G. Marx

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.

Mass measurements of very neutron-deficient Mo and Tc isotopes and their impact on rp process nucleosynthesis.

The masses of ten proton-rich nuclides, including the N=Z+1 nuclides ⁸⁵Mo and ⁸⁷Tc, were measured with the Penning trap mass spectrometer SHIPTRAP. Compared to the Atomic Mass Evaluation 2003 a systematic shift of the mass surface by up to 1.6 MeV is observed causing significant abundance changes of the ashes of astrophysical x-ray bursts. Surprisingly low α separation energies for neutron-deficient Mo and Tc are found, making the formation of a ZrNb cycle in the rp process possible. Such a cycle would impose an upper temperature limit for the synthesis of elements beyond Nb in the rp process.

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.

Precise mass measurements of exotic nuclei—the SHIPTRAP Penning trap mass spectrometer

The SHIPTRAP Penning trap mass spectrometer has been designed and constructed to measure the mass of short‐lived, radioactive nuclei. The radioactive nuclei are produced in fusion‐evaporation reactions and separated in flight with the velocity filter SHIP at GSI in Darmstadt. They are captured in a gas cell and transfered to a double Penning trap mass spectrometer. There, the cyclotron frequencies of the radioactive ions are determined and yield mass values with uncertainties ⩾4.5⋅10−8. More than 50 nuclei have been investigated so far with the present overall efficiency of about 0.5 to 2%.

SHIPTRAP is Trapping: A Capture and Storage Device on Its Way towards a RIB-Facility

First off-line tests at the ion trap facility SHIPTRAP took place. The facility is being set up to deliver very clean and cooled beams of singly-charged recoil ions (Rare Isotope Beam) produced at the SHIP (Separator for Heavy Ion Production) velocity filter at GSI, Darmstadt. SHIPTRAP consists of a gas cell for stopping and thermalizing high-energy recoil ions from SHIP, an 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. The progress in testing the sub-systems separately and in combinations is reported.

Delayed bunching for multi-reflection time-of-flight mass separation

Many experiments are handicapped when the ion sources do not only deliver the ions of interest but also contaminations, i.e., unwanted ions of similar mass. In the recent years, multi-reflection time-of-flight mass separation has become a promising method to isolate the ions of interest from the contaminants, in particular for measurements with low-energy short-lived nuclides. To further improve the performance of multi-reflection mass separators with respect to the limitations by space-charge effects, the simultaneously trapped ions are spatially widely distributed in the apparatus. Thus, the ions can propagate with reduced Coulomb interactions until, finally, they are bunched by a change in the trapping conditions for high-resolution mass separation. Proof-of-principle measurements are presented.

Cluster-Electron Interaction for Poly-Anion Production

By interaction with electrons in ion storage devices (ion-cyclotron-resonance and radio-frequency traps) negatively charged clusters of gold and aluminum have been produced up to the 6th and 10th charge state, respectively. The production of these poly-anions opens exciting new possibilities to measure their lifetimes, to monitor their relaxation schemes after laser radiation, as well as to probe their Coulomb barriers.

Extending Penning trap mass measurements with SHIPTRAP to the heaviest elements

Penning-trap mass spectrometry of radionuclides provides accurate mass values and absolute binding energies. Such mass measurements are sensitive indicators of the nuclear structure evolution far away from stability. Recently, direct mass measurements have been extended to the heavy elements nobelium (Z=102) and lawrencium (Z=103) with the Penning-trap mass spectrometer SHIPTRAP. The results probe nuclear shell effects at N=152. New developments will pave the way to access even heavier nuclides.

Mass measurements of exotic nuclides at SHIPTRAP

The Penning trap mass spectrometer SHIPTRAP is installed behind the velocity‐filter SHIP at GSI for high‐precision mass measurements of fusion‐evaporation residues. To facilitate an efficient stopping of the reaction products a buffer gas stopping cell is utilized. In an investigation of neutron‐deficient nuclides in the terbium‐to‐thulium region around A ≈ 146, 18 new or improved mass values have been obtained, resulting in a more accurate determination of the proton drip line for holmium and thulium. With the present performance of SHIPTRAP, a first direct mass measurement of transuranium elements in the nobelium region is within reach.