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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^sup 2,3^. 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 ^sup 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 ^sup 54^Ca and provide direct experimental evidence for the onset of a sizable subshell closure at neutron number 34 in isotopes far from stability.
The shell structure of the atomic nucleus was first successfully described more than 60 years ago1. However, the question of how robust the standard magic numbers are in unstable nuclei with a large excess of neutrons-often referred to as 'exotic' nuclei-has been one of the main driving forces behind recent nuclear structure studies that focus on changes in the shell structure, called 'shell evolution'. A noteworthy example is the disappearance of the N528 (neutron number 28) standard magic number in 42Si (ref. 4), a nucleus that lies far from the stable isotopes on the Segrè chart. On the contrary, exotic oxygen isotopes3 provide evidence for the onset of a new shell closure at N516, one that is not observed in stable nuclei. In both cases, the tensor force, a non-central component of the nuclear force, has a key role in describing the experimental spectra5.
The region of the Segrè chart around exotic calcium isotopes has also contributed valuable input to the understanding of nuclear shell evolution over recent years owing to experimental advances. Enhanced excitation energies of first JP521 states (spin,...