Abstract：Many elements' most sensitive spectral lines are mainly distributed in the ultraviolet band. Some important elements commonly used as detection targets, such as C, P, and S, even have spectral lines in the vacuum ultraviolet range below 200 nm. However, radiation in the vacuum ultraviolet band is easily absorbed by air and cannot be transmitted through air. Therefore, to detect the spectral lines in the ultraviolet range, a short optical path, and a streamlined Rowland circle optical system are commonly used as the dispersion system of the spectrometer. During the detection and analysis, it is also necessary to establish a relative vacuum environment inside the equipment cavity to reduce the weakening effect of the experimental environment on the intensity of the spectral lines in the vacuum ultraviolet range. However, even under these conditions, the spectral sensitivity of elements P and S in the vacuum ultraviolet range remains relatively weak. This study designed a cylindrical mirror focusing system at 30 mm in front of the image plane to achieve high sensitivity within the vacuum ultraviolet range. The system utilizes a cylindrical mirror with a material of CaF2, a central thickness of 5mm, and a sagittal radius of curvature of 22.06 mm as the core component. The optical system design was simulated and optimized using the simulation software Zemax. Furthermore, the simulated radiance results under non-sequential mode revealed that the cylindrical mirror focusing system significantly improved the imaging sensitivity for various wavelengths with minimal impact on the resolution. In the final experiment, a medium-low alloy steel detection sample was used, and the various spectral lines of elements in the ultraviolet range were excited using an electrical spark excitation light source to verify the spectrometer's performance. Mathematical models were established for the peak intensity of each element's spectrum and its actual content through linear fitting, reference fitting, and quadratic fitting, with fitting goodness offit (R2) values above 0.99. The cylindrical mirror focusing system significantly improved the sensitivity of the equipment in the vacuum ultraviolet range, with the response increased by more than 1 fold. The experimental results demonstrate that the design approach of the cylindrical mirror focusing system has a certain reference value for enhancing the sensitivity of the Rowland circle dispersion system.
Key words：Ultraviolet spectrum; Roland circular optical system; Cylindrical lens; Sensitivity; Goodness of fit
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