Search NASA⌕ Search

Engineering topics

Bessas, D.

Publications and source records attributed to Bessas, D..

8 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2018

Pressure-mediated structural transitions in bulk EuTiO 3

The high pressure structural phase diagram of EuTiO 3 is revisited by means of single crystal x-ray diffraction at pressures below 30GPa. We report a transition at about 3GPa with a surprisingly small lattice distortion. At 23GPa we have detected a yet unknown transition into a new orthorhombic phase (space group Ibam). Combining our observations on structures obtained on single crystals and polycrystalline samples with complementary methods, such as electrical resistivity and nuclear forward scattering of synchrotron radiation measured on polycrystalline samples, we address issues relevant to chemistry and disorder.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Rocking curve imaging of high quality sapphire crystals in backscattering geometry

Here, we report on the characterization of high quality sapphire single crystals suitable for high-resolution X-ray optics at high energy. Investigations using rocking curve imaging reveal the crystals to be of uniformly good quality at the level of ~10 -4 in lattice parameter variations, deltad/d. But, investigations using backscattering rocking curve imaging with lattice spacing resolution of deltad/d ~ 5.10 -8 shows very diverse quality maps for all crystals. Our results highlight nearly ideal areas with edge length of 0.2-0.5 mm in most crystals, but a comparison of the back re ection peak positions shows that even neighboring ideal areas exhibit a relative difference in the lattice parameters on the order of deltad/d = 10-20.10 -8 ; this is several times larger than the rocking curve width. Furthermore, the stress-strain analysis suggests that an extremely stringent limit on the strain at a level of ~100 kPa in the growth process is required in order to produce crystals with large areas of the quality required for X-ray optics at high energy.

36 MATERIALS SCIENCE↗
DOE OSTI2016

The sapphire backscattering monochromator at the Dynamics beamline P01 of PETRA III

Here, we report on a high resolution sapphire backscattering monochromator installed at the Dynamics beamline P01 of PETRA III. The device enables nuclear resonance scattering experiments on M ossbauer isotopes with transition energies between 20 and 60 keV with sub-meV to meV resolution. In a first performance test with 119 Sn nuclear resonance at a X-ray energy of 23.88 keV an energy resolution of 1.34 meV was achieved. Moreover, the device extends the field of nuclear resonance scattering at the PETRA III synchrotron light source to many further isotopes like 151 Eu, 149 Sm, 161 Dy, 125 Te and 121 Sb.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2014

Nuclear forward scattering of synchrotron radiation by 99 Ru

In this study, we measured nuclear forward scattering spectra utilizing the 99 Ru transition, 89.571(3) keV, with a notably mixed E2/M1 multipolarity. The extension of the standard evaluation routines to include mixed multipolarity allows us to extract electric and magnetic hyperfine interactions from 99 Ru-containing compounds. This paves the way for several other high-energy Mössbauer transitions, E~90 keV. Lastly, the high energy of such transitions allows for operando nuclear forward scattering studies in real devices.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗