How can the knowledge and technological progress provided by nuclear physics best be used to benefit society?
A national priority being addressed by the nuclear science community relates to stockpile stewardship and nuclear safeguards. Here, specific nuclear reaction and structure data is crucial and high-resolution γ−ray spectroscopy is often used to both tag the final states and map the relevant nuclear decay paths. Neutron-induced reactions on unstable nuclei are not only important for understanding the synthesis of heavy elements in stellar environments, but also for applied-science topics in nuclear energy, nuclear forensics, and stockpile stewardship. For these topics, neutron-capture reactions play a prominent role and the resulting complex reaction networks are difficult to calculate using modern reaction theory.
For over 10 years [Esc12] there has been considerable effort spent in using surrogate reactions to determine neutron-induced reactions on short-lived nuclei. For nuclei close to stability a variety of light-ion induced reactions, such as (p,p’), (p,d), (p,t), and (α,α’), have been utilized in the actinide region and for selected lighter-mass nuclei such as Gd and Y/Zr. For (n,f) reactions the agreement with directly measured cross sections is within ~5-10%. For (n,γ) reactions the surrogate approach is more challenging due to angular momentum differences between the neutron-capture and surrogate reactions as well as the challenge of quantifying the γ-ray exit channel using the observed discrete transitions. The existing surrogate data is often limited by the γ-ray sensitivity – the high efficiency and excellent sensitivity of GRETA will have a significant impact.
For isotopes more than two nucleons removed from stability the use of the surrogate approach requires rare-isotope beams and inverse-kinematics measurements. This requires beams with energies of 8-12 MeV/u to populate the compound nucleus with excitation energies greater than the neutron separation energy. Experiments require efficient arrays of charged particle and γ-ray detectors to identify the nuclear reaction of interest. In conjunction, a recoil separator would enable the identification of the heavy recoils emitted at small angles with velocities and masses close to those of the beam. The fragment separator would need to analyze 12-MeV/u beam-like recoils with a mass resolution of ±1 for a given Z. The γ rays emitted by the projectile-like nuclei in flight will be significantly broadened by Doppler shifts. The Doppler broadened γ-ray energy can be corrected by using the excellent tracking capabilities of GRETA. High-efficiency particle-γ-ray coincidence spectroscopy will be needed to disentangle such complex spectra. With GRETA coupled to an efficient silicon-based charged-particle array, surrogate experiments on fission fragments such as 95Sr and other key nuclei in reaction networks can be performed for the first time. An example reaction network is illustrated in Figure 1.