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.
Characterisation of the few doubly magic nuclei, known and predicted, provides a benchmark for our knowledge of the fundamental forces that drive the evolution of shell closures with proton-to-neutron asymme
Excited states in 38,40,42 Si nuclei have been studied via in-beam γ-ray spectroscopy with multinucleon removal reactions. Intense radioactive beams of 40 S and 44 S provided at the new facility of the RIKEN Radioactive Isotope Beam Factory enabled γ-γ coincidence measurements. A prominent γ line observed with an energy of 742 (8) 23.20.Lv, 27.40.+z, 29.38.Db Shell closures and collectivity are important properties that characterize the atomic nucleus. Interchange of their dominance along isotopic or isotonic chains has attracted much attention. The recent extension of the research frontier to nuclei far away from the valley of stability has revealed several new phenomena for neutronor proton-number dependent nuclear structure. For example, a weakening or even disappearance of shell closures occur in several neutron-rich nuclei at N = 8 [1][2][3] and N = 20 [4][5][6]. A well known example in the case of N = 20 is the so-called 'island of inversion ' [7] located around the neutron-rich nucleus 32 Mg. The low excitation energy of the first 2 + state E x (2 + 1 ) and large E2 transition probability [4][5][6] clearly indicate shell quenching in 32 Mg despite the fact that N = 20 is traditionally a magic number. The next magic number, N = 28, which appears due to the f 7/2 -f 5/2 spin-orbit splitting, has also been explored [8][9][10][11][12][13]. Weakening of the shell closure is seen by the decrease of the 2 With proton number Z = 14 and neutron number N = 28, the nuclear structure of 42 Si is of special interest. A simple but important question that arises is whether the weakening of the N = 28 shell closure continues, causing an enhancement of nuclear collectivity, or if shell stability is restored owing to a possible doubly magic structure. A study on 42 Si was made by a two-proton removal reaction experiment with radioactive 44 S beams at the NSCL [15]. The small two-proton removal cross sec-
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