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Today's most precise time and frequency measurements are performed with optical atomic clocks. However, it has been proposed that they could potentially be outperformed by a nuclear clock, which employs a nuclear transition instead of an atomic shell transition. There is only one known nuclear state that could serve as a nuclear clock using currently available technology, namely, the isomeric first excited state of (229)Th (denoted (229m)Th). Here we report the direct detection of this nuclear state, which is further confirmation of the existence of the isomer and lays the foundation for precise studies of its decay parameters. On the basis of this direct detection, the isomeric energy is constrained to between 6.3 and 18.3 electronvolts, and the half-life is found to be longer than 60 seconds for (229m)Th(2+). More precise determinations appear to be within reach, and would pave the way to the development of a nuclear frequency standard.

The first excited isomeric state of 229 Th possesses the lowest energy among all known excited nuclear states. The expected energy is accessible with today's laser technology and in principle allows for a direct optical laser excitation of the nucleus. The isomer decays via three channels to its ground-state (internal conversion, γ decay and bound internal conversion), whose strengths depend on the charge state of 229m Th. We report on the measurement of the internal-conversion decay half-life of neutral 229m Th. A half-life of 7 ± 1 µs has been measured, which is in the range of theoretical predictions and, based on the theoretically expected lifetime of ≈ 10 4 s of the photonic decay channel, gives further support for an internal conversion coefficient of ≈ 10 9 , thus constraining the strength of a radiative branch in the presence of IC.229 Th is the only known nucleus providing an excited isomeric state of sufficiently low energy to allow for direct nuclear optical laser excitation [1]. The possibility to drive the transition with laser technology has led to the proposal of a multitude of interesting applications. The predicted spectroscopic properties of the 229m Th ground-state transition make it a promising candidate for a nuclear optical clock that may outperform today's existing atomic clock technology [2][3][4]. As other ultra-precise optical clocks, a nuclear clock could be a tool in the search for dark matter [5], gravitational waves [6] as well as for geodesy [7]. Such a nuclear clock promises ultra-high sensitivity for potential time variations of fundamental constants [8]. There exist also proposals towards a nuclear γ-ray laser [9] based on the 229m Th ground-state transition and a nuclear qubit for quantum computing [10]. However, to enable laser excitation, precise knowledge on the spectroscopic properties of the transition, such as the lifetime and the excitation energy, is required. Since the first proposal of the existence of a low-energy isomeric state of 229 Th in 1976 [11] several indirect energy measurements [12][13][14] have been performed. With steadily improved detector energy resolution, these measurements pinned down the energy to 7.8 ± 0.5 eV [15] (λ ≈ 159 ± 10 nm). A direct half-life measurement of a photonic decay channel [16] has been controversially discussed [17].Several different experimental approaches have been pursued, aiming for a measurement of the isomer's properties or for an optical laser excitation of the nucleus [18][19][20][21][22][23][24][25][26][27]. However, despite significant experimental effort, no conclusive measurement of the isomeric half-life has been reported so far.As a result of the low excitation energy, the isomer can decay via three decay channels to its ground state, whose occurrence depends on the electronic surrounding of the nucleus [28][29][30][31]: when the binding energy of an electron E B in the surrounding of the nucleus is lower than the excitation energy of the isomer E I , the isomer decays preferably via internal conversion (IC) by emit...

The isotope Th is the only nucleus known to possess an excited stateTh in the energy range of a few electronvolts-a transition energy typical for electrons in the valence shell of atoms, but about four orders of magnitude lower than typical nuclear excitation energies. Of the many applications that have been proposed for this nuclear system, which is accessible by optical methods, the most promising is a highly precise nuclear clock that outperforms existing atomic timekeepers. Here we present the laser spectroscopic investigation of the hyperfine structure of the doubly charged Th ion and the determination of the fundamental nuclear properties of the isomer, namely, its magnetic dipole and electric quadrupole moments, as well as its nuclear charge radius. Following the recent direct detection of this long-sought isomer, we provide detailed insight into its nuclear structure and present a method for its non-destructive optical detection.

The elusive ‘thorium isomer’, i.e. the isomeric first excited state of 229Th, has puzzled the nuclear and fundamental physics communities for more than 40 years. With an exceptionally low excitation energy and a long lifetime it represents the only known candidate so far for an ultra-precise nuclear frequency standard (‘nuclear clock’), potentially able to outperform even today’s best timekeepers based on atomic shell transitions, and promising a variety of intriguing applications. This tutorial reviews the development of our current knowledge on this exotic nuclear state, from the first indirect evidence in the 1970s, to the recent breakthrough results that pave the way towards the realization of a nuclear clock and its applications in practical fields (satellite based navigational systems and chronometric geodesy) as well as fundamental physics beyond the standard model (the search for topological dark matter and temporal variations of fundamental constants).