229-Thorium is the only known candidate for a frequency standard based on a nuclear transition. Such a “nuclear clock” can be used for ultraprecise metrology, enhanced precision in GPS and geodesy or as a means to find answers to fundamental questions in science such as: “Have the fundamental constants of nature changed since the birth of the universe?” 229Th also proves to be a unique system for studying the interface of the atomic electron shell and the nucleus.
At GSI, an alternative approach to the physics of 229-Th is under development and a first experiment is scheduled in 2022 (experiment E142): We propose to investigate and utilize a phenomenon that is unique to very highly charged 229Th such as one-electron 229Th89+ or three-electron 229Th87+. In these thorium charge states, in addition to the ordinary hyperfine structure, the very strong magnetic field of ∼28 MT (and ∼3.5 MT, resp.) of the unpaired s-electron mediates a mixing of the F = 2 levels of ground state (g.s.) and isomeric state (i.s.). The mixing results in an additional small energy shift. But more notable, the lifetime of the i.s. decreases drastically by 5-6 orders of magnitude, from a few hours down to a few 10 ms. In two-electron 229Th88+ (2e) the effective field of the paired electrons is zero. As a consequence, mixing is “switched off” and tg is the same as for the bare nucleus. Three-electron 229Th87+ again shows strongly accelerated decay times but with 2.5 s to 4 s somewhat slower than for one-electron 229Th89+.
In essence, depending on the electronic configuration, the lifetime of the nucleus can be gradually or instantaneously manipulated on purpose. These manipulations of nuclear gamma-decay rates by orders-of-magnitude simply by altering the number or the configuration of electrons in the atomic shell is a behavior that contradicts the common notion of an atomic nucleus being inert against external perturbations.
Furthermore, the hyperfine quenched accelerated decay e.g. in 229Th89+ implies that the excitation probability with a laser is enhanced by these 5-6 orders of magnitude.
It is proposed to investigate hyperfine nuclear mixing using laser spectroscopy and dielectronic recombination at the ESR and the CRYRING storage rings. Precision and scope of application can be furthermore enhanced with extracted beams and dedicated experiments at the HITRAP facility. The 229Th ions will be produced in-flight between SIS18 and the ESR and cleanly separated in the ESR, i.e., without using the FRS.
Beyond the mere investigation of this very fascinating process, this hitherto disregarded effect has the potential for a breakthrough in 229Th physics and provide important input to the nuclear clock developments: The experiment would be (i) the first to excite the 8.28 eV-state with a laser, (b) yield a significantly improved value for the excitation energy, and (c) measure the g-lifetime, i.e., the transition strength of the M1-transition between g.s. and i.s.
Manuel Vogel - Atomic Physics Department