High-resolution Laboratory Measurements of K-shell X-Ray Line Polarization and Excitation Cross Sections in Helium-like S XV Ions

• Published in: The Astrophysical journal Vol. 914; no. 1; pp. 34 - 46
• Main Authors: Shah, Chintan, Hell, Natalie, Hubbard, Antonia, Gu, Ming Feng, MacDonald, Michael J., Eckart, Megan E., Kelley, Richard L., Kilbourne, Caroline A., Leutenegger, Maurice A., Porter, F. Scott, Brown, Gregory V.
• Format: Journal Article
• Language: English
• Published: Goddard Space Flight Center The American Astronomical Society 11.06.2021; American Astronomical Society; IOP Publishing; Institute of Physics (IOP)
• Subjects: 70 PLASMA PHYSICS AND FUSION TECHNOLOGY; Absorption cross sections; ASTRONOMY AND ASTROPHYSICS; Astrophysics; ATOMIC AND MOLECULAR PHYSICS; atomic data; Atomic data benchmarking; Atomic physics; atomic plasmas; atomic processes; Beta decay; Crystal structure; Electron beams; Electron capture; Electron impact excitation; Emission; Excitation; Experimental techniques; Helium; Ionization; Laboratory astrophysics; laboratory methods; line formation; Linear polarization; Plasma physics; Polarization; Spectrometers; Theoretical models; X-ray astronomy; Atomic Data Benchmarking; Laboratory Astrophysics; Experimental Techniques; X-Ray Astronomy; Atomic Data; Atomic Physics; Plasmas; Plasma Physics; Line: Formation; Atomic Processes; Theoretical Models
• ISSN: 0004-637X; 1538-4357
• DOI: 10.3847/1538-4357/abf1ea

We report measurements of electron-impact excitation cross sections for the strong K-shell n = 2 -> 1 transitions in S XV, using the LLNL EBIT-I electron beam ion trap, two crystal spectrometers, and the EBIT Calorimeter Spectrometer. The cross sections are determined by direct normalization to the well-known cross sections of radiative electron capture, measured simultaneously. Using contemporaneous polarization measurements with the two crystal spectrometers, whose dispersion planes are oriented parallel and perpendicular to the electron beam direction, the polarization of the direct excitation line emission is determined, and in turn the isotropic total cross sections are extracted. We further experimentally investigate various line-formation mechanisms, finding that radiative cascades and collisional inner-shell ionization dominate the degree of linear polarization and total line emission cross sections of the forbidden line, z.