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Seiferle, Benedict; Wense, Lars von der; Laatiaoui, Mustapha and Thirolf, Peter G. (2016): A VUV detection system for the direct photonic identification of the first excited isomeric state of Th-229. In: European Physical Journal D, Vol. 70, No. 3, 58

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With an expected energy of 7.6(5) eV, Th-229 possesses the lowest excited nuclear state in the landscape of all presently known nuclei. The energy corresponds to a wavelength of about 160 nm and would conceptually allow for an optical laser excitation of a nuclear transition. We report on a VUV optical detection system that was designed for the direct detection of the isomeric ground-state transition of Th-229. Th-229(m) ions originating from a U-233 alpha-recoil source are collected on a micro electrode that is placed in the focus of an annular parabolic mirror. The latter is used to parallelize the UV fluorescence that may emerge from the isomeric ground-state transition of Th-229. The parallelized light is then focused by a second annular parabolic mirror onto a CsI-coated position-sensitive MCP detector behind the mirror exit. To achieve a high signal-to-background ratio, a small spot size on the MCP detector needs to be achieved. Besides extensive ray-tracing simulations of the optical setup, we present a procedure for its alignment, as well as test measurements using a D-2 lamp, where a focal-spot size of approximate to 100 mu m has been achieved. Assuming a purely photonic decay, a signal-to-background ratio of approximate to 7000:1 could be achieved.

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