Jochen Ballof

The present work aimed at the synthesis of the hitherto unknown Am 231 and known Am 229 isotopes via the 4 n evaporation channels of the fusion reactions of Ca 48 + Re 187 and Ca 48 + Re 185 , respectively. The new isotope Am 231 , with a half-life of 47 − 19 + 85 s , was identified on the basis of five observed α -decay chains. It emits α particles with an energy of 7.11(2) MeV. An upper limit of 0.66 was estimated for the electron-capture decay branch of Am 231 . No α -decay chain from Am 229 was observed, corresponding to a cross-section level of 0.03 nb for the detection of a single event. The α -decay properties of Am 231 are discussed in the framework of the Geiger-Nuttall empirical relation.

The chemistry of the radioelement polonium has attracted increasing attention owing to its formation in accelerator-driven systems and its high radiotoxicity. Being the lighter homologue of the superheavy element livermorium, whose chemistry is still unexplored, studies of polonium provide a benchmark for verifying the structure of the periodic table at the heavy-element frontier. While the reactivity of elemental polonium towards various surfaces in inert or reducing atmospheres has been investigated previously, the reactivity of oxidized polonium towards quartz has not been explored in detail. Here, we report on gas–solid thermochromatography studies of polonium on quartz glass and α-Al₂O₃ surfaces in helium, as well as in oxygen- and water-containing atmospheres in the atom-at-a-time regime. We found that polonium chemically reacts in an oxygen-containing atmosphere, forming two oxidized species, which are less volatile than elemental polonium. The chemical reaction is influenced by the water vapour concentration in the carrier gas and the applied temperature. The adsorption enthalpy of elemental polonium on α-Al₂O₃ in pure helium gas was determined to be −85⁺⁴ ₋₃ kJ mol⁻¹, which is identical to the adsorption enthalpy on quartz as reported earlier. The results of the reported measurements will support future experiments with the superheavy element livermorium.

The apparent invariance of the strong nuclear force under combined charge conjugation and parity (CP) remains an open question in modern physics^1,2. Precision experiments with heavy atoms and molecules can provide stringent constraints on CP violation via searches for effects due to permanent electric dipole moments and other CP-odd properties in leptons, hadrons and nuclei³. Radioactive molecules have been proposed as highly sensitive probes for such searches⁴, but experiments with most such molecules have so far been beyond technical reach. Here we report the production and spectroscopic study of a gas-phase actinium molecule, ²²⁷AcF. We observe the predicted strongest electronic transition from the ground state, which is necessary for efficient readout in searches of symmetry-violating interactions. Furthermore, we perform electronic- and nuclear-structure calculations for ²²⁷AcF to determine its sensitivity to various CP-violating parameters, and find that a realistic, near-term experiment with a precision of 1 mHz would improve current constraints on the CP-violating parameter hyperspace by 3 orders of magnitude. Our results thus highlight the potential of ²²⁷AcF for exceptionally sensitive searches of CP violation.

Beyond the (quasi)stable and abundant elements lead and bismuth, all elements are radioactive, with polonium being the first of a series of radioelements up to primordial uranium. Interest in understanding its chemical behavior is increasing, not least due to its co-production in accelerator-driven systems and high radiotoxicity. Polonium is also the lighter homologue of the superheavy element livermorium, which has not been studied chemically to date. Polonium therefore acts as a benchmark to verify the structure of the periodic table at the heavy-element frontier. Here, we report on gas–solid thermochromatography studies of polonium in the atom-at-a-time regime under helium and hydrogen gas atmospheres. Quartz surfaces with different degrees of hydroxylation were used as a stationary phase. On quartz glass with low OH-concentrations, a volatile species interacting with an adsorption enthalpy of −85⁺³ ₋₂ kJ mol⁻¹ was found and assigned to elemental polonium. On more highly hydroxylated quartz glass, an additional deposition zone due to a species with an adsorption enthalpy of −139⁺⁶ ₋₅ kJ mol⁻¹ was observed and attributed to a polonium species formed by chemical reactions with the surface. Under our experimental conditions, chemical reactions of polonium in the solid phase dominate over reactions in the gas phase. Thus, the nature of the surface should be considered as an important parameter in future gas chromatography studies.

Man-made superheavy elements (SHE) are produced as energetic recoils in complete-fusion reactions and need to be thermalized in a gas-filled chamber for chemical studies. The ever-shorter half-lives and decreasing production rates of the elements beyond Fl (atomic number Z = 114)-the heaviest element chemically studied today-require the development of novel techniques for quantitative thermalization and fast extraction efficiency. The Universal high-density gas stopping Cell (UniCell), currently under construction, was proposed to achieve this. Within this work, we propose an Ion Transfer by Gas Flow (ITGF) device, which serves as a UniCell ejector to interface with a gas chromatography detector array for chemical studies. Detailed parameter optimizations, using gas dynamics and Monte Carlo ion-trajectory simulations, promise fast (within a few ms) and highly efficient (up to 100%) ion extraction across a wide mass range. These ions can then be transmitted quantitatively through the ITGF into the high-pressure environment needed for further chemical studies.

The apparent invariance of the strong nuclear force under combined charge conjugation and parity (CP) remains an open question in modern physics. Precision experiments with heavy atoms and molecules can provide stringent constraints on CP violation via searches for effects due to permanent electric dipole moments and other CP-odd properties in leptons, hadrons, and nuclei. Radioactive molecules have been proposed as highly sensitive probes for such searches, but experiments with most such molecules have so far been beyond technical reach. Here we report the first production and spectroscopic study of a gas-phase actinium molecule, $^{227}$AcF. We observe the predicted strongest electronic transition from the ground state, which is necessary for efficient readout in searches of symmetry-violating interactions. Furthermore, we perform electronic- and nuclear-structure calculations for $^{227}$AcF to determine its sensitivity to various CP-violating parameters, and find that a realistic, near-term experiment with a precision of 1 mHz would improve current constraints on the CP-violating parameter hyperspace by three orders of magnitude. Our results thus highlight the potential of $^{227}$AcF for exceptionally sensitive searches of CP violation.

Decay properties of the neutron-deficient Sg isotopes were studied at the gas-filled recoil separator TASCA at GSI, Darmstadt by using the Cr 52 + Pb 206 , 208 reactions. We report on the discovery of the new isotope Sg 257 . Its ground state, tentatively assigned to have 9 / 2 − [734] configuration, was observed to decay by both spontaneous fission and α -particle emission with a half-life of 12.6 − 2.3 + 3.7 ms . The alpha decay from this high-spin state was followed by fission of Rf 253 after 11 μ s . In Sg 259 , we observed strong evidence for the occurrence of a K -isomeric state for the first time in superheavy nuclei with 106 protons. The current experimental findings, discussed within the experimental systematics and theory, indicate that hitherto unseen features of the shell effect on the fission process may be found in the neutron-deficient superheavy nuclei. Published by the American Physical Society 2025

We report the discovery of the new isotope ^{252}Rf. With its extremely short half-life of 60_{-30}^{+90} ns, it expands the range of half-lives of the known superheavy nuclei by about 2 orders of magnitude. This nucleus was synthesized in its high-K isomeric state, for which we measured a half-life of 13_{-3}^{+4} μs. Our results confirm a smooth onset of decreasing ground-state spontaneous fission half-lives in the neutron-deficient Rf isotopes toward the isotopic border of 10^{-14} s, which is the time needed to form an atomic shell. Our findings set a new benchmark for further exploration of phenomena associated with high-K states and inverted fission stability in the heaviest nuclei.

We introduce a novel thick-target concept tailored to the extraction of refractory 4d and 5d transition metal radionuclides of molybdenum, technetium, ruthenium and tungsten for radioactive ion beam production. Despite the more than 60-year old history of thick-target ISOL mass-separation facilities like ISOLDE, the extraction of these most refractory elements as radioactive ion beam has so far not been successful. In ordinary thick ISOL targets, their radioisotopes produced in the target are stopped within the condensed target material. Here, we present a concept which overcomes limitations associated with this method. We exploit the recoil momentum of nuclear reaction products for their release from the solid target material. They are thermalized in a carbon monoxide-containing atmosphere, in which volatile carbonyl complexes form readily at ambient temperature and pressure. This compound serves as volatile carrier for transport to the ion source. Excess carbon monoxide is removed by cryogenic gas separation to enable low pressures in the source region, in which the species are ionized and hence made available for radioactive ion beam formation. The setup is operated in batch mode. Initially, we investigate the feasibility of the approach with isotopes of more than 35s half-life. At the cost of reduced efficiency, the concept could also be applied to isotopes with half-lives of at least one to 10s. We report parameter studies of the key processes of the method, which validate this concept and which define the parameters for the setup. This would allow for the first time the extraction of radioactive molybdenum, tungsten and several other transition metals at thick-target ISOL facilities.