Polarization of x-ray emission lines from heliumlike scandium as a probe of the hyperfine interaction

Abstract

We report the first measurements of the polarization of x rays emitted from bound-bound transitions in a highly ionized He-like ion. Polarization was measured for the decay to the ground state, Is 2 'So, of the He-like Sc 19+ levels \slp l P\> \s2p 3 Pi, ls2p 3 Pi, and \s2s 3 S\. The measurements were made with the Electron Beam Ion Trap at two electron-beam energies: 4.36 and 5.62 keV. Polarization of two of the lines is strongly influenced by the hyperfine interaction with the Sc nucleus, demonstrating that polarization measurements can be used to investigate hyperfine interactions in highly ionized atoms.PACS numbers: 32.30. Rj, 34.80.Kw Evidence for the hyperfine interaction in He-like ions has been seen in beam-foil spectroscopy, where hyperfine effects were found 1 to explain the measured lifetime of the \s2p 3 P2 level in V 21+ . The hyperfine interaction can mix "pure" atomic levels, changing the level lifetimes and even allowing decay by an otherwise forbidden transition. The hyperfine interaction also effects the populations of magnetic sublevels, thus affecting the polarization of the radiation emitted when a state decays. Measurements of the polarization of line emission can be used to test the theory of the hyperfine interaction, as well as provide information relevant to the understanding of solar flares and tokamaks, 2 where nonthermal electron distributions can produce polarized radiation.We have measured, for the first time, the polarization of electric dipole (E\), magnetic dipole (Ml), and magnetic quadrupole (M2) emission lines emitted from a highly ionized He-like ion. The particular emission lines that were observed are the £1 resonance line 1J 21 SO\s2p ] P\, the M2 quadrupole line Is 2 l S 0 -ls2p 3 /> 2 , the E\ intercombination line Is 2 x S$-\s2p 3 P\, and the Ml line Is 2l So-ls2s 3 S\ in Sc 19+ . These transitions are known as w, x, j>, and z, respectively, 3 and are important for solar and tokamak plasma diagnostics. 4,5 The emission lines of Sc 19+ appear in the x-ray region near 2.9 A and were observed with a Bragg crystal spectrometer attached to the Lawrence Livermore National Laboratory Electron Beam Ion Trap (EBIT). Their energies 6 are shown on an energy-level diagram in Fig. 1.Sc 19+ was chosen because it was the He-like ion most suitable for polarization studies with our apparatus. The He-like Sc (Z=21) w = 2 to « = 1 lines fall at wavelengths 7 that correspond to Bragg angles of about 45° for the Ge(220) (2^=4.00 A) curved crystal installed in our Johann spectrometer. 8 Sc is monoisotopic with an / = y nuclear spin. The hyperfine interaction with the nuclear magnetic moment of Sc can influence the polarization of the atomic transitions.EBIT is described in detail elsewhere. 9,10 Briefly, ions are injected into a cylindrical electrostatic well approximately 2 cm long and 70 /zm in diameter. The ions are ionized to high charge states and collisionally excited by a tunable (in energy) electron beam. The electron beam is nearly monoenergetic with a width of 50 eV -1s2p 1 P, ...

Observation of Hyperfine Mixing in Measurements of a Magnetic OctupoleDecay in Isotopically Pure Nickel-LikeXe129andXe132Ions

“…The hyperfine levels of the ground state of atomic hydrogen give rise to the 21-cm line used in radioastronomy to image our galaxy and were the basis for the first maser (see [6]). Hyperfine line splitting is of importance in photopumped x-ray lasers [7], and hyperfine interaction affects the polarization of spectral lines used in plasma diagnostics [8]. Hyperfine interaction is sufficient to break symmetry in atoms and ions with a J=0 excited level above a J=0 ground level (typically in two-electron ions), and this can turn a very long-lived level (for example, 1s2p 3 P o 0 in heliumlike ions) into a shorter one [9], in some cases drastically so [10].…”

Time-dependent X-ray polarization analysis for anisotropic distribution of hot electrons in ultrahigh intensity laser plasmas

“…Theoretical results: d, present results in He-like Cl ion; B, He-like Ti ions [22]. Experimental results: C, He-like Sc ion [8]; -, He-like Fe ion [9]; *, He-like Ti ion [10]; Â, Helike Si ion [11]. …”

Measurements of density dependent intensity ratios of extreme ultraviolet line emission from Fe X, XI, and XII

“…The measured intensity of the emitted radiation from these ions strongly depends on the angle of observation relative to the beam axis. There have been several studies reported in the literature for the polarization of radiation emitted from ions trapped in an EBIT (Henderson et al 1990;Beiersdorfer et al 1996;Taḱacs et al 1996;Gu et al 1999;Liang et al 2009a;Hu et al 2014b;Shah et al 2015). In the present study, we measured EUV spectra using CoBIT at 90…”

“…• with respect to the electron beam, the polarization of the emitted line radiation may therefore effect the resulting line intensities as discussed by (Liang et al 2009a;Beiersdorfer et al 1996;Henderson et al 1990). Liang et al (2009a) estimated polarization effect for the transitions in Fe VII−Fe XIV and found that for most of the observed lines, the contribution is less than 10% to the resultant intensity.…”