Synthesis of morphology-controlled silver nanostructures by electrodeposition

Nanostructured silver was obtained by potentiostatic electrolysis.The effects of ionized surfactant(sodium dodecanesulphonate)and the substrate(Cu and Ti)on the morphology of depositions were investigated.It is found that morphologies of silver nanostructures can be simply controlled via change of the substrate.Spherical Ag nanoparticles with narrow size distribution were obtained by electrodeposition in Ag NO3-SDS aqueous solution on copper substrate.In the case of titanium substrate,silver dendrite structures were obtained.Despite of different morphologies,XRD and TEM results showed that the as-prepared samples belong to face-centered cubic silver structure with good crystallinity.The formation mechanism of different silver nanostructures was discussed.

Enhanced oxidation resistance of Ce by addition of Ga and nanocrystallization

Ce is prone to catastrophic oxidation at room temperature and its oxidation resistance is difficult to be improved by alloying.Herein,we found that the oxidation resistance of active metal Ce can be significantly improved by the addition of 20 at.%Ga.Focused ion beam lift-out technique and scanning transmission electron microscopy analysis disclosed that a discontinuous Ga-rich layer was generated beneath the oxide layer in the coarse-grained Ce-Ga alloy.The Ga-rich layer formed by selective oxidation of Ce acts as a diffusion barrier for Ce outward diffusion and ceases the O/M interfacial reaction when a critical concentration of Ga(75 at.%)is reached.After nanocrystallization,uniform distribution of Ga was achieved.After oxidation,a relatively continuous Ga-rich layer was formed which further enhanced the oxidation resistance.The introduction of noble elements combining with nanocrystallization may provide a novel strategy for the protection of metals with high activity and poor oxidation resistance.

Flux variations of cosmic ray air showers detected by LHAASO-KM2A during a thunderstorm on June 10,2021

The Large High Altitude Air Shower Observatory(LHAASO)has three sub-arrays,KM2A,WCDA,and WFCTA.The flux variations of cosmic ray air showers were studied by analyzing the KM2A data during a thunderstorm on June 10,2021.The number of shower events that meet the trigger conditions increases significantly in atmospheric electric fields,with a maximum fractional increase of 20%.The variations in trigger rates(increases or decreases)were found to be strongly dependent on the primary zenith angle.The flux of secondary particles increased significantly,following a trend similar to that of shower events.To better understand the observed behavior,Monte Carlo simulations were performed with CORSIKA and G4KM2A(a code based on GEANT4).We found that the experimental data(in saturated negative fields)were in good agreement with the simulations,assuming the presence of a uniform electric field of-700 V/cm with a thickness of 1500 m in the atmosphere above the observation level.Due to the acceleration/deceleration by the atmospheric electric field,the number of secondary particles with energy above the detector threshold was modified,resulting in the changes in shower detection rate.

Colletotrichum species associated with cultivated citrus in China

There have been considerable advances in the understanding of species concepts in the genus Colletotrichum.This has lead to the need to carry out fresh surveys of Colletotrichum species associated with important hosts.Colletotrichum species are associated with Citrus plants as saprobes,important pre-harvest and post-harvest pathogens,as well as endophytes.In this study,a total of 312 Colletotrichum strains were isolated from leaves,shoots and fruits of cultivated Citrus and Fortunella species with or without disease symptoms across the main citrus production areas in China.The morphology of all strains were studied and multilocus(ACT,TUB2,CAL,GAPDH,GS,ITS)phylogeny established.Strains were from four important species complexes of Colletotrichum,namely C.gloeosporioides species complex,C.boninense species complex,C.acutatum species complex and a final group including C.truncatum,which was rare on Citrus species.The species belonging to the C.gloeosporioides species complex comprised C.gloeos porioides and C.fructicola,the C.boninense complex comprised C.karstii and a new species C.citricola and the C.acutatum complex included a new species,C.citri.The ability of strains to cause anthracnose on citrus fruits was tested by inoculation and strains of Colletotrichum gloeosporioides,C.fructicola and C.truncatum were pathogenic.

Observation of the Crab Nebula with LHAASO-KM2A−a performance study

A sub-array of the Large High Altitude Air Shower Observatory(LHAASO),KM2A is mainly designed to observe a large fraction of the northern sky to hunt for γ-ray sources at energies above 10 TeV.Even though the detector construction is still underway,half of the KM2A array has been operating stably since the end of 2019.In this paper,we present the KM2A data analysis pipeline and the first observation of the Crab Nebula,a standard candle in very high energy γ-ray astronomy.We detect γ-ray signals from the Crab Nebula in both energy ranges of 10-100 TeV and>100 TeV with high significance,by analyzing the KM2A data of 136 live days between December 2019 and May 2020.With the observations,we test the detector performance,including angular resolution,pointing accuracy and cosmic-ray background rejection power.The energy spectrum of the Crab Nebula in the energy range 10-250 TeV fits well with a single power-law function dN/dE=(1.13±0.05stat±0.08sys)×10^(-14).(E/20 TeV)-309±0.06stat±0.02syscm^(-2) s^(-1) TeV^(-1).It is consistent with previous measurements by other experiments.This opens a new window of γ-ray astronomy above 0.1 PeV through which new ultrahigh-energy γ-ray phenomena,such as cosmic PeVatrons,might be discovered.

Geometrical reconstruction of fluorescence events observed by the LHAASO experiment

The LHAASO-WFCTA experiment,which aims to observe cosmic rays in the sub-EeV range using the fluorescence technique,uses a new generation of high-performance telescopes.To ensure that the experiment has ex-cellent detection capability associated with the measurement of the energy spectrum,the primary composition of cosmic rays,and so on,an accurate geometrical reconstruction of air-shower events is fundamental.This paper de-scribes the development and testing of geometrical reconstruction for stereo viewed events using the WFCTA(Wide Field of view Cherenkov/Fluorescence Telescope Array)detectors.Two approaches,which take full advantage ofthe WFCTA detectors.are investigated.One is the stereo-angular method,which uses the pointing of triggered SiPMs in the shower trajectory,and the other is the stereo-timing method,which uses the triggering time of the fired SiPMs.The results show that both methods have good geometrical resolution:the resolution of the stereo-timing method is slightly better than the stereo-angular method because the resolution of the latter is slightly limited by the shower track length.

A numerical study on the influence of grain boundary oxides on dwell fatigue crack growth of a nickel-based superalloy

A theoretical treatment on the oxide-controlled dwell fatigue crack growth of aγ'strengthened nickelbased superalloys is presented.In particular,this study investigates the influence of an externally applied load and variations in theγ'dispersion on the grain boundary oxide growth kinetics.A dislocation-based viscoplastic constitutive description for high temperature deformation is used to simulate the stress state evolution in the vicinity of a crack at elevated temperature.The viscoplastic model explicitly accounts for multimodalγ'particle size distributions.A multicomponent mass transport formulation is used to simulate the formation/evolution of an oxide wedge ahead of the crack tip,where stress-assisted vacancy diffusion is assumed to operate.The resulting set of constitutive and mass transport equations have been implemented within a finite element scheme.Comparison of predicted compositional fields across the matrix/oxide interface are compared with experiments and shown to be in good agreement.Simulations indicate that the presence of a fineγ'size distribution has a strong influence on the predicted ow stress of the material and consequently on the relaxation in the vicinity of the crack-tip/oxide wedge.It is shown that a unimodal dispersion leads to reduced oxide growth rates(parabolic behavior)when compared to a bimodal one.Stability conditions for oxide formation are investigated and is associated with the prediction of compressive stresses within the oxide layer just ahead of the crack tip,which become progressively negative as the oxide wedge develops.However,mechanical equilibrium requirements induce tensile stresses at the tip of the oxide wedge,where failure of the oxide is predicted.The time taken to reach this critical stress for oxide failure has been calculated,from which dwell crack growth rates are computationally derived.The predicted rates are shown to be in good agreement with available experimental data.

Performance of LHAASO-WCDA and observation of the Crab Nebula as a standard candle

The first Water Cherenkov detector of the LHAASO experiment(WCDA-1)has been operating since April 2019.The data for the first year have been analyzed to test its performance by observing the Crab Nebula as a standard candle.The WCDA-1 achieves a sensitivity of 65 mCU per year,with a statistical threshold of 5 cr.To accomplish this,a 97.7%cosmic-ray background rejection rate around 1 TeV and 99.8%around 6 TeV with an ap proximate photon acceptance of 50%is achieved after applying an algorithm to separate gamma-induced showers.The angular resolution is measured using the Crab Nebula as a point source to be approximately 0.45°at 1 TeV and better than 0.2°above 6 TeV,with a pointing accuracy better than 0.05°.These values all match the design specifications.The energy resolution is found to be 33%for gamma rays around 6 TeV.The spectral energy distribution of the Crab Nebula in the range from 500 GeV to 15.8 TeV is measured and found to be in agreement with the results from other TeV gamma ray observatories.

A dynamic range extension system for LHAASOWCDA-1

Purpose The main scientific goal of LHAASO-WCDA is to survey gamma-ray sources with energy from 100 GeV to 30 TeV.To observe high-energy shower events,especially to measure the energy spectrum of cosmic rays from 100 TeV to 10 PeV,a dynamic range extension system(WCDA++)is designed to use a 1.5-inch PMT with a dynamic range of four orders of magnitude for each cell in WCDA-1.Method The dynamic range is extended by using these PMTs to measure the effective charge density in the core region of air shower events,which is an important parameter for identifying the composition of primary particles.Result and Conclusion The system has been running for more than one year.In this paper,the details of the design and performance of WCDA++are presented.

Line-of-shower trigger method to lower energy threshold for GRB detection using LHAASO-WCDA

Purpose Observation of high energy and very high emission from Gamma Ray Bursts(GRBs)is crucial to study the gigantic explosion and the underline processes.With a large field-of-view and almost full duty cycle,the Water Cherenkov Detector Array(WCDA),a sub-array of the Large High Altitude Air Shower Observatory(LHAASO),is appropriate to monitor the very high energy emission from unpredictable transients such as GRBs.Method Nevertheless,the main issue for an extensive air shower array is the high energy threshold which limits the horizon of the detector.To address this issue a new trigger method is developed in this article to lower the energy threshold of WCDA for GRB observation.Result The proposed method significantly improves the detection efficiency of WCDA for gamma-rays around the GRB direction at 10-300 GeV.The sensitivity of the WCDA for GRB detection with the new trigger method is estimated.The achieved sensitivity of the quarter WCDA array above 10 GeV is comparable with that of Fermi-LAT.The data analysis process and corresponding fluence upper limit for GRB 190719C is presented as an example.

Reconstruction of Cherenkov image bymultiple telescopes of LHAASO-WFCTA

Introduction One of main scientific goals of the Large High Altitude Air Shower Observatory(LHAASO)is to accurately measure the energy spectra of different cosmic ray compositions around the‘knee’region.The Wide Field-of-View(FoV)Cherenkov Telescope Array(WFCTA),which is one of the main detectors of LHAASO and has 18 telescopes,is built to achieve this goal.Multiple telescopes are put together and point to connected directions for a larger FoV.Method Telescopes are deployed spatially as close as possible,but due to their own size,the distance between two adjacent telescopes is about 10 m.Therefore,the Cherenkov lateral distribution and the parallax between the two telescopes should be considered in the event building process for images crossing over the boundaries of FoVs of the telescopes.An event building method for Cherenkov images measured by multiple telescopes of WFCTA is developed.The performance of the shower measurements using the combined images is evaluated by comparing with showers that are fully contained by a virtual telescope in simulation.Results and conclusion It is proved that the developed event building process can help to increase the FoV of WFCTA by 30%while maintaining the same reconstruction quality,compared to the separate telescope reconstruction method.