ALBERT

Description: 〈span〉〈div〉ABSTRACT〈/div〉To ensure the accuracy of future seismological studies using horizontal‐component data recorded by broadband seismic stations in Africa and environs, we investigate the sensor orientation of 1075 stations belonging to 41 seismic networks deployed in and around the African continent in the past three decades. We applied three independent waveform‐based orientation estimation methods that involve the measurement of 〈span〉P〈/span〉‐wave particle motion based on the principal component analysis, minimizing the 〈span〉P〈/span〉‐wave energy on the transverse component of motion, and measuring intermediate‐period Rayleigh‐wave arrival angles from teleseismic earthquakes. We found that 34.3%–43.5% of the stations are well oriented within 3°, 40%–48.2% have sensor misorientation values between 3° and 10°, whereas 16.5%–18% of the stations are misaligned by more than 10°, most likely true sensor misorientation. The fairly high correlation coefficients (0.71–0.93) and very small mean (−0.01°–0.06°) and median (−0.04°–0.3°) differences suggest a high consistency among the estimates from the three methods. Likewise, the comparison of our results with reported orientations in the metadata at 33 stations demonstrates the robustness of the results obtained in this study. Likewise, the increase in the cross‐correlation coefficients and reduced time shifts between the Rayleigh‐wave signals on the vertical and Hilbert‐transformed radial components when the sensor misorientation angles are corrected show the importance of this study. An investigation of the time dependence of the estimated misorientation angles over the validation period reveals that the sensor orientation remained fairly constant for most stations included in the study. The nearly 180° sensor misorientation angles obtained at some stations led to the suspicion of possible polarity reversal of the seismometer components and/or channel mislabeling that was confirmed with a network manager for two of the seismic stations. Result of this study serves as a reference for future data users and a reminder to seismic network managers to decrease the number of errors that may lead to misorientations in future deployments.〈/span〉

Description: 〈span〉〈div〉ABSTRACT〈/div〉To ensure the accuracy of future seismological studies using horizontal‐component data recorded by broadband seismic stations in Africa and environs, we investigate the sensor orientation of 1075 stations belonging to 41 seismic networks deployed in and around the African continent in the past three decades. We applied three independent waveform‐based orientation estimation methods that involve the measurement of 〈span〉P〈/span〉‐wave particle motion based on the principal component analysis, minimizing the 〈span〉P〈/span〉‐wave energy on the transverse component of motion, and measuring intermediate‐period Rayleigh‐wave arrival angles from teleseismic earthquakes. We found that 34.3%–43.5% of the stations are well oriented within 3°, 40%–48.2% have sensor misorientation values between 3° and 10°, whereas 16.5%–18% of the stations are misaligned by more than 10°, most likely true sensor misorientation. The fairly high correlation coefficients (0.71–0.93) and very small mean (−0.01°–0.06°) and median (−0.04°–0.3°) differences suggest a high consistency among the estimates from the three methods. Likewise, the comparison of our results with reported orientations in the metadata at 33 stations demonstrates the robustness of the results obtained in this study. Likewise, the increase in the cross‐correlation coefficients and reduced time shifts between the Rayleigh‐wave signals on the vertical and Hilbert‐transformed radial components when the sensor misorientation angles are corrected show the importance of this study. An investigation of the time dependence of the estimated misorientation angles over the validation period reveals that the sensor orientation remained fairly constant for most stations included in the study. The nearly 180° sensor misorientation angles obtained at some stations led to the suspicion of possible polarity reversal of the seismometer components and/or channel mislabeling that was confirmed with a network manager for two of the seismic stations. Result of this study serves as a reference for future data users and a reminder to seismic network managers to decrease the number of errors that may lead to misorientations in future deployments.〈/span〉

Description: Nanowire growth on heteroepitaxial GaP/Si(111) by metalorganic vapor phase epitaxy requires the [-1-1-1] face, i.e., GaP(111) material with B-type polarity. Low-energy electron diffraction (LEED) allows us to identify the polarity of GaP grown on Si(111), since (2×2) and (1×1) surface reconstructions are associated with GaP(111)A and GaP(111)B, respectively. In dependence on the pre-growth treatment of the Si(111) substrates, we were able to control the polarity of the GaP buffers. GaP films grown on the H-terminated Si(111) surface exhibited A-type polarity, while GaP grown on Si surfaces terminated with arsenic exhibited a (1×1) LEED pattern, indicating B-type polarity. We obtained vertical GaAs nanowire growth on heteroepitaxial GaP with (1×1) surface reconstruction only, in agreement with growth experiments on homoepitaxially grown GaP(111).

Description: We have developed two techniques for time-resolved x-ray diffraction from bulk polycrystalline materials during dynamic loading. In the first technique, we synchronize a fast detector with loading of samples at strain rates of ∼10 3 –10 4 s −1 in a compression Kolsky bar (split Hopkinson pressure bar) apparatus to obtain in situ diffraction patterns with exposures as short as 70 ns. This approach employs moderate x-ray energies (10–20 keV) and is well suited to weakly absorbing materials such as magnesium alloys. The second technique is useful for more strongly absorbing materials, and uses high-energy x-rays (86 keV) and a fast shutter synchronized with the Kolsky bar to produce short (∼40 μs) pulses timed with the arrival of the strain pulse at the specimen, recording the diffraction pattern on a large-format amorphous silicon detector. For both techniques we present sample data demonstrating the ability of these techniques to characterize elastic strains and polycrystalline texture as a function of time during high-rate deformation.

Description: Low temperature radioluminescence and thermoluminescence spectra of ZnO track numerous changes produced by copper ion implantation into the surface layer. A significant, but unexpected, feature is that the bulk crystal becomes modified by the stress generated in the surface layer. This is reflected by the energy of intrinsic band gap emission. There are also differences in the spectra and peak temperatures of the thermoluminescence components, consistent with such a structural relaxation. The copper implant layer is both absorbing and reflective, so this introduces major distortions on the radioluminescence component from the bulk region, since the bulk luminescence signals are transmitted through, or reflected from, the implant layer. The temperature dependence of the spectra includes anomalies that are typical of changes driven by phase transitions of nanoparticle inclusions. Overall, the features of bulk relaxation, spectral distortion, and detection of nanoparticle inclusions are rarely considered for ion implanted luminescence studies, but the data suggest they are almost inevitable in a wide range of implanted materials.