Yang Yang

Nuclear excitation by electron capture (NEEC) is an important nuclear excitation mechanism which still lacks conclusive experimental verification. This is primarily attributed to strong background x-/$γ$-ray noise and competing nuclear excitation processes which would overshadow the signals in various environments that NEEC takes place. Here, we propose an experimental approach to observe the NEEC process within a background-free environment. Through collisions with a highly-compressed mono-energetic electron beam in an electron beam ion trap, nuclei may get excited to a long-lived isomeric state via the NEEC process. Subsequently, ions can be extracted and Penning-trap mass spectrometry employed to unambiguously detect the isomer. Our study focuses on the promising candidate $^{189}\mathrm{Os}$, demonstrating measurable detection rates of the NEEC process and discussing the feasibility of the proposed approach. This new approach for observing the NEEC process may be realized in the near future.

The wavelength and rate of the $5p-5s$ transition of W XIV - W XVI ions have been calculated by the relativistic configuration interaction (RCI) method with the implementation of Flexible Atomic code (FAC). A reasonable collisional-radiative model (CRM) has been constructed to simulate the $5p - 5s$ transition spectrum of W XIV - W XVI ions which had been observed in electron beam ion trap (EBIT) device. The results are in reasonable agreement with the available experimental and theoretical data, and might be applied to identify the controversial spectra. The confusion on the assignment of the ionization stage are solved in the present work.

A low-energy, compact and superconducting electron beam ion trap (the Shanghai-Wuhan EBIT or SW-EBIT) for extraction of highly charged ions is presented. The magnetic field in the central drift tube of the SW-EBIT is approximately 0.21 T produced by a pair of high-temperature superconducting coils. The electron-beam energy of the SW-EBIT is in the range of 30-4000 eV, and the maximum electron-beam current is up to 9 mA. Acting as a source of highly charged ions, the ion-beam optics for extraction is integrated, including an ion extractor and an einzel lens. A Wien filter is then used to measure the charge-state distribution of the extracted ions. In this work, the tungsten ions below the charge state of 15 have been produced, extracted, and analyzed. The charge-state distributions and spectra in the range of 530-580 nm of tungsten ions have been measured simultaneously with the electron-beam energy of 279 eV and 300 eV, which preliminarily indicates that the 549.9 nm line comes from $W^{14+}$.

We have recorded extreme ultraviolet spectra from $\mathrm{W^{11+}}$ to $\mathrm{W^{15+}}$ ions using a new flat field spectrometer installed at the Shanghai high temperature superconducting electron beam ion trap. The spectra were recorded at beam energies ranging between 200 eV and 400 eV and showed spectral lines/transition arrays in the 170 - 260 Å region. The charge states and spectra transitions were identified by comparison with calculations using a detailed relativistic configuration interaction method and collisional-radiative model, both incorporated in the Flexible Atomic Code. Atomic structure calculations showed that the dominant emission arises from $5d$ $\rightarrow$ $5p$ and $5p$ $\rightarrow$ $5s$ transitions. The work also identified the ground-state configuration of $W^{13+}$ as $4f^{13}5s^2$ both theoretically and experimentally.