Surrogate Reactions at Heavy-Ion Storage Rings: The NECTAR Project

Introduction Neutron-induced reaction cross-sections of radioactive nuclei are essential for nuclear astrophysics and for applications in nuclear technology. However, these data are often subject to significant uncertainties or simply not available. The reason is the difficulty to produce samples containing the radioactive nuclei of interest. Neutron-induced reactions are also very difficult to describe theoretically, mainly because we are not able to predict accurately how the nucleus deexcites (i.e., how it releases the internal energy acquired after the capture of a neutron). The excited nucleus may decay by the emission of γ rays, the emission of a neutron, or by fission, if the excited nucleus is heavy enough. These three deexcitation modes compete with each other and have different probabilities. The latter probabilities depend on fundamental properties of the nucleus, such as nuclear-level densities, γ and particle transmission coefficients, or fission barriers, which are very difficult to calculate if experimental data are not available. Nuclear rEaCTions At storage Rings (NECTAR) aims to circumvent these problems by using the surrogate reaction method in inverse kinematics. In standard measurements in direct kinematics, a beam of neutrons interacts with a heavy, radioactive nucleus at rest. In NECTAR, the kinematics of the nuclear reaction are inverted and the heavy, unstable nucleus is put in the beam to bombard a light nucleus. Because free neutron targets are not available, we use targets of light nuclei such as protons or deuterons. By appropriately choosing the projectile nucleus we can produce the excited nucleus that is formed in the neutron-induced reaction of interest with inelastic-scattering or transfer reactions. The probabilities as a function of the nucleus excitation energy for the different deexcitation modes, which can be measured with the alternative or surrogate reaction, are particularly useful to constrain the models describing the fundamental nuclear properties mentioned above and eventually lead to much more accurate theoretical predictions for neutron-induced reactions [1]. Figure 1 shows the idea behind the surrogate-reaction method. The use of inverse kinematics makes it possible to study very unstable nuclei by using radioactive ion beams. It also makes possible the detection of the heavy products of the decay of the excited nucleus. This simplifies significantly the determination of the γ and neutron emission probabilities because the detection efficiencies for the heavy products can be much larger than the detection efficiencies for γ rays or neutrons. However, the deexcitation probabilities change very rapidly with excitation energy at the particle and at the fission thresholds. The excitation-energy resolution required to scan this rapid evolution is a few 100 keV, which is quite difficult to achieve for heavy nuclei in inverse kinematics due to long-standing target issues. Indeed, to infer the excitation energy we need to know accurately the energy of the beam and the target residue at the interaction point, as well as the angle between them. However, the required large target density and thickness lead to important energy loss and straggling effects that translate into a significant uncertainty in all these quantities. In addition, the presence of target windows and impurities induces a strong background.