Physicists have found evidence of a new "magic number" of neutrons in an unstable isotope of calcium. Using the Radioactive Ion Beam Factory at RIKEN in Japan, they have isolated calcium nuclei containing 34 neutrons – the first time 34 has been seen as a magic number. The discovery could improve our understanding of astrophysical processes, such as nucleosynthesis, that involve highly unstable and short-lived nuclei.

Magic nuclei are those with full "shells" of nucleons (protons or neutrons) and they tend to be stable against radioactive decay. A familiar example is the helium-4 nucleus, which has a magic number of both protons (two) and neutrons (two). It is therefore termed "doubly magic" and is extremely stable. Other magic numbers include 8, 20, 28, 50, 82 and 126.

However, there are significant exceptions to this shell model of the nucleus and its magic numbers. In particular, unstable nuclei that have a large imbalance in their numbers of protons and neutrons do not seem to conform. In neutron-rich silicon-42, for example, 28 neutrons (N = 28) is no longer a magic number, whereas N = 16 does appear to be magic in neutron-rich oxygen isotopes.

Predicted in 2001

Studies of neutron-rich calcium nuclei have already shown that N = 32 is a magic number – and theoretical calculations done in 2001 suggest that N = 34 should also be magic. Now, physicists working at RIKEN have found experimental evidence for this magic number at 34.

The experiment was carried out by a team led by David Steppenbeck at the University of Tokyo. The researchers began by firing a beam of scandium and titanium nuclei at a solid target to create large numbers of short-lived nuclei. These nuclei are often in highly excited energy states and, as they decay, emit gamma rays, which the team detected. In much the same way that an atom can be identified by the light it emits, the presence of a particular nucleus – and some information about its internal structure – can be gleaned from its gamma-ray spectrum.

Sub-shell closing

The experiment revealed that the first excited state of calcium-54 is at a relatively high energy and this is indicative of a "sub-shell" closing at 34 neutrons. Nuclear sub-shells are analogous to the familiar atomic sub-shells (s, p, d and so on); if the energy gap between sub-shells is large, then the closure of that shell corresponds to a magic number.

The discovery should provide physicists with important information about interactions between nucleons in highly unstable nuclei. While such nuclei are not encountered in daily life, they can play an important role in the process of nucleosynthesis whereby heavy elements, such as iron and nickel, are forged in violent astrophysical events such as supernovae.

"Enriching our knowledge of the structures of highly unstable nuclei and the nucleon–nucleon forces that drive nuclear-shell evolution and the appearance or disappearance of the nuclear magic numbers in radioactive nuclei plays an important role in understanding astrophysical processes such as nucleosynthesis in stars," explains Steppenbeck.

The research is described in Nature.