In-beam γ-ray spectroscopy towards the nucleon driplines

Abstract

Abstract. The contribution of in-beam γ-ray spectroscopy to nuclear structure research in recent years will be described with the example of the experimental efforts focused on neutronrich Cr and Fe nuclei around neutron number N = 40. Tremendous progress has been made at facilities around the world with complementary spectroscopic techniques, providing important benchmarks for developing shell-model effective interactions and elucidating the driving forces behind shell evolution in the exotic regime. IntroductionThe shell structure of the atomic nucleus is one of the cornerstones for a comprehensive understanding of this strongly correlated fermionic many-body quantum system. Large stabilizing energy gaps between groups of single-particle levels at certain, so-called "magic", fillings of the corresponding single-particle orbitals with protons and/or neutrons are part of the foundation of the nuclear shell model. Doubly-magic nuclei define the model spaces for large-scale calculations. The structure of stable nuclei is rather well described by the traditional nuclear shell model, while significant modifications have been encountered for short-lived, exotic species with unbalanced proton and neutron numbers: New shell gaps develop conventional shell closures vanish. Current theoretical and experimental efforts are focused on investigating how the shell structure changes far away from stability and how these changes relate to the isospin dependence of the N N force. One important experimental task is the quantification of changes in the nuclear structure from the measurement of experimental observables that are calculable and that ultimately allow to discriminate between different theoretical approaches.Major driving forces of shell evolution in neutron-rich nuclei that have been identified so far are the spin-isospin parts of the N N interaction [1], in particular the monopole part of the tensor force [2]. The roles of the central part and 3N forces have been emphasized recently as well [3,4]. Toward the driplines -in the regime of weak nucleon binding -the density dependence of the spin-orbit force and couplings to the continuum will become important [5]. Examples for the shell evolution in neutron-rich nuclei driven by the tensor force are the emergence of new shell gaps at N = 14, 16 and N = 32 in the neutron-rich sd and f p shell, respectively [1, 2].

Unexpectedly large charge radii of neutron-rich calcium isotopes

“…The calcium isotopic chain (Z = 20) is a unique nuclear system to study how protons and neutrons interact inside the atomic nucleus: two of its stable isotopes are magic in both their proton and neutron number ( 40 Ca and 48 Ca), while experimental evidence of doubly-magic features in two short-lived calcium isotopes have been reported recently, based on precision measurements of nuclear masses for 52 Ca (N = 32) 6 and 2 + excitation energies for 54 Ca (N = 34) 7 . Possibly, additional doubly-magic isotopes might exist even further away from stability 10 . As a local change in the behaviour of the charge radius is expected in doubly-magic nuclei 11 , it is important to pin down the charge radius in these exotic isotopes to understand how shell structure evolves and impacts the limits of stability.…”