The synthesis of elements from iron to uranium that takes place in stars can only be understood through the study of neutron reactions on radioactive nuclei. Neutron cross sections of radioactive nuclei are also important for industrial purposes, e.g. for more sustainable and efficient energy production and in the search for novel therapeutic radionuclides for medical diagnostic and treatment.
Direct measurements of neutron capture or neutron-induced fission on unstable isotopes are very difficult for several reasons, one reason is the lack of radioactive targets available. The most promising approach to overcome the difficulties is to use surrogate reactions in regular or inverse kinematics. The surrogate reaction produces the compound nucleus of interest by a different reaction than the neutron-capture reaction (Figure 1). The different decay probabilities (gamma, neutron-emission and fission) of the compound nucleus are used to tune model parameters that will lead to much more accurate predictions of the desired neutron cross sections. In addition, performing reactions using the inverse kinematics technique will allow to access short-lived nuclei and detect all the decay channels (gamma, neutron-emission and fission) simultaneously. Moreover, the study of surrogate reactions in storage rings would provide data of exceptional quality data due to the very good emittance and purity of the stored beam. The setup proposed to measure all the decay channels is shown in Figure 2.
A heavy-ion storage ring is an ensemble of beam pipes and electro-magnetic devices arranged in a closed geometry where the heavy ions turn with high frequencies, about 1 MHz at 10 MeV/u. The storage of heavy ions needs minimizing atomic reactions between the stored beam and the residual gas inside the ring. Therefore, heavy-ion storage beams are operated at ultra-high vacuum (UHV) conditions (10^-11 to 10^-12 mbar). The presence of an electron color allows to keep the beam conditions each turn, even after passing a ultra-thin gas targets (10^13 atoms/cm^2). This type of target is of great advantage for surrogate reactions since the beam will only interact with the desired material and in a well-defined interaction zone. The outstanding beam quality, along with the very small straggling induced by the ultra-thin target enables a very precise measurement of the excitation energy of the decaying nucleus.
The NU-RING project proposed to set the basis to perform surrogate-reaction experiments in inverse kinematics using storage rings. We have focused our efforts on the prepare for experiments at the ESR and the CRYRING storage rings of GSI/FAIR. Nevertheless, we have proposed solutions useful and applicable to other existing and future storage rings.
The goals of Nu-Ring were four: a) definition of the set-up to be used for the surrogate measurements using simulations; b) study of the response of solar cells to heavy ions above 1 MeV/u, evaluating their suitability to work as heavy particle detectors; c) evaluate the use of solar cells in the UHV environment; d) produce high-quality benchmark data in direct kinematics by measuring the γ-emission probability of 208Pb induced by the 208Pb(d,d’) reaction.
After the 7 months of the project we have proven the feasibility of the study of the 238U(d,d') reaction at the CRYRING storage ring and we have also perform first tests on solar cells. These preliminary tests indicate solar cells are suitable to be used as heavy ion detectors at the storage ring.
