After one year of operation the heavy ion storage ring TSR at the Heidelberg Max-Planck Institut für Kernphysik has reached full performance. As designed 1000 turns are accumulated by a combination of multiturn and rf stacking. Due to phase space compression by an electron cooler the momentum spread of the beams is Δp/p = 10−5 −10−4 depending on the heating by intrabeam scattering. The cooled beam lifetime is pushed to the limits set by charge exchange processes as electron capture for bare nuclei and electron stripping for incompletely stripped ion beams. As the vacuum pressure is P ≤ 10−10 Torr at present, beam lifetimes range from τ = 36 h for 21 MeV protons to 20 s for 7 MeV Be+. Intensities of up to 18 mA (3 × 1010 particles) C6+ beam have been stacked by applying phase space cooling during injection. For these high intensities the splitting of the longitudinal Schottky noise signal showed irregular behaviour with respect to the expected Δƒ ∼ I12 scaling law.

An overview of atomic physics experiments at the heavy ion Test Storage Ring (TSR) is given. Highly charged ions up to fully stripped silicon have been stored at energies between 4 and 12 MeV/u. The enhancement of the beam intensity by stacking, the beam lifetime, and electron cooling of these ion beams are discussed. Radiative and state‐selective dielectronic recombination rates of hydrogen‐like oxygen ions with free electrons from the electron cooler were measured. Beam noise spectra are being investigated with regard to collective effects caused by the Coulomb interaction in the cold ion beams. Resonance fluorescence from stored single‐charged ions was observed using tunable narrow‐band lasers. First indications of laser cooling in a storage ring were seen.

The Heidelberg heavy ion test storage ring TSR started operation in May 1988. The lifetimes of the ion beams observed in the first experiments can be explained by interactions with the residual gas. Multiple Coulomb scattering, single Coulomb scattering, electron capture and electron stripping are the relevant processes. Electron cooling of ions as heavy as O8+ has been observed for the first time. With increasing particle number, the longitudinal Schottky noise spectrum becomes dominated by collective waves for cooled beams, allowing a determination of velocities of sound. After correcting for these coherent distortions fo the Schottky spectrum, the longitudinal beam temperature could be extracted. The observed longitudinal equilibrium beam temperatures increase strongly with the charge of the ions. For a cooled C6+ beam, temperatures a factor of 120 higher were measured compared to a proton beam with the same particle number. The shrinking of the beam diameter due to electron cooling was observed with detectors which measured the profile of charge-changed ions behind a bending magnet. A strong laser-induced fluorescence was detected when storing metastable 7Li+ ions in the ring. Via the Doppler effect a very accurate measurement of the ion velocity profile could be performed. First attempts to observe laser cooling failed, probably due to heating effects from intrabeam scattering and a coupling between longitudinal and transversal motion in the beam. Several experiments under preparation are outlined.

Commissioning of the Heidelberg Test Storage Ring (TSR) started in May 1988. The TSR is a low-energy cooler storage ring for heavy ions with energies up to 30 MeV/amu at a charge-to-mass ratio 1/A=0.5. Phase space cooling for coasting beams as well as for bunched beams is routinely done by electron cooling. As the ring is fed by a tandem linac combination, stored intensities of up to 1×1010 particles are obtained by combined stacking into transversal and longitudinal phase space (multiturn injection and RF stacking). This stacking method gives 800 times the number of stored ions compared to single-turn injection. Cooling oxygen and carbon beams resulted in a typical emittance of 0.3 π mm-mrad and a momentum spread of Δp/p=10-4. The equilibrium was mainly determined by intrabeam scattering, and the heating in the residual gas was mainly determined by multiple scattering. An overall increase of phase space density by six orders of magnitude was observed, similar to cooling results at proton machines. Results on the first year of operation with heavy ions at the TSR are reported

Angular and multiplicity distributions for the γ-decay of 164 Er compound nuclei have been measured with the Darmstadt-Heidelberg Crystal Ball. Combining this information with the fusion cross section of (500 ± 50)mb, a compound nuclear spin distribution has been derived which almost exhausts the unitarity limit for lower partial waves. Near the limiting angular momentum of (52 ± 2)ℏ the distribution of partial wave cross sections exhibits a steeper fall-off than predicted by barrier fluctuation and coupled channel calculations.

Angular momentum distributions for the dominant decay channels of $^{156}\mathrm{Er}$ compound nuclei have been studied with the Darmstadt-Heidelberg "crystal ball" detector in nearly mass-symmetric ($^{64}\mathrm{Ni}$+$^{92}\mathrm{Zr}$) and asymmetric ($^{12}\mathrm{C}$+$^{144}\mathrm{Sm}$) entrance channels. Strong differences in the $\ensuremath{\alpha}n$ yield and $\frac{{2}_{n}}{{3}_{n}}$ cross-section ratios are observed at the same excitation energy (47 MeV) and spin in $^{156}\mathrm{Er}$. This effect indicates that there is memory of the entrance channel during the particle-evaporation stage of the compound-nucleus decay. The dominant exit channels do not exhibit high-angular-momentum components.

The high energy component above 10 MeV in the γ decay following the 128Te + 34S compound nucleus reaction is studied by exploiting the new experimental possibilities provided by the NaI crystal ball spectrometer.

A modularized NaI detector with close to 4π geometry can provide unique information on the decay γ-radiation from highly excited nuclei. GSI Darmstadt, the Max-Planck-Institute (MPI) for Nuclear Physics and the University of Heidelberg have joined in a collaboration to realize such a detector. The detector has the shape of a spherical shell with a free inner radius of 25 cm and a thickness of 20 cm and comprises 162 individual modules of equal solid angle. This contribution explains our special choice of configuration and gives an outline of the present status of the mechanical and electronic assembly.

γ-transitions preceding the fission decay of the shape isomers in 238U and 240Pu have been measured with the crystal ball. Lines at 616 keV and 808 keV are observed in 238U and 240Pu, respectively, which are tentatively interpreted as deexcitations of the β-vibrational band in the second minimum.