Eur. Phys. J. A (2012) 48: 52
https://doi.org/10.1140/epja/i2012-12052-3

Regular Article - Experimental Physics

The search for the existence of ^229mTh at IGISOL

¹ Department of Physics, University of Jyväskylä, P.O. Box 35 (YFL), FI-40014, Jyväskylä, Finland
² Institute of Physics, University of Mainz, Staudingerweg 7, 50099, Mainz, Germany
³ Petersburg Nuclear Physics Institute, 188350, St-Petersburg, Russia

^* e-mail: volker.t.sonnenschein@jyu.fi

Received: 30 March 2012
Accepted: 3 April 2012
Published online: 23 April 2012

Abstract

An overview of preparatory work aiming at the identification of the low-lying 7.6 eV isomer in ²²⁹Th through a measurement of its hyperfine structure is presented. A ²³³U recoil gas cell has been developed and has undergone several iterations in order to improve the efficiency of extracting a low-energy beam of ²²⁹Th⁺ ions. Spectroscopic studies on stable ²³²Th have been carried out to establish an efficient laser ionization scheme. The latter will be applied in connection with the gas cell in order to improve the extraction efficiency by accessing the neutral atomic fraction. In addition to that of the ²²⁹Th ground state, the hyperfine structure of the isomer can then be determined either by collinear laser spectroscopy on the extracted and accelerated ions or by direct in-source/in-jet high-resolution Resonance Ionization Spectroscopy (RIS). The first experiments using high-resolution RIS have been performed and isotope shifts of ²²⁸Th, ²²⁹Th and ²³⁰Th relative to ²³²Th were measured on an atomic 7s ² → 7s7p ground-state transition at 380.42 nm. A template of the ground-state hyperfine structure of ²²⁹Th has been established for a 261.24 nm UV transition. This is an important step towards identification of the isomeric state.