The research program of the atomic physics group at GSI takes advantage of the unique possibilities arising from the availability of intense beams of highly energetic and/or highly charged ions up to naked uranium U92+ and short-lived isotopes far off stability. Powerful accelerator devices such as the heavy-ion synchrotron and the experimental storage ring (ESR) open many new experimental possibilities. Besides conventional atomic physics methods like for instance, x-ray spectroscopy, laser spectroscopy and the investigation of atomic collision processes, mass measurements and half-life measurements of short-lived radioactive isotopes are performed in close collaboration with the nuclear physics division. Atomic physics activities can be roughly split into the following topics:
- Spectroscopy of highly charged ions
- Dynamics and reactions of highly charged ions
- Determination of Ground state properties of atomic nuclei with atomic physics methods
The workhorse of atomic physics at GSI is the ESR. In the ESR, highly-charged ions can be stored and investigated over a long period of time. Here, electron capture from an electron beam or during collisions with hydrogen atoms from a gas jet is investigated, to mention only two examples. Important information about the electronic structure of ions can be obtained from the measured reaction rates and from the observation of simultaneous photon emission. This enables precise tests of quantum electro dynamics, which is the fundamental theory of the interaction between charged particles. So far, QED has passed all tests in the range of low electric and magnetic fields and is the prototype of all theories that we use nowadays to describe the fundamental forces between the fundamental particles. QED tests in the regime of extremely strong fields, however, are still rare and by far not as accurate as in the low-field regime. Here, highly charged ions are perfect objects for studies since an electron in U91+ experiences electric fields about one million times stronger than in hydrogen. But very often the tools and techniques required for such precision experiments have to be developed first. For example, detectors for x-rays are being developed which will allow us to determine the polarisation of photons in the x-ray regime that are emitted in the collision process with hydrogen. The atomic physics group fosters close collaborations with external partners, which are often involved in these developments.
Presently a new experimental facility (HITRAP) is being built. Using HITRAP, the ions stored in the ESR can be - after deceleration in the ESR - completely stopped and stored in a Penning trap. Then they can be extracted as very slow and cold particles and be made available for further experiments. Another trapping facility ( SHIPTRAP) has recently started operation with the ultimate goal to measure the mass of superheavy elements.
The atomic physics division has close connections to national and international networks and collaborates with many universities and research centres worldwide. Moreover, several Helmholtz "Nachwuchsgruppen" (Young Investigator Groups) were established.
In the frame of the GSI expansion towards an international research facility (FAIR) – starting in 2007 – atomic physics will set up new experiments at the upcoming accelerator SIS100/300 as well as the super fragment separator and at the new experimental storage ring (NESR). The two large collaborations – SPARC and FLAIR – are mainly supported by the atomic physics division. The SPARC collaboration aims at the investigation of the physics of strong electromagnetic fields with highly charged ions, where FAIR offers unique possibilities. As an example, experiments are being discussed which take advantage of the extremely short and strong field pulses (sub-attoseconds), in order to investigate the correlated multi particle dynamics of atomic systems. Moreover, the investigation of quantum electrodynamical effects in strong fields will be realized with a precision not yet gained so far. FLAIR will provide a completely new field of activity: experiments with slow anti protons, available in a large amount and allowing novel experiments with anti matter. But also for the investigation of exotic nuclei new perspectives are opened: The super fragment separator offers the possibility to produce new isotopes of many elements for the first time, which than could be investigated with the help of laser spectroscopy (LaSpec), with mass spectrometry in particle traps (MATS) or in the new experimental storage ring (NESR, ILIMA). Additionally it will be possible to investigate reactions with relativistic radioactive beams (R3B) or exotic nuclides with light ions (EXL) at the new facility.