The low-energy, long-lived isomer in ²²⁹Th, first studied in the 1970s as an exotic feature in nuclear physics, continues to inspire a multidisciplinary community of physicists. It has stimulated innovative ideas and studies that expand the understanding of atomic and nuclear structure of heavy elements and of the interaction of nuclei with bound electrons and coherent light. Using the nuclear resonance frequency, determined by the strong and electromagnetic interactions inside the nucleus, it is possible to build a highly precise nuclear clock that will be fundamentally different from all other atomic clocks based on resonant frequencies of the electron shell. The nuclear clock will open opportunities for highly sensitive tests of fundamental principles of physics, particularly in searches for violations of Einstein’s equivalence principle and for new particles and interactions beyond the standard model. It has been proposed to use the nuclear clock to search for variations of the electromagnetic and strong coupling constants and for dark matter searches. The ²²⁹Th nuclear optical clock still represents a major challenge in view of the tremendous gap of nearly 17 orders of magnitude between the present uncertainty in the nuclear transition frequency (about 0.2 eV, corresponding to ∼48 THz) and the natural linewidth (in the mHz range). Significant experimental progress has been achieved in recent years, which will be briefly reviewed. Moreover, a research strategy will be outlined to consolidate our present knowledge about essential ^229mTh properties, to determine the nuclear transition frequency with laser spectroscopic precision, realize different types of nuclear clocks and apply them in precision frequency comparisons with optical atomic clocks to test fundamental physics. Two avenues will be discussed: laser-cooled trapped ²²⁹Th ions that allow experiments with complete control on the nucleus–electron interaction and minimal systematic frequency shifts, and Th-doped solids enabling experiments at high particle number and in different electronic environments.

The Gamma Factory (GF) is an ambitious proposal, currently explored within the CERN Physics Beyond Colliders program, for a source of photons with energies up to ≈400 MeV and photon fluxes (up to ≈10¹⁷ photons s⁻¹) exceeding those of the currently available gamma sources by orders of magnitude. The high‐energy (secondary) photons are produced via resonant scattering of the primary laser photons by highly relativistic partially‐stripped ions circulating in the accelerator. The secondary photons are emitted in a narrow cone and the energy of the beam can be monochromatized, down to 10⁻³–10⁻⁶ level, via collimation, at the expense of the photon flux. This paper surveys the new opportunities that may be afforded by the GF in nuclear physics and related fields.

The excitation of the 8 eV ${}^{229m}\mathrm{Th}$ isomer through the electronic bridge mechanism in highly charged ions is investigated theoretically. By exploiting the rich level scheme of open $4f$ orbitals and the robustness of highly charged ions against photoionization, a pulsed high-intensity optical laser can be used to efficiently drive the nuclear transition by coupling it to the electronic shell. We show how to implement a promising electronic bridge scheme in an electron beam ion trap starting from a metastable electronic state. This setup would avoid the need for a tunable vacuum ultraviolet laser. Based on our theoretical predictions, determining the isomer energy with an uncertainty of ${10}^{-5}\mathrm{eV}$ could be achieved in one day of measurement time using realistic laser parameters.

The photofission cross section of ${}^{238}$U was measured at sub-barrier energies as a function of the $\gamma$-ray energy using a monochromatic, high-brilliance, Compton-backscattered $\gamma$-ray beam. The experiment was performed at the High Intensity $\gamma$-ray Source (HI$\gamma$S) facility at beam energies between $E_{\gamma }=4.7$ MeV and 6.0 MeV and with $\sim 3\%$ energy resolution. Indications of transmission resonances have been observed at $\gamma$-ray beam energies of $E_{\gamma }=5.1$ MeV and 5.6 MeV with moderate amplitudes. The triple-humped fission barrier parameters of ${}^{238}$U have been determined by fitting empire-3.1 nuclear reaction code calculations to the experimental photofission cross section.

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).