In this work, as part of the EXL project nuclear reactions are investigated in inverse kinematics experiments with stored ion-beams. These experiments were carried out at the heavy ion storage ring ESR at GSI with stored 58Ni (at 100 and 150 MeV/u) and 20Ne (at 50 MeV/u) beams. The 58Ni beam was impinged on an internal gas-jet target of helium, while for the 20Ne experiment the internal gas-jet target utilized was hydrogen. The recoil particles produced in different reaction channels were measured with a dedicated detector setup compatible with the Ultra High Vacuum (UHV) in the storage ring. This detector setup included double-sided silicon-strip detectors operating as active barriers between the UHV of the storage ring and an auxiliary vacuum of internal pockets of the experimental chamber. This permitted successful measurements of low energy recoils (hundreds of keV), since both target and detectors were windowless.
Nuclear reaction channels like elastic scattering, excitation of isoscalar giant resonances and neutron pick-up were observed in these experiments. The angular distributions for elastic scattering were analyzed with optical potentials deduced from density-folding models. In particular, the elastic differential cross sections for 58Ni+ alpha were fitted by using the t-rho rho potential which is based on the optical limit approximation of the Glauber theory. The resulting RMS point matter-radii are 3.68(10) fm and 3.64(9) fm for the measurements at 100 and 150 MeV/u, respectively. These results are in very good agreement with the literature values.
The excitation of isoscalar giant resonances has been studied for the first time in a stored-beam experiment. In the double-differential cross section measured for 58Ni+ alpha at 100 MeV/u, a well-defined peak in the energy range from 15 to 30 MeV was obtained.
A fit with a Lorentz function for the Iso-Scalar Giant Monopole Resonance (ISGMR) component resulted in a centroid of 19.27(61) MeV and a width of 6.45(51) MeV. Moreover, a multipole decomposition analysis was performed to extract the ISGMR contribution. The extracted strength of the ISGMR exhausts 79^{+12}_{-11} \% of the Energy-Weighted Sum Rule (EWSR). The results are consistent with the analysis of other experiments performed in the past in normal kinematics as well as theoretical predictions.
A neutron pick-up reaction was measured in the experiment with the stored 20Ne beam. In this experiment, the contribution of the transfer to the ground state and to low-lying states of 19Ne were not kinematically separable. In order to disentangle the different components, a multipole decomposition analysis was applied to the experimental data. Spectroscopic factors were deduced from the analysis of the differential cross section for this transfer reaction. These results are in very good agreement with the predictions from shell model calculations.
The feasibility to perform different types of nuclear reactions with stored ion beams and an internal target by using in-ring detection is successfully demonstrated in this work. This is a paramount milestone toward further EXL experiments with radioactive beams at GSI and in future at FAIR
