The preferred de-excitation pathway of low-energy excited nuclear states is internal conversion (IC). Instead of de-exciting to the ground state via emission of a gamma, they couple to electronic states of similar energy. Their excitation energy is transferred to the electron, which typically leaves the atom and carries away the excess energy. Such a process is common for nuclear states up to 1 MeV, and its probability generally increases with decreasing excitation energy. For the U-235 isomer at 70 eV, IC is virtually the only de-excitation pathway. The nucleus does not only couple to electronic states of the atom itself, but also the the environment: the IC rate depends on the electronic states in the neighborhood (charge state, work function of the material the atom is placed on, molecule/chemisty, …). Indeed, for U-235, a mild dependence of the isomer half-life on the nature of the host material has been observed.
The nuclear optical clock, envisioned by nuClock researchers, relies on optical excitation and optical detection of the isomeric transition. As such, IC is thought of as a competing, undesired process that needs to be suppressed. The IC rate (or equivalently, the half-life of the isomer) is expected to depend dramatically on the charge state of the Th-229 atom or ion. For the neutral thorium atom, the first ionization energy is smaller than the isomer energy: the isomer may pass its energy to a valence electron quickly, which then leaves the atom. This process was used for the first direct detection of the isomer in the earlier work of the LMU group. For atoms in higher charge states (more specifically, with an ionization energy way larger than the isomer energy), the IC channel should be heavily suppressed, and the lifetime of the isomer is expected to reach its radiative limitation of about 1000 s.
The LMU group set out to measure the isomer half-life in neutral Th-299. Using much of the technology available from their earlier work, they accumulate about 400 ions of Th2+ or Th3+ in an ion trap (about 2% of them in the isomeric state), and gently slam this bunch of ions onto an MCP detector. The ionic impact (neutralization of the ions on the detector surface) produces a rather large signal, which decays in a fraction of a microsecond. Afterwards, the isomeric nuclei are free to de-excite via coupling to the electrons of the Th atom (or of the detector surface). The electrons are ejected with a small kinetic energy (e.g. the isomer energy minus the work function of the material) and generate a signal on the MCP. By summing over many 10’000 bunches, the signal of the IC electrons forms a very beautiful exponential decay with a half-life of 7(1) µs, corresponding to a lifetime of about 10 µs. Such an exponential decay was observed only for Th-229, but not for any other Th or U isotope.
The half-life of the isomer matches theoretical expectations very well (predicted reduction of the 1000-seconds radiative half-life by a factor of 10^9). The half-life of other charge states (e.g. Th+ or Th3+) is of interest as well, but cannot be measured with the present technique. The layout of the experiment, however, allows to deduce certain bounds on these values: < 10 ms for Th+, and > many minutes for Th2+ and Th3+. It is still unclear how the surprisingly short half-life in Th+ is related to the isomer energy.
Learning that the isomer half-life is at the expected order of magnitude is reassuring for the international research community working towards the nuclear clock. Although a more precise measurement of the half-life would not aid the development of the nuclear clock much, it would add to similar work on the isomer in U-235. So far, the LMU group worked with only one surface material. It would be insightful to replace the present nickel alloy by a range of other materials, e.g. metals with different work functions, or insulators. Such a measurement would explain whether the IC is dominated by the Th atom’s electronic shell, or by the surface around it. Ultimately, researchers at LMU plan to perform the experiment in free space (without any surface), e.g. by neutralizing the ion bunch with a beam of cesium atoms.
The findings of this experiment have been published today with Phys. Rev. Lett. (even as an Editors’ Suggestion!) and can be found here.