• The 1s Ground State in Pionic Lead Atoms
  
  
• Is 19C a Halo Nucleus?
  
  
• Vanishing Lines
  
  
• Nuclear Waste Transmutation and Incineration
  
  
• A New Spectrometer for Precision Tests of QED
  
  

Pionic atoms are exotic systems in which a negatively charged pion is bound in an atom-like orbital. However, in comparison with electron orbitals, pionic orbitals have almost 300 times smaller diameters, which is a consequence of the large difference in mass of the two particles. For heavy atoms with high nuclear charge number and comparatively large nuclear radius, this leads to strong overlap of the low-lying pionic states with the nucleus. Thus, pions in these low-lying states do not only experience the long-range Coulomb force, but also the short-range nuclear force. This is, why particle and nuclear physicists are so interested in low-lying pionic states: They offer a closer look into the complex interactions between pions and nuclear matter, in particular into the s-wave pion-nuclear potential. More...



The low-lying pionic states were populated and identified via the transfer reaction 206Pb (d,3He) 205PbXp-. The pionic reaction occurs at a neutron of the 206Pb nucleus which is transferred as proton to the d-projectile to form 3He, while the pion remains in a quasi-stable low-lying state.

The study of exotic nuclei far from the valley of beta stability is one of the central topics of present-day nuclear structure physics. Outstanding structural phenomena have been observed, such as the formation of neutron halos for very light nuclei like 11Li and 11Be. The extreme neutron excess in these nuclei causes a clusterisation into an ordinary core nucleus and a veil of halo neutrons-forming exceptionally dilute neutron matter. There are speculations that halo structures may also occur for heavier nuclei. A particular interesting case is 19C because of its very loosely bound valence neutron. Recent experiments performed at the GSI fragment separator (FRS) have brought further insight into the structure of this nucleus. The results reveal an extended neutron density distribution, corroborating the assumption of a 18C-core surrounded by a one-neutron halo. More...



One-neutron removal cross section for 19C and 12C. Shown are the momentum distribution of 18C fragments after the breakup of 19C (green line) and of 11C fragments after the breakup of 12C (yellow line). The momentum distributions center around the projectile momentum indicated by pz=0. The much smaller width and much higher yield measured for the 18C fragments supports the assumption of a one-neutron halo for 19C. The dased line indicate the halfwidths of the two distributions, respectively.

Investigations of e+ and e+e- emission in central collisions of heavy ions near the Coulomb barrier performed at GSI by the groups ORANGE and EPOS during the eighties had turned out puzzling line structures in the energy spectra. No conventional explanations for these structures were found. For many years, they had been discussed in relation with new phenomena; in particular the spontaneous pair creation in transient overcritical electric fields and‹since 1986‹the decay of a so far unknown neutral particle. In an attempt to clarify the situation, a new generation of experiments with substantially improved spectrometers and significantly higher efficiencies was performed between 1993 and 1995. But the former line structures could not be reproduced. More...

The spectrometers ORANGE (top) and EPOS II (bottom) designed for the spectroscopy of positrons and electron-positron pairs in heavy ion collisions. Both set-ups use parallel-plate avalanche counters (PPAC´s) to detect the scattered heavy ions. The leptons have to be detected in a background of g radiation and d electrons, which dominates by many orders of magnitude. The spectrometers use the deflection of the charged leptons in a strong magnetic field to transport the particles out of the region of intense g radiation in close proximity to the target. In the ORANGE spectrometer this is achieved by means of two toroidal, in case of EPOS II through a solenoidal magnetic field.





Nuclear Waste Transmutation and Incineration

Accelerator-driven subcritical reactors may offer a path to solve the nuclear waste problem by transmutation of long-lived fission products into short-lived or stable isotopes, and by incineration of the transuranic isotopes via fission reaction. In principle, they would also allow incineration of the several thousands of tons of civil and weapon grade plutonium. GSI carries out basic research to assemble data on spallation and fission reactions that are relevant for the design of such hybrid reactor systems. More...



Accelerator-driven reactor for the incineration of long-lived radioactive isotopes. The accelerator produces an intense (50 mA) beam of relativistic (1 GeV) protons which generates spallation neutrons in a lead target. These neutrons control the neutron balance in a subcritical reactor, operated in the thorium-uranium-cycle. The system can be used to incinerate nuclear waste that is added to the thorium-uranium-fuel.

One of the central topics of the atomic physics program at GSI are precision tests of quantum-electrodynamics (QED) in strong electric and magnetic fields. A major part of the experiments performed in this direction has concentrated on the X-ray spectroscopy of hydrogen-like heavy ions like U91+ at the Experimental Storage Ring (ESR). Employing segmented Ge detectors, the QED contributions to the 1s groundstate energy of U91+ could be determined with an accuracy of about 12 eV. For a critical test of QED, however, an accuracy of the order of 1 eV has to be obtained. In order to reach this ambitious goal, a crystal spectrometer is presently being built up at the ESR which is tuned to X-ray energies in the region 50 to 100 keV. This corresponds to wavelengths between 25 and 12 picometers. The project is carried out in cooperation with the Universities of Jena and Siegen, and the Forschungszentrum Jülich. More...


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Schematic view of the spectrometer arrangement. The X-rays emerge from the interaction zone of the ion beam with the gas jet and are Bragg-reflected by a cylindrically curved Si-crystal on to position sensitive detectors. By measuring the wavelength difference to a known spectral line from a calibration source, the X-ray energy can be detected with high accuracy.