Evidence for a new nuclear ‘magic number’ from the level structure of 54Ca

Abstract

Atomic nuclei are finite quantum systems composed of two distinct types of fermion--protons and neutrons. In a manner similar to that of electrons orbiting in an atom, protons and neutrons in a nucleus form shell structures. In the case of stable, naturally occurring nuclei, large energy gaps exist between shells that fill completely when the proton or neutron number is equal to 2, 8, 20, 28, 50, 82 or 126 (ref. 1). Away from stability, however, these so-called 'magic numbers' are known to evolve in systems with a large imbalance of protons and neutrons. Although some of the standard shell closures can disappear, new ones are known to appear. Studies aiming to identify and understand such behaviour are of major importance in the field of experimental and theoretical nuclear physics. Here we report a spectroscopic study of the neutron-rich nucleus (54)Ca (a bound system composed of 20 protons and 34 neutrons) using proton knockout reactions involving fast radioactive projectiles. The results highlight the doubly magic nature of (54)Ca and provide direct experimental evidence for the onset of a sizable subshell closure at neutron number 34 in isotopes far from stability.

Collectivity of neutron-rich Cr and Fe toward N=50

“…Although the absolute energy scale depends on the overall mass region, and may be influenced by other structural properties (e.g., effects of triaxiality or single magicity), the evolution of the E(2 + 1 ) over a series of isotopes is often the first indicator for the evolution of structures, and can test or guide nuclear theory. A recent example is the location of a new doubly-magic Ca isotope at N = 34 through measurement of its E(2 + 1 ) values [7] by γ-spectroscopy after proton-knockout at the RIKEN Radioactive Ion Beam Factory (RIBF).…”

Ab InitioCoupled-Cluster Effective Interactions for the Shell Model: Application to Neutron-Rich Oxygen and Carbon Isotopes

“…Shell structure influences the locations of the neutron and proton drip lines and the stability of matter. Examples of changes in shell structure are the appearance of new magic numbers N = 14 and N = 16 in the neutron-rich oxygen isotopes [1,2], and the emergence of an N = 34 sub-shell closure in 54 Ca [3][4][5][6].Phenomenological shell-model Hamiltonians such as the sd Hamiltonian of Brown and Wildenthal [7,8] (abbreviated USD) and the p-sd Hamiltonian of Warturburton and Brown [9] (abbreviated WBP), have successfully described properties of nuclei with proton number Z and neutron number N less than about 20. To understand the origin of shell structure, however, researchers are now trying to derive the shell model from realistic nucleonnucleon (NN) and three-nucleon forces (3NFs), without further phenomenology [3,10,11].…”

“…Shell structure influences the locations of the neutron and proton drip lines and the stability of matter. Examples of changes in shell structure are the appearance of new magic numbers N = 14 and N = 16 in the neutron-rich oxygen isotopes [1,2], and the emergence of an N = 34 sub-shell closure in 54 Ca [3][4][5][6].…”

Are There Signatures of Harmonic Oscillator Shells Far from Stability? First Spectroscopy of Zr110

“…The magic numbers for stable nuclei were first successfully described by invoking a one-body square-well and spin-orbit potential [4,5]; the latter was eventually replaced by a harmonic oscillator potential with l 2 term to obtain proper angular momentum splittings [6]. However, studies of radioactive nuclei over the past decades have shown that the magic numbers are not universal across the nuclear chart [7][8][9][10]. Despite intensive e↵ort, the theoretical description of these structural changes is not yet fully understood and the mechanisms that drive structural evolution di↵er between models.…”