Thorium-doped CsI: Implications for the thorium nuclear clock transition

For Th229m isomer nucleus with an anomalously low nuclear excitation energy, (currently accepted value of 7.8±0.5eV), the bound internal conversion (BIC) decay process is caused by the excitation of a valence electron that is sensitive to the electronic structure of the atomic-sized neighborhood. We analyze an experiment where an impacting Th ion is neutralized by a negatively charged ion-receiving cesium iodide (CsI) surface, to obtain the minimal nuclear excitation energy Emin necessary for BIC promotion of a Th-impurity valence electron in a CsI matrix. We analyze two cases: CsI with Th deposited into the bulk and with Th deposited on the CsI surface. In the bulk we consider band gap effects while on the surface we consider the work function. The energy to pull the electron to the surface is 1.5-2.2eV depending on the surface plane, while to promote to the conduction band in the bulk it is 1.5eV. Therefore we conclude that the Th-surface interaction can significantly reduce the lower bound of 6.3 eV, as estimated from the direct observation of the Th-clock transition [von der Wense et al., Nature (London) 533, 47 (2016)NATUAS0028-083610.1038/nature17669]. We suggest coating the multichannel plate with differing materials leading to different Th-impurity gaps and work functions can further narrow the uncertainty interval.