Abstract
We have observed the emission spectra in collisions of bare oxygen ions with a helium gas target in the soft x-ray region with a window-less silicon drift detector at the collision energy range of 48–80 keV. The dominant soft x-ray emission corresponds to the 1s–2p transition of hydrogen-like oxygen O7+ produced by the single-electron charge exchange reaction. Other emission lines are the 1s–3p, 1s–4p and 1s–5p transitions of O7+, and also the 1s2–1s2p transition of O6+ produced by the true double-electron capture. The cascades from the upper states result in a large population of the 2p state, even though the direct capture into the 2p state is extremely scarcer than those into the 3p, 4p and 5p states.
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1. Introduction
In the 1990s, the soft x-ray emission whose intensity fluctuated in a cycle of a few days was observed during the all-sky survey by the ROSAT (Röntgensatellit, an x-ray observatory launched in 1990) [1]. It was found that this phenomenon is due to solar wind charge exchange (SWCX) [2]. SWCX means the electron capture of multiple charged ions constituting the solar wind in collisions with neutral matter within the heliosphere and has been regarded as a dominant mechanism of soft x-ray emission in the solar system [3]. In order to analyze the x-ray spectra observed with the observatory satellites quantitatively in detail, the x-ray emission cross sections of the SWCX processes are needed for astrophysics. Already some experiments have been performed for this purpose [4–7]. We also measured the emission spectra in collisions of hydrogen-like oxygen and nitrogen ions with a helium gas target at a collision energy around 100 keV in previous work [8]. In this work, we observed the charge exchange emission spectra in the collision of O8+ with the He gas target within the solar wind velocity region, namely 200–900 km s−1, which corresponds to 0.2–4.2 keV u−1 in collision energies.
2. Experiment
The bare oxygen ion O8+ was produced in a 14.25 GHz electron cyclotron resonance ion source with introduction of 18O2 gas into a plasma chamber. The ions, extracted from the plasma by an electric potential of 10 kV, were fed into a collision gas cell after the charge-state separation by a 110° double-focusing dipole magnet. We applied the high voltage to the collision cell in order to achieve the solar wind velocity of ions. The target gas pressure in the cell was kept lower than 1.3 × 10−3 Pa to avoid multiple collisions of each ion with the targets. The ion beam current had an average value of about 0.4 nA, which was measured by a Faraday cup located behind the collision chamber. The soft x-ray emission spectra were measured by a window-less silicon drift detector (SDD) at the magic angle, namely 54.7° from the ion-beam axis.
3. Results and discussion
The soft x-ray spectrum in collision of O8+ ions with He gas at a collision energy of 80 keV is shown in figure 1. The dominant peak at 654 eV corresponds to the 1s 2S–2p 2P transition of O7+ ions, and the small peaks of the 1s–3p (775 eV), 1s–4p (817 eV) and 1s–5p (837 eV) transitions of O7+ ions could be distinguished after the deconvolution using Gaussian functions with a full-width at half-maximum of 71 eV for each transition. Also the 1s2 1S0–1s2p 1P1 transition of O6+ ions at 574 eV, which is produced by two-electron capture, was found. This result shows a better separation of each transition than our previous experiment, because the SDD has a better energy resolution of about 120 eV than the Si(Li) detector.
Figure 1. Soft x-ray emission spectrum measured with the SDD in the collision of O8+ ions with He gas at a collision energy of 80 keV.
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Standard image High-resolution imageWe have also observed the spectra at different collision energies of 48 and 64 keV. The relative intensities of each transition in measured spectra are shown in figure 2. As can be seen in this figure, the energy dependence of the fractions is quite small. However, the increase of the 1s2–1s2p transition at the lower collision energies implies a larger cross section of the true double-electron capture in much lower energies. These fractions exhibit subtle differences with the previous results by using a high-purity Ge solid-state detector with a 7.5 μm thick Be window at an observation angle of 90° from the ion-beam axis [5]. The difference is due to the transmission of the Be window, which strongly depends on the energy of the soft x-rays. Otherwise, it will cause significant angular distribution of the emission.
Figure 2. Intensity fractions of each transition in the observed emission spectra as a function of collision energy.
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According to the classical over the barrier model [9], the dominant electron capture level in collisions of O8+ with He is the principal quantum number n = 4. This primitive prediction agrees with the theoretical results of the two-center atomic orbital close coupling (TC-AOCC) method. According to the TC-AOCC calculation, the state-selective capture cross sections for each nℓ state have the largest values in the 4f state, and the 4d state has similar large values in the solar wind velocity range. Although the 1s–2p transition is the dominant one in the observed spectra, the capture cross sections of the 2p state are extremely small in these calculations. This discrepancy can be understood by the cascade of the population from the upper to the lower state by the allowed transitions.
Acknowledgments
This work was partially supported by the Grant-in Aid for Scientific Research (numbers 21246017 and 23244083) from the Japan Society for the Promotion of Science and the JSPS-CAS Core-University Program in the field of 'Plasma and Nuclear Fusion'.