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RISS 인기검색어
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- 저자 : Marco Das (검색결과 3 건)
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Antarctic ice shelf potentially stabilized by export of meltwater in surface river
Bell, Robin E.,Chu, Winnie,Kingslake, Jonathan,Das, Indrani,Tedesco, Marco,Tinto, Kirsty J.,Zappa, Christopher J.,Frezzotti, Massimo,Boghosian, Alexandra,Lee, Won Sang Nature Publishing Group 2017 Nature Vol. No.
<P>Meltwater stored in ponds(1) and crevasses can weaken and fracture ice shelves, triggering their rapid disintegration(2). This ice-shelf collapse results in an increased flux of ice from adjacent glaciers(3) and ice streams, thereby raising sea level globally(4). However, surface rivers forming on ice shelves could potentially export stored meltwater and prevent its destructive effects. Here we present evidence for persistent active drainage networks-interconnected streams, ponds and rivers-on the Nansen Ice Shelf in Antarctica that export a large fraction of the ice shelf's meltwater into the ocean. We find that active drainage has exported water off the ice surface through waterfalls and dolines for more than a century. The surface river terminates in a 130-metre-wide waterfall that can export the entire annual surface melt over the course of seven days. During warmer melt seasons, these drainage networks adapt to changing environmental conditions by remaining active for longer and exporting more water. Similar networks are present on the ice shelf in front of Petermann Glacier, Greenland, but other systems, such as on the Larsen C and Amery Ice Shelves, retain surface water at present. The underlying reasons for export versus retention remain unclear. Nonetheless our results suggest that, in a future warming climate, surface rivers could export melt off the large ice shelves surrounding Antarctica-contrary to present Antarctic ice-sheet models(1), which assume that meltwater is stored on the ice surface where it triggers ice-shelf disintegration.</P>
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Koroidov, Sergey,Hong, Kiryong,Kjaer, Kasper S.,Li, Lin,Kunnus, Kristjan,Reinhard, Marco,Hartsock, Robert W.,Amit, Das,Eisenberg, Richard,Pemmaraju, C. Das,Gaffney, Kelly J.,Cordones, Amy A. American Chemical Society 2018 Inorganic Chemistry Vol.57 No.21
<P>The valence electronic structure of several square planar Ni-centered complexes, previously shown to catalyze the hydrogen evolution reaction, are characterized using S K-edge and Ni L-edge X-ray absorption spectroscopy and electronic structure calculations. Measurement of the atomic Ni 3d and S 3p contributions enables assessment of the metal-ligand covalency of the electron accepting valence orbitals and yields insight into the ligand-dependent reaction mechanisms proposed for the catalysts. The electron accepting orbital of the Ni(abt)<SUB>2</SUB> (abt = 2-aminobenzenethiolate) catalyst is found to have large ligand character (80%), with only 9% S 3p (per S) character, indicating delocalization over the entire abt ligand. Upon two proton-coupled reductions to form the Ni(abt-H)<SUB>2</SUB> intermediate, the catalyst stores 1.8 electrons on the abt ligand, and the ligand N atoms are protonated, thus supporting its role as an electron and proton reservoir. The electron accepting orbitals of the Ni(abt-H)<SUB>2</SUB> intermediate and Ni(mpo)<SUB>2</SUB> (mpo = 2-mercaptopyridyl-<I>N</I>-oxide) catalyst are found to have considerably larger Ni 3d (46-47%) and S 3p (17-18% per S) character, consistent with an orbital localized on the metal-ligand bonds. This finding supports the possibility of metal-based chemistry, resulting in Ni-H bond formation for the reduced Ni(abt-H)<SUB>2</SUB> intermediate and Ni(mpo)<SUB>2</SUB> catalyst, a critical reaction intermediate in H<SUB>2</SUB> generation.</P><P>The electronic structure of Ni-centered hydrogen evolution catalysts with noninnocent ligands was characterized using S and Ni edge X-ray absorption spectroscopies. The electron-accepting valence orbitals involved in catalysis are found to have high (>50%) ligand character. Ligand-dependent differences in electronic structure provide insight into the previously proposed reaction mechanisms of the catalysts.</P> [FIG OMISSION]</BR>
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Babs M.F. Hendriks,Roald S. Schnerr,Gianluca Milanese,Cécile R.L.P.N. Jeukens,Sandra Niesen,Nienke G. Eijsvoogel,Joachim E. Wildberger,Marco Das 대한영상의학회 2019 Korean Journal of Radiology Vol.20 No.2
Objective: To evaluate the radiation dose for pregnant women and fetuses undergoing commonly used computed tomography of the pulmonary arteries (CTPA) scan protocols and subsequently evaluate the simulated effect of an optimized scan length. Materials and Methods: A total of 120 CTPA datasets were acquired using four distinctive scan protocols, with 30 patients per protocol. These datasets were mapped to Cristy phantoms in order to simulate pregnancy and to assess the effect of an effective radiation dose (in mSv) in the first, second, or third trimester of pregnancy, including a simulation of fetal dose in second and third trimesters. The investigated scan protocols involved a 64-slice helical scan at 120 kVp, a high-pitch dual source acquisition at 100 kVp, a dual-energy acquisition at 80/140 kVp, and an automated-kV-selection, high pitch helical scan at a reference kV of 100 kVref. The effective dose for women and fetuses was simulated before and after scan length adaptation. The original images were interpreted before and after scan length adaptations to evaluate potentially missed diagnoses. Results: Large inter-scanner and inter-protocol variations were found; application of the latest technology decreased the dose for non-pregnant women by 69% (7.0–2.2 mSv). Individual scan length optimization proved safe and effective, decreasing the fetal dose by 76–83%. Nineteen (16%) cases of pulmonary embolism were diagnosed and, after scan length optimization, none were missed. Conclusion: Careful CTPA scan protocol selection and additional optimization of scan length may result in significant radiation dose reduction for a pregnant patient and her fetus, whilst maintaining diagnostic confidence.
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