Excitation of the 229Th nucleus using the hole in inner electronic shells

M. G. Kozlov, A. V. Oleynichenko, D. Budker, D. A. Glazov, Y. V. Lomachuk, V. M. Shabaev, A. V. Titov, I. I. Tupitsyn, and A. V. Volotka

Petersburg Nuclear Physics Institute of NRC “Kurchatov Institute”, Gatchina, Leningrad District 188300, Russia

The ²²⁹Th nucleus has a long-lived isomeric state A* at 8.338(24) eV [Kraemer et al, Nature, 617, 706 (2023)]. This state is connected to the ground state by an M1 transition. For a hydrogenlike Th ion in the 1s state the hyperfine structure splitting is about 0.7 eV. This means that the hyperfine interaction can mix the nuclear ground state with the isomeric state with a mixing coefficient β about 0.03. If the electron is suddenly removed from this system, the nucleus will be left in the mixed state. The probability to find the nucleus in the isomeric state A* is equal to β² ∼ 10⁻³. For the 2s state the effect is roughly two orders of magnitude smaller. An atom with a hole in the ns shell with n = 1 or 2 is similar to the hydrogenlike atom, only the hole has a short lifetime τ. After the hole is filled, there is a non-zero probability P*(ns) to find the nucleus in the A* state. We found that P*(1s) =1E-5 and P*(2s)=2E-5. The reason why the probability is higher for the 2s hole is the much longer lifetime. The lifetime of the isomeric nuclear state for the bare nucleus is expected to be of the order of 1000 seconds. In the neutral and singly charged thorium the lifetime is orders of magnitude smaller because of the internal conversion. In order to study nuclear isomeric state we need to place thorium ion in the crystal with the broad band gap where internal conversion is suppressed. One example of such a crystal is ThF₄. Then we can produce the holes in thorium 2s shell by irradiating this crystal with 20 - 25 keV X-ray beam. This way one can produce 10⁶ isomeric nuclei, or more per accumulation cycle of about 1000 sec. This opens new possibilities to study such properties of the isomeric state A*, as its energy, charge radius, and the hyperfine constants. An accurate knowledge of the transition energy is crucial for making an optical clock on this transition. Other parameters are necessary to use this transition to search for "new physics", in particular, to study possible variation of the fundamental constants.