Evolution of Shell Structure Far From Stability
The shell structure of the atomic nucleus near stability is well established. The average nuclear potential is well parameterized and phenomenological frameworks describe many experimental facts. As one moves away from stability, however, decreasing nucleon binding energy and the large proton-to-neutron asymmetry lead to modifications in the nuclear potential and the spin-isospin component of the nucleon-nucleon interaction drives changes to the single-particle energies. Together with the increased role of many-body correlations, these changes to the single-particle structure lead to the disappearance of established shell closures and the appearance of new ones. Such evolution has been experimentally verified already at current facilities with present-day detectors, for example the disappearance of the N=20 shell gap leading to the island-of-inversion near 32Mg [War90], or the new shell gaps at N=32 and N=34 in the Ca isotopes [Huc85, Gad06, Ste13].
Beyond these changes to shell structure, which can be at least partially captured within phenomenological models, theoretical descriptions of the atomic nucleus are pushing towards more microscopic and even ab-initio approaches. Already in light systems, such as the C and O isotopes where such calculations have been tractable for several years, microscopic calculations have shown the need to include forces beyond two-body interactions, with an accurate description of basic nuclear properties such as binding energies or masses requiring inclusion of three-nucleon (3N) interactions [Ham13]. Such descriptions are now available up to the Ca isotopic chain [Hol12]. Future experimental work in nuclear structure will have the critical task of not only tracking modifications to single-particle structure but also providing the spectroscopic data required to validate the predictions of such microscopic calculations to fully understand and quantify the role of higher-order interactions. Not only is such exploration of structural evolution critical for the development of a predictive model of atomic nuclei, but the driving forces and many-body correlations at play can have a profound impact on the number of bound isotopes that exist for a given element.
Sensitive in-beam measurements and the extension of scientific reach to the most exotic nuclei will depend directly on the resolving power of the γ-ray spectrometer used. GRETA will have superior resolving power for fast-beam experiments compared to any other γ-ray detector. The resolution and spectral quality of a tracking detector has been shown with the outstanding performance of GRETINA at NSCL; the completion of GRETINA to GRETA at FRIB will provide unparalleled sensitivity to answer the most pressing questions in nuclear structure physics in terms of the evolution of single-particle structure and nuclear forces with far reaching consequences for the limits of existence on-the nuclear chart.
Rare isotopes not quite as far toward the driplines will be available for the first time at intensities that allow for unprecedented detailed spectroscopic studies. This will include, for example, spin determination from γ-ray angular distributions, a powerful and model-independent approach that is applicable for a variety of reactions and in different energy regimes. GRETA’s 4π angular coverage paired with its position resolution will significantly extend the reach of this technique. The sensitivity of GRETA as a polarimeter will provide complementary information on the multipolarity of observed transitions. The precise knowledge of spin values is critical in tracking the evolution of nuclear structure across the nuclear chart.
A unique observable that quantifies the interplay of collective and single-particle degrees of freedom is the gyromagnetic ratio or g-factor. Different approaches to determine g-factors in exotic nuclei have been used for rare-isotope beams: HVTF (High-Velocity Transient Field) [Stu06] and RIV (Recoil in Vacuum) [Stu13], both involve the precise measurement of angular distributions, which will benefit tremendously from the angular coverage and position sensitivity of GRETA. Other important observables are excited-state lifetimes. As demonstrated by precision measurements with GRETINA at NSCL [Iwa14], the mm-scale position resolution and high detection efficiency greatly extends the reach of the approach, which relies on the clean separation of 2 or more peaks in a γ-ray spectrum.
The combination of pioneering in-beam γ-ray spectroscopy with fast beams from FRIB, and the detailed spectroscopy possible closer to stability with GRETA at FRIB and ATLAS/ANL will be important steps towards realizing a predictive model of the atomic nucleus, valid also in the exotic regime.