Extraction and trapping of ^229mTh ions for measuring nuclear-decay half-lives of various ionic species of ^229mTh and observing electronic bridge process

The first excited state of the ²²⁹Th nucleus, ^229mTh, has a low excitation energy of 8.3 eV, which allows the variation of its decay mode (internal conversion (IC) [1], gamma-ray emission [2], and electronic bridge (EB)) depending on the chemical environment. The EB process, which is an exotic nuclear decay through the transition of an atomic electron, has never been observed thus far. The half-life of the EB process is expected to largely different from other decay modes; hence, we aim to measure the nuclear-decay half-lives of some candidates such as ^229mTh⁺ and ²²⁹ThO²⁺ to find the EB process of ^229mTh.

We developed an extraction and trapping apparatus for ^229mTh ions. ^229mTh^2+,3+ ions emitted from a ²³³U source are extracted by a radiofrequency-carpet gas cell into an ion trap, where ^229mTh⁺ and molecular ^229mTh ions are produced by chemical reactions with reactive gases. The produced ions are then extracted, mass-separated, and placed on a multichannel plate (MCP) detector. The half-lives of ^229mTh can be determined from the number of trapped ^229mTh (the number of IC electrons measured by the MCP detector) as a function of the trapping time.

We successfully extracted ²²⁹Th^3+,2+ and observed the IC electrons emitted from ^229mTh neutralized on a MCP detector. Moreover, we were able to produce ²²⁹Th⁺ and ²²⁹ThO²⁺ by the reaction with impurities in Ar gas. We will soon try to measure the half-lives of these ^229mTh species.

[1] L. von der Wense et al., Nature 533, 47 (2016).

[2] S. Kraemer et al, Nature 617, 706 (2023).