlattice structure are discussed in more detail. The results show that, due to the sp-d electron interaction between conduction band electrons and doped Mn ions, giant Zeeman splitting occurs. It is shown that a significant spin-dependent transmission and the position and width of the resonant-transmission-band of spin-dependent electron can be manipulated by adjusting the magnetic- and light-field. Considering the light field irradiation, the resonance band of electron is deformed and broadened with the increase of the light field intensity. For the case of a strong magnetic field, the transmission coefficient (TC) in the low-energy region is almost zero when the light field is not added, but with the increase of light intensity, the TC increased significantly in the zone increases significantly, that is, a quasi-bound band appears. These features are due to the energy exchange between electrons and the light field when electrons tunnel through the superlattice structure under light irradiation. In addition, light and magnetic fields can significantly change the spin polarization of electrons. Under a certain magnetic field intensity (<i>B</i> = 2 T), the light field significantly changes the spin polarization of electrons, the main effect is that the width of the spin polarization platform narrows and oscillatory peaks are accompanied on both sides of the platform. This effect is strengthened with the increase of the light field intensity. However, when the magnetic field is stronger (<i>B</i> = 5 T), the opposite is true. These show that the spin polarization can be modulated by the light field. Finally, the tunneling time of spin-up and spin-down electrons is studied by the evolution of Gaussian wave packets in the structure. The results show that the tunneling time depends on a spin of electrons, and it can be seen that the tunneling time of the spin-down electron is shorter than that of the spin-up electron in the superlattice structure. These remarkable properties of spin polarized transport may be beneficial for the devising tunable spin filtering devices based on diluted magnetic semiconductor/semiconductor superlattice structure.

Measurement of Cosmic-ray Energy Spectrum with the TALE Detector in Hybrid Mode

The TA Low-energy Extension (TALE) experiment extends the energy range observed by the TA experiment to below 1016 eV. We aim to study the transition from galactic to extragalactic cosmic rays. The TALE detector is a hybrid apparatus composed of fluorescence telescopes and surface detectors. The surface detectors are arranged to be suitable for hybrid energy spectrum measurements in the low-energy region. We measured the energy spectrum with the 392 hours observation data of the TALE hybrid detector. This energy spectrum measurement will play an important role in understanding the transition from cosmic rays of galactic origin to those of extragalactic origin.

Interpreting the cosmic ray spectrum and composition measurements across the ankle and up to the highest energies with the data of the Pierre Auger Observatory

In this work we present a combined fit of the energy spectrum and mass composition data above 6×1017 eV as measured by the Pierre Auger Collaboration and we show that our data can be reproduced by the superposition of two (or more) components, either Galactic or extragalactic. We use a simple astrophysical model with two extragalactic components, one dominant at high energy and the other at low energy, whose superposition generates the so-called “ankle” feature. We find that the component dominating the low (high) energy region is required to be emitted from the sources with a very soft (hard) spectrum. In our best fit scenario the sources contributing above the ankle emit a mixed and increasingly heavier mass composition, whereas a mix of protons and intermediate-mass nuclei is observed below the ankle, which reach the Earth almost una↵ected by the propagation in the intergalactic space. The possible presence of a Galactic population providing the intermediate-mass contribution at low energy is also explored. In order to evaluate our capability to constrain astrophysical models, we finally discuss the impact on the fit results of the main experimental systematic uncertainties and of the assumptions on the quantities a↵ecting propagation, both in the atmosphere and in the intergalactic medium.

Density Functional Theory Calculation of N–H2S Hydrate

This study establishes a high-symmetry water cage structure model (structure N) based on the density functional theory first-principles technique. Three high-symmetry H2S hydrates (N–H2S hydrate) are examined in terms of their geometric structures, electrical characteristics, and optical characteristics. The structure of three N–H2S hydrates is most stable when H2S is in the center of water cage with small hydrogen bonds, and the molecular orientation of H2S has little effect on the structure of hydrate. The energy band of H2S enters forbidden band, resulting in the band gap of N–H2S hydrate being much less than that of water cage. A formation energy of −0.128 eV is reached when H2S DOS shift to the higher energy zone and the water cage DOS shift more to the lower energy region. The three N–H2S hydrates also possess various reflectivity characteristic peaks in the energy range of 5–12.5 eV, which might be exploited for electromagnetic detection of N–H2S hydrates. The findings of this research may serve as a theoretical foundation for related experiments.

Optical properties of multi-layers structured PEO/PVP solid polymer membranes doped with sodium perchlorate

In this present research work, fabrication of multi-layers structured (MLS) polymer membranes has been proposed to understand the variation of the optical bandgap and determine the types of electronic transitions as a function of sodium salt doping concentration. Solid MLS polymer membranes based on PEO/PVP have been prepared by the well-known solution cast technique. The recorded absorption, transmittance and reflectance spectra revealed that, incident energy absorption increased upon increase of salt dopant concentration and a shift of absorption edge towards lower energy region suggests the good complexation between the host polymers and dissociated salt (Na+, ClO4-) ions, which in turn the energy bandgap decrement expected. A detailed investigation has been carried out on the variation of refractive index of all the MLS polymer membranes as a function of salt doping concentration. The miscibility between the sodium salt and the individual polymers present in MLS polymer membranes can be understood from the linear relationship between the refractive index and the volume fraction of the added salt. The increase of extinction coefficient at high wavelengths was observed. The optical band gap measured from the plots of (αhυ)x versus photon energy (hυ) was compared to that determined from the optical dielectric loss.

Environmental Sound Classification With Low-Complexity Convolutional Neural Network Empowered by Sparse Salient Region Pooling

Environmental Sound Classification (ESC) is an important field in a broad range of applications, such as smart cities, audio surveillance, and health care. Recently, Convolutional Neural Networks (CNNs) have taken the lead from traditional approaches and have produced promising results. However, the achieved improvements are often accompanied by increasing depth, complexity, and size of the network, which prevents their usage in many practical applications. In this work, our goal is to empower a small-size low-complexity CNN model to achieve superior performance. To this end, we concentrate on the importance of global pooling technique, which is less investigated in ESC. In most previous works, models utilize global average pooling layer which does not consider regional saliency, and thus weakens the salient time-frequency regions contributions to the classification, and also to the training of convolutional kernels. We propose a novel global pooling method, called Sparse Salient Region Pooling (SSRP), which computes the channel descriptors using a sparse subset of features, and guides the model to effectively learn from the more salient time-frequency regions. Experimental results demonstrate that the proposed model with only 700K parameters yields accuracies of 86.7% on ESC-50 and 94.8% on ESC-10, which are comparable to that of the state-of-the-art methods. Compared to the baseline model, our model achieves absolute improvement of 21.8% in accuracy on ESC-50, with 98% smaller model size. Our visual analyses show that SSRP intensifies the responses of low-energy regions such that they contribute even more than high-energy regions to the classification of specific sound classes.

In English

The hexagon-shape graphene nanoflakes (GNFs) limited by zigzag edges only (with doubly and triply coordinated atoms) have unique increased reactivity. Despite the high systems symmetry (D6h) the Carbon atoms in GNFs occupy non-equivalent positions. Can such physical and chemical characteristics of GNFs, which depend of the atom position in the cluster, definition? This characteristic together with the simplicity of its calculation makes it possible to predict the properties of nanoflakes obtained from GNFs by introducing single and multiatomic vacancies into them or by replacing Carbon atoms with electron withdrawing and electron donating atoms. This characteristic includes the C1s core-level binding energy shifts, the maxima of which characterize the C atoms of a certain type. The proposed work is devoted to quantum chemical calculations of the electronic density of states (DOS) of pristine hexagon-shape GNF C96 (multiplicity, M=5), their saturated counterpart –polycyclic aromatic hydrocarbon(PAH) C96H24 (M=1) and their derivatives with one and two single vacancies in the ground electronic state (GES). All calculations were performed using the density functional theory (DFT) method with the involvement of the valence-split basis set 6-31G (d,p). Systems with open shells were considered using the UB3LYP exchange-correlation functional. The obtained spectra were fitted using Gaussian curve fitting program to determine the binding energy for each peak. The Gaussian function distribution of the theoretically calculated C1s core-level binding energy shifts of GNFs testified the presence of six peaks, each of which refers to a certain type of Carbon atoms. The C1s peak with the highest binding energy (-285.57 eV) is caused by contributions from the doubly coordinated edge cyclic chain (ECC) Carbon atoms. The C1s orbitals of the central hexagon (CHex) atoms and the first cyclic chain (FCC) atoms form delocalized molecular orbitals (MOs) in different parts of the cluster. The analogous spectrum of PAH C96H24 is slightly shifted to the region of lower binding energies and contains only two well-defined peaks. The peak with a higher binding energy (-284.36 eV) is generated by the 1s states of the CHex atoms and the atoms of the FCC, which are bounded to the CHex atoms. The electronic DOS difference in C1s core-level spectra of GNF C96 (M=5) and their saturated counterpart PAH C96H24 is established due to the presence of two weakly bounded π-systems in GNF and common conjugated system in PAH. The electronic DOS of defect-containing cluster C96-1(1) (M=3) (one CHex atom has been removed from the C96nanoflake) is generated by the C1s core-level atoms of the second cyclic chain (SCC), which are located at the different distances from the center of the nanoflake. The peak of the lowest intensity (-284.63 eV) appears in the spectrum as a reflection of the appearance of doubly coordinated Carbon atoms surrounding the single vacancy in the C96-1(1) nanoflake. The analysis of the electronic DOS of the C1s core-level spectrum of the C96-2(1) nanoflakeis shown, that doubly coordinated Carbon atoms, concentrated around two single vacancies, are essentially non-equivalent. If the MO with the lowest binding energy is localized on two of them – the MO with the highest binding energy is localized on the third atoms (one around each single vacancy). The electronic C1s core-level DOS spectrum of defect-containing molecular systems with one C96-1(1)H24 and two C96‑2(1)H24 single vacancies are similar to the analogous spectrum of PAH C96H24. In the first of them – one additional maximum appears due to C1s atoms surrounding the single vacancy. In the second – there are two additional maxima, each of which is generated by C1s core-level atoms adjacent to individual vacancies.

Design of a Two-Region Arc Plasma ion source for Cs-free H-

The Korea Atomic Energy Research Institute (KAERI) is developing a new concept of cesium-free negative hydrogen (deuterium) ion source based on a Two-Region Arc Plasma (TRAP) ion source for a neutral beam injection (NBI) system in a fusion tokamak. The plasma generator of the ion source consists of magnetic cusp field bucket as an anode and filaments as a cathode. The main feature of the TRAP ion source is that the generated plasma has two regions (a high-energy electron region and a low-energy electron region) without an external magnetic filter field. This plasma is generated by optimization of the filament electron emission and magnetic cusp field. The plasma generator of the TRAP ion source has been designed to study the effects of two-region plasma generation and the resulting negative ion generation. An extractor (also called the pre-accelerator) was also designed. The extractor consists of four grids (G1, G2, G3, G4) and permanent magnets are employed to suppress co-extracted electron currents. The design of the TRAP ion source is finished and fabrications are currently in progress. This study discusses the main considerations and simulation results for the design of the TRAP ion source.