Observation of the radiative decay of the 229Th nuclear clock isomer.

The radionuclide thorium-229 features an isomer with an exceptionally low excitation energy that enables direct laser manipulation of nuclear states. It constitutes one of the leading candidates for use in next-generation optical clocks1-3. This nuclear clock will be a unique tool for precise tests of fundamental physics4-9. Whereas indirect experimental evidence for the existence of such an extraordinary nuclear state is substantially older10, the proof of existence has been delivered only recently by observing the isomer's electron conversion decay11. The isomer's excitation energy, nuclear spin and electromagnetic moments, the electron conversion lifetime and a refined energy of the isomer have been measured12-16. In spite of recent progress, the isomer's radiative decay, a key ingredient for the development of a nuclear clock, remained unobserved. Here, we report the detection of the radiative decay of this low-energy isomer in thorium-229 (229mTh). By performing vacuum-ultraviolet spectroscopy of 229mTh incorporated into large-bandgap CaF2 and MgF2 crystals at the ISOLDE facility at CERN, photons of 8.338(24) eV are measured, in agreement with recent measurements14-16 and the uncertainty is decreased by a factor of seven. The half-life of 229mTh embedded in MgF2 is determined to be 670(102) s. The observation of the radiative decay in a large-bandgap crystal has important consequences for the design of a future nuclear clock and the improved uncertainty of the energy eases the search for direct laser excitation of the atomic nucleus.

Direct detection of the (229)Th nuclear clock transition.

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.

Atom-at-a-time laser resonance ionization spectroscopy of nobelium.

Optical spectroscopy of a primordial isotope has traditionally formed the basis for understanding the atomic structure of an element. Such studies have been conducted for most elements and theoretical modelling can be performed to high precision, taking into account relativistic effects that scale approximately as the square of the atomic number. However, for the transfermium elements (those with atomic numbers greater than 100), the atomic structure is experimentally unknown. These radioactive elements are produced in nuclear fusion reactions at rates of only a few atoms per second at most and must be studied immediately following their production, which has so far precluded their optical spectroscopy. Here we report laser resonance ionization spectroscopy of nobelium (No; atomic number 102) in single-atom-at-a-time quantities, in which we identify the ground-state transition 1S01P1. By combining this result with data from an observed Rydberg series, we obtain an upper limit for the ionization potential of nobelium. These accurate results from direct laser excitations of outer-shell electrons cannot be achieved using state-of-the-art relativistic many-body calculations that include quantum electrodynamic effects, owing to large uncertainties in the modelled transition energies of the complex systems under consideration. Our work opens the door to high-precision measurements of various atomic and nuclear properties of elements heavier than nobelium, and motivates future theoretical work.

Laser Resonance Chromatography of Superheavy Elements.

Optical spectroscopy constitutes the historical path to accumulate basic knowledge on the atom and its structure. Former work based on fluorescence and resonance ionization spectroscopy enabled identifying optical spectral lines up to element 102, nobelium. The new challenges faced in this research field are the refractory nature of the heavier elements and the decreasing production yields. A new concept of ion-mobility-assisted laser spectroscopy is proposed to overcome the sensitivity limits of atomic structure investigations persisting in the region of the superheavy elements. The concept offers capabilities of both broadband-level searches and high-resolution hyperfine spectroscopy of synthetic elements beyond nobelium.

Mobility of the Singly-Charged Lanthanide and Actinide Cations: Trends and Perspectives.

The current status of gaseous transport studies of the singly-charged lanthanide and actinide ions is reviewed in light of potential applications to superheavy ions. The measurements and calculations for the mobility of lanthanide ions in He and Ar agree well, and they are remarkably sensitive to the electronic configuration of the ion, namely, whether the outer electronic shells are 6s, 5d6s or 6s2. The previous theoretical work is extended here to ions of the actinide family with zero electron orbital momentum: Ac+ (7s2, 1S), Am+ (5f77s 9S°), Cm+ (5f77s2 8S°), No+ (5f147s 2S), and Lr+ (5f147s2 1S). The calculations reveal large systematic differences in the mobilities of the 7s and 7s2 groups of ions and other similarities with their lanthanide analogs. The correlation of ion-neutral interaction potentials and mobility variations with spatial parameters of the electron distributions in the bare ions is explored through the ionic radii concept. While the qualitative trends found for interaction potentials and mobilities render them appealing for superheavy ion research, lack of experimental data and limitations of the scalar relativistic ab initio approaches in use make further efforts necessary to bring the transport measurements into the inventory of techniques operating in "one atom at a time" mode.