D. Ackermann

A triplet of differently shaped spin-zero states in the atomic nucleus 186 Pb

Understanding the fundamental excitations of many-fermion systems is of significant current interest. In atomic nuclei with even numbers of neutrons and protons, the low-lying excitation spectrum is generally formed by nucleon pair breaking and nuclear vibrations or rotations. However, for certain numbers of protons and neutrons, a subtle rearrangement of only a few nucleons among the orbitals at the Fermi surface can result in a different elementary mode: a macroscopic shape change. The first experimental evidence for this phenomenon came from the observation of shape coexistence in 16O (ref. 4). Other unexpected examples came with the discovery of fission isomers and superdeformed nuclei. Here we find experimentally that the lowest three states in the energy spectrum of the neutron deficient nucleus 186Pb are spherical, oblate and prolate. The states are populated by the α-decay of a parent nucleus; to identify them, we combine knowledge of the particular features of this decay with sensitive measurement techniques (a highly efficient velocity filter with strong background reduction, and an extremely selective recoil-α-electron coincidence tagging method). The existence of this apparently unique shape triplet is permitted only by the specific conditions that are met around this particular nucleus.

A case study of collectivity, transfer and fusion enhancement

Abstract Precise fusion and quasi-elastic scattering excitation functions have been measured for the systems 40 Ca + 90,96 Zr at energies spanning the Coulomb barrier. From both the fusion and the quasi-elastic scattering data representations of the barrier distributions have been extracted. The barrier distributions of the two reactions are qualitatively very different. While the lighter system is well described by coupled-channels calculations including multi-phonon excitations, this coupling scheme fails completely for the heavier system, possibly due to multi-neutron transfer.

Direct Mapping of Nuclear Shell Effects in the Heaviest Elements

Pinning Down Nuclear Shells The nuclei of heavy atoms are destabilized by proton repulsions, and, conversely, the quantum-mechanical shell effects help to stabilize them. There are theoretical models for predicting the masses of yet-to-be-discovered superheavy elements, based on such shell effects, and these models can be tested by studying the shells of known actinide nuclei. The problem is that current mass values determined from studying radioactive decay products have substantial errors. Minaya Ramirez et al. (p. 1207, published online 9 August; see the Perspective by Bollen) were able to collect a sufficient number of nuclei of lawrencium and nobelium isotopes in an ion trap to determine their masses directly by mass spectroscopy. These results will be helpful in predicting the heaviest possible element. Highly precise mass measurements of nobelium and lawrencium isotopes provide insight into superheavy element stability. Quantum-mechanical shell effects are expected to strongly enhance nuclear binding on an “island of stability” of superheavy elements. The predicted center at proton number Z = 114, 120, or 126 and neutron number N = 184 has been substantiated by the recent synthesis of new elements up to Z = 118. However, the location of the center and the extension of the island of stability remain vague. High-precision mass spectrometry allows the direct measurement of nuclear binding energies and thus the determination of the strength of shell effects. Here, we present such measurements for nobelium and lawrencium isotopes, which also pin down the deformed shell gap at N = 152.

Superheavy Element Flerovium (Element 114) Is a Volatile Metal

The electron shell structure of superheavy elements, i.e., elements with atomic number Z ≥ 104, is influenced by strong relativistic effects caused by the high Z. Early atomic calculations on element 112 (copernicium, Cn) and element 114 (flerovium, Fl) having closed and quasi-closed electron shell configurations of 6d(10)7s(2) and 6d(10)7s(2)7p1/2(2), respectively, predicted them to be noble-gas-like due to very strong relativistic effects on the 7s and 7p1/2 valence orbitals. Recent fully relativistic calculations studying Cn and Fl in different environments suggest them to be less reactive compared to their lighter homologues in the groups, but still exhibiting a metallic character. Experimental gas-solid chromatography studies on Cn have, indeed, revealed a metal-metal bond formation with Au. In contrast to this, for Fl, the formation of a weak bond upon physisorption on a Au surface was inferred from first experiments. Here, we report on a gas-solid chromatography study of the adsorption of Fl on a Au surface. Fl was produced in the nuclear fusion reaction (244)Pu((48)Ca, 3-4n)(288,289)Fl and was isolated in-flight from the primary (48)Ca beam in a physical recoil separator. The adsorption behavior of Fl, its nuclear α-decay product Cn, their lighter homologues in groups 14 and 12, i.e., Pb and Hg, and the noble gas Rn were studied simultaneously by isothermal gas chromatography and thermochromatography. Two Fl atoms were detected. They adsorbed on a Au surface at room temperature in the first, isothermal part, but not as readily as Pb and Hg. The observed adsorption behavior of Fl points to a higher inertness compared to its nearest homologue in the group, Pb. However, the measured lower limit for the adsorption enthalpy of Fl on a Au surface points to the formation of a metal-metal bond of Fl with Au. Fl is the least reactive element in the group, but still a metal.

Strong isotopic dependence of the fusion of 40Ca + 90,96Zr

Abstract Precise fusion excitation functions measured for the reactions 40Ca + 90,96Zr yield barrier distributions which are qualitatively very different. While the lighter system is well described by coupled-channels calculations including multi-phonon excitations, this coupling scheme fails completely for the heavier system, possibly due to the role of transfer channels.

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 1S0 1P1. 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.

First on-line test of SHIPTRAP

Abstract The ion trap facility SHIPTRAP is installed behind the separator for heavy ion reaction products (SHIP) at GSI, which is well known for the discovery of new super-heavy elements produced in cold fusion reactions. SHIPTRAP consists out of a gas cell for stopping the recoil ions delivered by SHIP and two linear radio frequency quadrupole (RFQ) structures for cooling and accumulating the ions. In a first Penning trap the radionuclides of interest get further cooled and isobaric contaminants are removed. The second Penning trap is intended for high-precision mass measurements or identification of the stored ions before providing them to further downstream experiments. During a first on-line experiment in 2001, ions from SHIP were stopped in the gas cell and transferred into the RFQ structures. Accumulation and cooling could be demonstrated.

The SHIPTRAP project: A capture and storage facility at GSI for heavy radionuclides from SHIP

SHIPTRAP is an ion trap facility which is being set up to deliver very clean and cool beams of singly-charged recoil ions produced at the SHIP velocity filter at GSI Darmstadt. SHIPTRAP consists of a gas cell for stopping and thermalizing high-energy recoil ions from SHIP, a rf ion guide for extraction of the ions from the gas cell, a linear rf trap for accumulation and bunching of the ions, and a Penning trap for isobaric purification. The physics programme of the SHIPTRAP facility comprises mass spectrometry, nuclear spectroscopy, laser spectroscopy and chemistry of transeinsteinium elements.

New Short-Lived Isotope 221U and the Mass Surface Near N=126

Two short-lived isotopes ^{221}U and ^{222}U were produced as evaporation residues in the fusion reaction ^{50}Ti+^{176}Yb at the gas-filled recoil separator TASCA. An α decay with an energy of E_{α}=9.31(5)  MeV and half-life T_{1/2}=4.7(7)  μs was attributed to ^{222}U. The new isotope ^{221}U was identified in α-decay chains starting with E_{α}=9.71(5)  MeV and T_{1/2}=0.66(14)  μs leading to known daughters. Synthesis and detection of these unstable heavy nuclei and their descendants were achieved thanks to a fast data readout system. The evolution of the N=126 shell closure and its influence on the stability of uranium isotopes are discussed within the framework of α-decay reduced width.

The discovery of a prolate-oblate-spherical shape triple of spin 0(+) states in the atomic nucleus Pb-186.

Two excited J”=O+ states in ‘@Pb populated in the a-decay of lgoPo have been identified through a-particle/conversion electron coincidences in an experiment at the velocity filter SHIP. The parent lgoPo nuclei have been produced in the ‘42Nd(52Cr,4n)‘g0Po complete fusion reaction. a-particle energies and branching ratios have been measured and hindrance factors were deduced. The observed states have been interpreted as the band heads of the known prolate and (yet unobserved) oblate rotational bands in lssPb.