Experiments at the Limits of Stability:
Exploring Physical and Chemical Properties of Superheavy Elements
at TASCA
Synthesis of the heaviest elements
In the middle of the last century, extrapolations based on the nuclear shell model led to the prediction of the existence of heavy elements in a region of the nuclear chart far away from all nuclei that were known at the time. These elements, which exist only thanks to nuclear shell effects, were coined "superheavy elements" (SHE). The shell effects lead to stabilization against immediate spontaneous fission (SF); a macroscopic liquid drop fission barrier is no longer present.
Beyond the closed spherical shells at Z=82 and N=126, which give rise to the doubly-magic stable Pb-208, theoretical calculations suggest the next shell closures at Z=114, 120, or 126, and N=172 or, more frequently, N=184. On the way to this long-sought "island of stability of superheavy elements", deformed shell closures have been identified at N=152, Z=108 and N=162.
Nowadays, elements with atomic number Z≥104 are called SHE, and the heaviest one which is claimed to be observed comprises 118 protons, Z=118.
At TASCA, we have recently measured the production and decay of the most long-lived known isotopes of element 114, synthesized in the Ca-48 + Pu-244 fusion reaction, leading to 114-288 and 114-289 in the 4n and 3n evaporation channels, respectively. Results are published in Phys. Rev. Lett. 104, 252701 (2010) and Phys. Rev. C 83, 054618 (2011) .
Nuclear Structure of the Heaviest Elements
The intense beams available at the GSI and the highly efficient gas-filled separator TASCA are ideally suited for nuclear spectroscopy studies in the focal plane of TASCA by means of particle registration (evaporation residues, α particles, conversion electrons) and γ-ray measurements, either in prompt or delayed coincidence. Such studies allow investigating the decay of isomeric states, which help shedding light on the underlying nuclear structure of the heaviest elements. These studies require much higher statistics than the observation of single decay chains from the heaviest claimed elements and therefore, most recent successes at TASCA were obtained using the "TAsca Small Image mode Spectroscopy" multicoincidence setup TASISpec, which comprises five silicon detectors for particle radiation and 23 Ge crystals for g-rays. The setup is described in more detail in Nucl. Instr. Meth. A, 622, 164 (2010). Current TASISpec research focuses on α-γ-(γ-), α-c.e.-γ-, and c.e.-γ-γ-coincidence spectroscopy in the No-region (Z~102), aiming at nuclear structure studies of SHE up to about hassium (Z=108).
Direct Determination of the Atomic Number of the Heaviest Elements
An important aspect of the reports on the observation of the new elements beyond Z=112 in Ca-48-induced reactions is that their decay chains proceed exclusively through nuclei that were not known before. All these decay chains terminate by SF of previously unknown isotopes. It is thus highly desirable to directly measure the atomic number of these new nuclei. A well-known method is to measure the energies of characteristic X-rays, which are a fingerprint of Z, according to Moseley's law. Good candidates are odd-even nuclei, where α decay often populates excited nuclear states in the daughter nucleus. These states deexcite by the emission of γ-rays or conversion electrons (c.e.), and the emission of c.e. is followed by the emission of characteristic X-rays. These X-ray energies can be calculated with high enough accuracy for element-specific assignment also in the SHE region. With its high efficiency to measure a particles and coincident X-rays, TASISpec is ideal for such an endeavor at Z>112.
Chemical Studies of SHE
Chemical studies of superheavy elements – which are often called the "transactinides" as they follow the actinide series in the periodic table of the elements – trace the influence of relativistic effects on the electronic shell structure. These effects may give rise to changes in chemical behavior of these elements compared to the properties of their lighter homologs in the periodic table. Current research at TASCA addresses the question whether element 114 behaves chemically similarly to its lighter homolog lead (Pb) or rather like a noble gas (e.g., Rn). To obtain a better understanding as well as a correct interpretation of the experimental data, fully-relativistic quantum chemical calculations are being performed. In other experiments, which exploit physical preseparation, i.e., the combination of a recoil separator (in our experiments: TASCA) with chemistry setups, novel chemical compound classes of superheavy elements are studied. Current research focuses on carbonyl complexes, which would represent the first metal-organic SHE compounds.