W.R. Plaß

Observation of non-exponential orbital electron capture decays of hydrogen-like 140Pr and 142Pm ions

Abstract We report on time-modulated two-body weak decays observed in the orbital electron capture of hydrogen-like 140 Pr 59+ and 142 Pm 60+ ions coasting in an ion storage ring. Using non-destructive single ion, time-resolved Schottky mass spectrometry we found that the expected exponential decay is modulated in time with a modulation period of about 7 seconds for both systems. Tentatively this observation is attributed to the coherent superposition of finite mass eigenstates of the electron neutrinos from the weak decay into a two-body final state.

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.

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.

LARGE-SCALE MASS MEASUREMENTS OF SHORT-LIVED NUCLIDES WITH THE ISOCHRONOUS MASS SPECTROMETRY AT GSI

Precise mass measurements of short-lived exotic nuclei are very important for the understanding of basic nuclear structure physics and astrophysical nucleosynthesis in nature, as well as for the test and the development of theoretical nuclear mass models. At GSI, the Isochronous Mass Spectrometry (IMS) dedicated to mass measurements of short-lived nuclides was developed. In this contribution, the IMS technique is briefly reviewed. Recently, the first large-scale measurement on the 238U fission fragment was done successfully. The measured mass values are in excellent agreement with the recent Penning trap data, however, they show a systematical deviation from the values in the latest atomic mass evaluation. Some representative results from this experiment will be presented, including their impact on nuclear structure physics and astrophysical r-process nucleosynthesis.

Technique for Resolving Low-lying Isomers in the Experimental Storage Ring (ESR) and the Occurrence of an Isomeric State in 192Re

A recent experiment using projectile fragmentation of a 197Au beam on a 9Be target, combined with the fragment recoil separator and experimental storage ring at ring at GSI, has uncovered an isomeric state in 192Re at 267(10) keV with a half-life of ~60 s. The data analysis technique used to resolve the isomeric state from the ground state is discussed.

Direct observation of long-lived isomers in Bi212

Long-lived isomers in (212)Bi have been studied following (238)U projectile fragmentation at 670 MeV per nucleon. The fragmentation products were injected as highly charged ions into a storage ring, giving access to masses and half-lives. While the excitation energy of the first isomer of (212)Bi was confirmed, the second isomer was observed at 1478(30) keV, in contrast to the previously accepted value of >1910 keV. It was also found to have an extended Lorentz-corrected in-ring half-life >30 min, compared to 7.0(3) min for the neutral atom. Both the energy and half-life differences can be understood as being due a substantial, though previously unrecognized, internal decay branch for neutral atoms. Earlier shell-model calculations are now found to give good agreement with the isomer excitation energy. Furthermore, these and new calculations predict the existence of states at slightly higher energy that could facilitate isomer deexcitation studies.

High-performance multiple-reflection time-of-flight mass spectrometers for research with exotic nuclei and for analytical mass spectrometry

A class of multiple-reflection time-of-flight mass spectrometers (MR-TOF-MSs) has been developed for research with exotic nuclei at present and future accelerator facilities such as GSI and FAIR (Darmstadt), and TRIUMF (Vancouver). They can perform highly accurate mass measurements of exotic nuclei, serve as high-resolution, high-capacity mass separators and be employed as diagnostics devices to monitor the production, separation and manipulation of beams of exotic nuclei. In addition, a mobile high-resolution MR-TOF-MS has been developed for in situ applications in analytical mass spectrometry ranging from environmental research to medicine. Recently, the MR-TOF-MS for GSI and FAIR has been further developed. A novel RF quadrupole-based ion beam switchyard has been developed that allows merging and splitting of ion beams as well as transport of ions into different directions. It efficiently connects a test and reference ion source and an auxiliary detector to the system. Due to an increase in the kinetic energy of the ions in the time-of-flight analyzer of the MR-TOF-MS, a given mass resolving power is now achieved in less than half the time-of-flight. Conversely, depending on the time-of-flight, the mass resolving power has been increased by a factor of more than two.

Mass and Lifetime Measurements in Storage Rings

Masses of nuclides covering a large area of the chart of nuclides can be measured in storage rings where many ions circulate at the same time. In this paper the recent progress in the analysis of Schottky mass spectrometry data is presented as well as the technical improvements leading to higher accuracy for isochronous mass measurements with a time‐of‐flight detector. The high sensitivity of the Schottky method down to single ions allows to measure lifetimes of nuclides by observing mother and daughter nucleus simultaneously. In this way we investigated the decay of bare and H‐like 140Pr. As we could show the lifetime can be even shortened compared to those of atomic nuclei despite of a lower number of electrons available for internal conversion or electron capture.All these techniques will be implemented with further improvements at the storage rings of the new FAIR facility at GSI in the future.Masses of nuclides covering a large area of the chart of nuclides can be measured in storage rings where many ions circulate at the same time. In this paper the recent progress in the analysis of Schottky mass spectrometry data is presented as well as the technical improvements leading to higher accuracy for isochronous mass measurements with a time‐of‐flight detector. The high sensitivity of the Schottky method down to single ions allows to measure lifetimes of nuclides by observing mother and daughter nucleus simultaneously. In this way we investigated the decay of bare and H‐like 140Pr. As we could show the lifetime can be even shortened compared to those of atomic nuclei despite of a lower number of electrons available for internal conversion or electron capture.All these techniques will be implemented with further improvements at the storage rings of the new FAIR facility at GSI in the future.

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%.

Development and test of iron-free quadrupole lenses with high magnetic flux densities

Abstract Iron-free magnetic quadrupole lenses have been developed for the focusing of energetic bunched heavy-ion beams. These devices are operated in a pulsed mode and provide very strong magnetic fields. A magnetic flux density of more than 14 T has been reached in a 100 mm long quadrupole with a 20 mm wide aperture, which corresponds to a magnetic flux density of ∼1400 T/m. The pulse duration of the applied electric current is approximately 300 μs with a flat top of several μs. The calculated and measured field properties of the quadrupoles are presented. In a first test experiment with a fast-extracted 650 MeV/u 197 Au 79+ beam (bunch length ∼500 ns) at GSI the focusing properties could be demonstrated. As a possible application the ion-optical design of a condenser lens system will be presented. With this system the phase-space enlargement in a nuclear reaction target can be significantly reduced, thus leading to an increased collection efficiency of a fragment separator or, more general, of any ion-optical device. In particular this is advantageous for injection of secondary beams into a storage ring.