Highly accurate and precise electronic structure calculations of heavy radioactive atoms and their molecules are important for several research areas, including chemical, nuclear, and particle physics. Ab initio quantum chemistry can elucidate structural details in these systems that emerge from the interplay of relativistic and electron correlation effects, but the large number of electrons complicates the calculations, and the scarcity of experiments prevents insightful theory-experiment comparisons. Here we report the spectroscopy of the 14 lowest excited electronic states in the radioactive molecule radium monofluoride (RaF), which is proposed as a sensitive probe for searches of new physics. The observed excitation energies are compared with state-of-the-art relativistic Fock-space coupled cluster calculations, which achieve an agreement of ≥99.64% (within ~12 meV) with experiment for all states. Guided by theory, a firm assignment of the angular momentum and term symbol is made for 10 states and a tentative assignment for 4 states. The role of high-order electron correlation and quantum electrodynamics effects in the excitation energies is studied and found to be important for all states.

The quantum-mechanical nuclear-shell structure determines the stability and limits of the existence of the heaviest nuclides with large proton numbers Z ≳ 100 (refs. ^1–3). Shell effects also affect the sizes and shapes of atomic nuclei, as shown by laser spectroscopy studies in lighter nuclides⁴. However, experimental information on the charge radii and the nuclear moments of the heavy actinide elements, which link the heaviest naturally abundant nuclides with artificially produced superheavy elements, is sparse⁵. Here we present laser spectroscopy measurements along the fermium (Z = 100) isotopic chain and an extension of data in the nobelium isotopic chain (Z = 102) across a key region. Multiple production schemes and different advanced techniques were applied to determine the isotope shifts in atomic transitions, from which changes in the nuclear mean-square charge radii were extracted. A range of nuclear models based on energy density functionals reproduce well the observed smooth evolution of the nuclear size. Both the remarkable consistency of model prediction and the similarity of predictions for different isotopes suggest a transition to a regime in which shell effects have a diminished effect on the size compared with lighter nuclei.

RAdiation-Detected Resonance Ionization Spectroscopy (RADRIS) is a versatile method for highly sensitive laser spectroscopy studies of the heaviest actinides. Most of these nuclides need to be produced at accelerator facilities in fusion-evaporation reactions and are studied immediately after their production and separation from the primary beam due to their short half-lives and low production rates of only a few atoms per second or less. Only recently, the first laser spectroscopic investigation of nobelium ($Z=102$) was performed by applying the RADRIS technique in a buffer-gas-filled stopping cell at the GSI in Darmstadt, Germany. To expand this technique to other nobelium isotopes and for the search for atomic levels in the heaviest actinide element, lawrencium ($Z=103$), the sensitivity of the RADRIS setup needed to be further improved. Therefore, a new movable double-detector setup was developed, which enhances the overall efficiency by approximately 65% compared to the previously used single-detector setup. Further development work was performed to enable the study of longer-lived (t${}_{1/2}>1$ h) and shorter-lived nuclides (t${}_{1/2}<1$ s) with the RADRIS method. With a new rotatable multi-detector design, the long-lived isotope ${}^{254}$Fm (t${}_{1/2}=3.2 \mathrm{h}$) becomes within reach for laser spectroscopy. Upcoming experiments will also tackle the short-lived isotope ${}^{251}$No (t${}_{1/2}=0.8 \mathrm{s}$) by applying a newly implemented short RADRIS measurement cycle.

Atomic nuclei serve as prime laboratories for investigations of complex quantum phenomena, where minor nucleon rearrangements cause significant structural changes. ¹⁹⁰Pb is the heaviest known neutron-deficient Pb isotope that can exhibit three distinct shapes: prolate, oblate, and spherical, with nearly degenerate excitation energies. Here we report on the combined results from three state-of-the-art measurements to directly observe these deformations in ¹⁹⁰Pb. Contrary to earlier interpretations, we associate the collective yrast band as predominantly oblate, while the non-yrast band with higher collectivity follows characteristics of more deformed, predominantly prolate bands. Direct measurement of the \(E0({0}_{2}^{+}\to {0}_{1}^{+})\) transition and γ-e⁻ coincidence relations allowed us to locate and firmly assign the \({0}_{2}^{+}\) state in the level scheme and to discover a spherical \({2}_{3}^{+}\) state at 1281(1) keV with \(B(E2;{2}_{3}^{+}\to {0}_{1}^{+})=1.2(3)\) W.u. These assignments are based purely on observed transition probabilities and monopole strength values, and do not rely on model calculations for their interpretation.

We report on high-resolution laser spectroscopy of Fm 255 ( T 1 / 2 = 20 h ), one of the heaviest nuclides available from reactor breeding. The hyperfine structures in two different atomic ground-state transitions at 398.4 nm and 398.2 nm were probed by in-source laser spectroscopy at the RISIKO mass separator in Mainz, using the perpendicularly illuminated laser ion source and trap (PI-LIST) high-resolution ion source. Experimental results were combined with hyperfine fields from various atomic calculations, in particular using multiconfiguration Dirac-Hartree-Fock theory, as implemented in 18. In this manner, the nuclear magnetic dipole and electric quadrupole moments were derived to be μ = − 0.75 ( 5 ) μ N and Q s = + 5.84 ( 13 ) eb , respectively. The magnetic moment indicates occupation of the ν 7 / 2 [ 613 ] Nilsson orbital, while the large quadrupole moment confirms strong, stable prolate deformation consistent with systematics in the heavy actinides. Comparisons with available expectation values from nuclear theory show good agreement, providing a stringent benchmark for the used theoretical models. These results revise earlier data and establish Fm 255 as a reference isotope for future high-resolution studies.

Recoil-ion sources of ²³⁹ Pu and ²⁴⁰ Pu, providing recoiling alpha-decay daughters ^235m U and ²³⁶ U, were prepared by molecular plating. Comprehensive radiographic, microscopic and spectroscopic characterisation revealed thickness-dependent morphology, homogeneity, and activity distributions. Since neither recoil ion can be readily detected, ²³⁹ Pu sources spiked with ²²⁸ Th were also prepared to provide readily detectable ²²⁴ Ra daughter ions. Geant4 simulations, validated against ²²⁴ Ra surrogate measurements, reproduce recoil escape efficiencies within ~ 3%. ²³⁹ Pu and ²⁴⁰ Pu recoil-ion sources with thicknesses of ~ 40–55 nm produce the highest recoil rate per unit area and highest layer quality across the 24–108 nm thickness range.