The extension of the cosmic-ray spectrum beyond 1 petaelectronvolt (PeV; 10
electronvolts) indicates the existence of the so-called PeVatrons-cosmic-ray factories that accelerate particles to PeV ...energies. We need to locate and identify such objects to find the origin of Galactic cosmic rays
. The principal signature of both electron and proton PeVatrons is ultrahigh-energy (exceeding 100 TeV) γ radiation. Evidence of the presence of a proton PeVatron has been found in the Galactic Centre, according to the detection of a hard-spectrum radiation extending to 0.04 PeV (ref.
). Although γ-rays with energies slightly higher than 0.1 PeV have been reported from a few objects in the Galactic plane
, unbiased identification and in-depth exploration of PeVatrons requires detection of γ-rays with energies well above 0.1 PeV. Here we report the detection of more than 530 photons at energies above 100 teraelectronvolts and up to 1.4 PeV from 12 ultrahigh-energy γ-ray sources with a statistical significance greater than seven standard deviations. Despite having several potential counterparts in their proximity, including pulsar wind nebulae, supernova remnants and star-forming regions, the PeVatrons responsible for the ultrahigh-energy γ-rays have not yet been firmly localized and identified (except for the Crab Nebula), leaving open the origin of these extreme accelerators.
We investigate polarization-dependent ultrafast photocurrents in the Weyl semimetal TaAs using terahertz (THz) emission spectroscopy. Our results reveal that highly directional, transient ...photocurrents are generated along the noncentrosymmetric c axis regardless of incident light polarization, while helicity-dependent photocurrents are excited within the ab plane. This is consistent with earlier static photocurrent experiments, and demonstrates on the basis of both the physical constraints imposed by symmetry and the temporal dynamics intrinsic to current generation and decay that optically induced photocurrents in TaAs are inherent to the underlying crystal symmetry of the transition metal monopnictide family of Weyl semimetals.
Novel worm-like Ag/ZnO core–shell heterostructural composites were fabricated using a two-step chemical method. As-prepared silver nanowires were soaked in a solution of zinc acetate and ...triethanolamine to form worm-like Ag/ZnO core–shell composites under ultrasonic irradiation. Samples were characterized by field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), photoluminescence spectroscopy, and UV–vis spectrophotometer. The results show that the core–shell composites are composed of single-crystal Ag nanowires serving as the core, on which dense ZnO particles grow as the shell. The surface plasmon absorption band of Ag/ZnO composites is distinctly broadened and red shifted to monometallic Ag nanowires. The PL intensity of Ag/ZnO heterostructural composites varies and has the minimum intensity for the sample prepared with Ag of 2.8 atom %. Moreover, photocatalytic tests show that the Ag/ZnO composites exhibit higher photocatalytic activity compared to pure ZnO particles.
Some gamma-ray bursts (GRBs) have a tera-electron volt (TeV) afterglow, but the early onset of this has not been observed. We report observations with the Large High Altitude Air Shower Observatory ...(LHAASO) of the bright GRB 221009A, which serendipitously occurred within the instrument's field of view. More than 64,000 photons >0.2 TeV were detected within the first 3000 seconds. The TeV flux began several minutes after the GRB trigger and then rose to a peak ~10 seconds later. This was followed by a decay phase, which became more rapid ~650 seconds after the peak. We interpret the emission using a model of a relativistic jet with half-opening angle of ~0.8°. This is consistent with the core of a structured jet and could explain the high isotropic energy of this GRB.
Knowledge of the in-situ stress field in the rock mass at depth is important to provide stress boundary conditions for underground excavation design, stability evaluation, and the optimization of ...ground support systems. In the present study, in-situ stress measurement using hydraulic fracturing method is conducted in the Beishan area in Gansu Province, China, a preferred candidate site for China's high-level radioactive waste repository. Information on the stress magnitudes and orientations in three sub-areas is obtained from 136 measurement points in nine sub-vertical boreholes at depths ranging from 30 to 700m, and the distribution characteristics of the in-situ stress field is analyzed. The pressure–time records obtained from the test intervals with pre-existing fractures are discussed according to the acoustic borehole televiewer (BHTV) images. The measurement results indicate that the regional stress field is dominated by horizontal stress within most of the measurement depth ranges. A piecewise distribution of the principal stresses reveals approximately three stress domains with increasing depth. At shallow and medium depths, the stress fields are characterized by σH>σh>σv and σH>σv>σh, respectively, where σH, σh, and σv are maximum horizontal, minimum horizontal, and vertical stresses, respectively. With further increase of depth, the maximum principal stress changes from horizontal to vertical (i.e. σv>σH>σh). Fracture impression results reveal that the maximum horizontal stress is dominantly oriented in the NNE–NEE direction, which is in agreement with the orientation of regional tectonic stress field generated from movement of the Earth's tectonic plates. In addition, it is found that the orientations of σH are affected by the geological structures significantly.
•Geological features of the Beishan area are described.•The results of the hydro-fracturing in-situ stress measurements are presented.•The distribution characteristics of the in-situ stress filed are analyzed.•The regional stress field associated with the geological evolution is discussed.
A mobilized dilation angle model for rocks Zhao, X.G.; Cai, M.
International journal of rock mechanics and mining sciences (Oxford, England : 1997),
04/2010, Letnik:
47, Številka:
3
Journal Article
Recenzirano
Experimental and field observations of rock failure show that the failure process is closely associated with rock dilation, an indicator of volumetric increase during rock deformation. The most ...common concept used to describe dilation is the dilation angle. The conventional Mohr–Coulomb model considering strain-softening often makes an assumption of constant dilation, but it is observed that the approach is not successful in characterizing the nonlinear deformation behavior of rocks. In the present study, based on published data acquired from modified triaxial compression tests with volumetric strain measurement, a mobilized dilation angle model considering the influence of both confining stress and plastic shear strain is established. Based on the model response and in combination with the grain size description and the uniaxial compressive strength, the model parameters for four rock types (coarse-grained hard rock, medium-grained hard rock, fine-medium-grained soft rock, and fine-grained soft rock) are suggested. For coal and quartzite representing fine-grained soft rock, and coarse-grained hard rock, respectively, the dilation angle model is used to predict the volumetric-axial strain relationships, and the predictions are found to be in good agreement with experimental results.
•Crack initiation stress (σci) and crack damage stress (σcd) of the Beishan granite under uniaxial and triaxial compressions are determined.•3D distributions of AE events in the rock samples at ...different loading stages in the compression tests are obtained.•Methods of indentifying σci and σcd at different confinements are discussed.•Confinement-dependent damage stress characteristics are analyzed.
Rockburst is a sudden and violent failure of rocks and it often occurs in hard rocks in highly stressed ground. Strainburst is classified as one type of rockburst and it often occurs in rocks near or ...at the excavation boundary. Deep insight into the strainburst phenomenon is essential for safe underground construction at depth. In this paper, an experimental laboratory study on the strainburst behavior of Beishan granite is presented. Based on in-situ stress measurement data from the Beishan area in China, a series of tests under different unloading rates were performed to investigate the strainburst process using a true-triaxial strainburst test system which was equipped with an acoustic emission (AE) monitoring system. In addition, a high-speed video camera was used to record and visualize the initiation and ejection of rock fragments as well as the sudden dynamic failure (strainburst) of the test samples. AE characteristics associated with the cumulative energy and frequency–amplitude distributions were analyzed. Characteristics of the microscopic structure of a fragment generated from one test were observed using a scanning electron microscope. The experimental results indicate that the degree of violence during failure and the associated AE energy release in the strainburst process are dependent on the unloading rate. When the unloading rate is high, the rock is prone to strainburst. On the other hand, as the unloading rate decreases, the failure mode changes from strainburst to spalling. In addition, the cumulative AE energy is not sensitive to unloading rates greater than 0.05 MPa/s. When the unloading rate is less than 0.05 MPa/s, the cumulative AE energy shows a marked decreasing trend during rock failure.
Using femtosecond time-resolved x-ray diffraction, we investigated optically excited coherent acoustic phonons in the Weyl semimetal TaAs. The low symmetry of the (112) surface probed in our ...experiment enables the simultaneous excitation of longitudinal and shear acoustic modes, whose dispersion closely matches our simulations. We observed an asymmetry in the spectral line shape of the longitudinal mode that is notably absent from the shear mode, suggesting a time-dependent frequency chirp that is likely driven by photoinduced carrier diffusion. We argue on the basis of symmetry that these acoustic deformations can transiently alter the electronic structure near the Weyl points and support this with model calculations. Our study underscores the benefit of using off-axis crystal orientations when optically exciting acoustic deformations in topological semimetals, allowing one to transiently change their crystal and electronic structures.
PDF
