Eur. Phys. J. D (2016) 70: 58
https://doi.org/10.1140/epjd/e2016-60653-4

Regular Article

A VUV detection system for the direct photonic identification of the first excited isomeric state of ²²⁹Th

¹ Ludwig-Maximilians-Universität München, Am Coulombwall 1, Garching, Germany
² GSI Helmholtzzentrum für Schwerionenforschung GmbH, Planckstr. 1, Darmstadt, Germany
³ Helmholtz Institut Mainz, Johann-Joachim-Becherweg 36, Mainz, Germany

^a e-mail: benedict.seiferle@physik.uni-muenchen.de

Received: 17 November 2015
Received in final form: 1 February 2016
Published online: 15 March 2016

Abstract

With an expected energy of 7.6(5) eV, ²²⁹Th 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 ²²⁹Th. ^229(m)Th ions originating from a ²³³U α-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 ²²⁹Th. 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 D₂ lamp, where a focal-spot size of ≈100 μm has been achieved. Assuming a purely photonic decay, a signal-to-background ratio of ≈7000:1 could be achieved.