Our body, the Earth, the Sun and everything we know in the Universe is composed of chemical elements: hydrogen, oxygen... One of the fundamental questions is to understand how the heaviest elements found on Earth, such as uranium, were formed. NECTAR (NuclEar reaCTions At storage Rings) is designed to provide the scientific community with data to answer this question.
Chemical elements are produced by nuclear reactions in stars. The heaviest elements are produced in nuclear reactions where a neutron is captured by a nucleus. When the target nucleus is heavy, it can break in to two lighter nuclei, i.e. fission, after capturing the neutron. These reactions also take place in a nuclear reactor, thus their importance also for industrial applications. Today, we know rather well the probability for neutron induced reactions on stable target nuclei, but these data are often not known for unstable target nuclei. The reason is the difficulty to produce samples containing the radioactive nuclei of interest. Neutron-induced reactions are also very difficult to calculate theoretically, mainly because we are not able to describe accurately how the nucleus de-excites, i.e. how it releases the internal energy acquired after the capture of a neutron. The excited nucleus may decay by the emission of energetic photons (known as gamma rays), neutron emission or fission. These three different de-excitation modes have different probabilities, which depend on fundamental properties of the nucleus such as level densities, particle and gamma transmission coefficients or fission barriers, which are very difficult to calculate.
NECTAR aims to circumvent these problems by using the surrogate reaction method in inverse kinematics. In standard measurements a beam of neutrons interacts with a heavy, radioactive nucleus at rest. In NECTAR, the kinematics of the nuclear reaction is inverted and the heavy, unstable nucleus is to be put in the beam to bombard a light nucleus. Since free neutron targets are not available, we use targets of light nuclei such as protons or deuterons (a nucleus made of a proton and a neutron). By appropriately choosing the projectile nucleus we can produce the nucleus that is formed in the neutron-induced reaction of interest. The probabilities for the different de-excitation 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 to inform much more accurate theoretical predictions for neutron-induced reactions. One of the figures shows the principle of the surrogate reaction method.
NECTAR experiments are to be performed with the heavy-ion storage rings of the GSI/FAIR facility (
https://www.gsi.de/en/(s’ouvre dans une nouvelle fenêtre)) in Germany, which offer the ideal conditions for investigating surrogate reactions. The storage rings at GSI/FAIR provide high-quality radioactive beams, which can be used in conjunction with ultra-thin gas-jet targets. This and the capability of storage rings to separate the beam-like residues produced after the nuclear reaction will enable for measurements of decay probabilities with unrivalled accuracies. However, the ultra-high vacuum (UHV) conditions inside the storage rings pose severe constraints to in-ring detection systems. NECTAR proposes a completely new solution to cope with this issue, which is to use solar cells.
Solar cells, the devices that are routinely used to convert sunlight into electricity, have significant advantages with respect to the traditionally-used silicon detectors. First, they are much more resistant to radiation damage, which represents a huge advantage in the UHV environment of the storage rings as it reduces significantly the need for breaking the UHV for detector troubleshooting during the beam period. Moreover, solar cells are much more cost effective.
The objective of NECTAR is to develop a set-up and a methodology to simultaneously measure fission, gamma- and neutron-emission probabilities induced by surrogate reactions in inverse kinematics at the storage rings of the GSI/FAIR facility.
