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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 685 records · Page 38
All sourcesNASA NTRSDOE OSTI
DOE OSTI2011

High-pressure Study of Silane to 150 GPa

We present an extensive study of the optical, electronic, and structural properties of silane (SiH{sub 4}) to 150 GPa through the use of Raman spectroscopy, optical microscopy, synchrotron infrared reflectivity, optical absorption, and synchrotron x-ray diffraction measurements. To mitigate possible contamination from previously reported metal hydride formation, we performed experiments using gold-lined sample gaskets, finding molecular silane remains in the transparent and insulating P2{sub 1}/c structure until {approx}40 GPa. Silane shows a partial loss of crystallinity above {approx}50 GPa and appears to visibly darken. The darkening is plausibly the result of a loss of molecular character with many enthalpically competitive pathways available, including decomposition, combined with the absorptive nature of the sample. Above 100 GPa we observed crystallization into structures partially consistent with the previously reported nonmolecular I4{sup -} 2d and I4{sub 1}/a types. In the absence of decomposition, silane remains partially transparent and nonmetallic to at least 150 GPa with a band gap constrained between 0.6 and 1.8 eV. Under pressure, silane is sensitive to irradiation from x-rays and lasers, and may easily decompose into metallic silicon. We suggest that previous reports of metallization starting from molecular SiH{sub 4} arise from decomposition, and superconductivity may originate from hydrogen-doped silicon. While silane may readily decompose, the inherent metastability provides access to a wide range of path- and sample-history-dependent states and suggests a unique range of physical properties for hydrogen-rich silicon alloys.

36 MATERIALS SCIENCE↗
DOE OSTI2011

Search for a 2-quasiparticle high-K isomer in {sup 256}Rf

The energies of 2-quasiparticle (2-qp) states in heavy shell-stabilized nuclei provide information on the single-particle states that are responsible for the stability of superheavy nuclei. We have calculated the energies of 2-qp states in {sup 256}Rf, which suggest that a long-lived, low-energy 8{sup -} isomer should exist. A search was conducted for this isomer through a calorimetric conversion electron signal, sandwiched in time between implantation of a {sup 256}Rf nucleus and its fission decay, all within the same pixel of a double-sided Si strip detector. A 17(5)-{mu}s isomer was identified. However, its low population, {approx}5(2)% that of the ground state instead of the expected {approx}30%, suggests that it is more likely a 4-qp isomer. Possible reasons for the absence of an electromagnetic signature of a 2-qp isomer decay are discussed. These include the favored possibility that the isomer decays by fission, with a half-life indistinguishably close to that of the ground state. Another possibility, that there is no 2-qp isomer at all, would imply an abrupt termination of axially symmetric deformed shapes at Z=104, which describes nuclei with Z=92-103 very well.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2015

High-spin terminating states in the N = 88 Ho 155 and Er 156 isotones

Here, the 124 Sn( 37 Cl,6nγ) fusion-evaporation reaction at a bombarding energy of 180 MeV has been used to significantly extend the excitation level scheme of 155 67 Ho 88 . The collective rotational behavior of this nucleus breaks down above spin I ~ 30 and a fully aligned noncollective (band terminating) state has been identified at I π = 79/2 – . Comparison with cranked Nilsson–Strutinsky calculations also provides evidence for core-excited noncollective states at I π = 87/2 – and (89/2+) involving particle-hole excitations across the Z = 64 shell gap. A similar core-excited state in 156 68 Er 88 at I π = (46 + ) is also presented.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2015

Persistence of collective behavior at high spin in the N = 88 nucleus Tb 153

Excited states in the N = 88 nucleus 153 Tb were observed up to spin ~40 in an experiment utilizing the Gammasphere array. The 153 Tb states were populated in a weak α4n evaporation channel of the 37 Cl + 124 Sn reaction. Two previously known sequences were extended to higher spins, and a new decoupled structure was identified. The πh 11/2 band was observed in the spin region where other N = 88 isotopes exhibit effects of prolate to oblate shape changes leading to band termination along the yrast line, whereas 153 Tb displays a persistent collective behavior. Furthermore, minor perturbations of the very highest state in both signatures of this h 11/2 band are observed, which perhaps signal the start of the transition towards band termination.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2011

Emittance-Exchange-Based High Harmonic Generation Scheme for a Short-Wavelength Free Electron Laser

Generation of short-wavelength radiation by a free-electron laser using up-frequency conversion of an electron bunch density modulation is currently an area of active research. We propose a new scheme for producing the longitudinal electron bunch density modulation similar to the recently proposed echo-enabled harmonic generation but based on an emittance exchange beam line and a multislit mask. Beam line analysis and start-to-end simulation are presented.

43 PARTICLE ACCELERATORS↗