Researchers at Michigan State University’s National Superconducting Cyclotron Laboratory (NSCL) have reported the first measurement of the half-life of nickel-78 (78Ni). With completely filled proton and neutron shells, 78Ni is doubly magic and also neutron-rich, and is an important element for understanding heavy-metal nucleosynthesis.
Doubly magic nuclei are of fundamental interest to nuclear physics, as their simplified structure makes it feasible for them to be modelled. In addition, neutron-rich nuclei play an important role in the astrophysical rapid neutron-capture process, or “r process”. The r process is responsible for the origin of about half of the elements heavier than iron in the universe, yet its exact mechanism is still unknown. 78Ni is the only doubly magic nucleus that provides an important “waiting point” in the path of the r process, where the reaction sequence halts to wait for the decay of the nucleus.
There are 10 doubly magic nuclei (excluding super-heavy ones), and only four of these are far from stability: 48Ni, 78Ni, 100Sn and 132Sn. Of these, (neutron-poor) 48Ni and (neutron-rich) 78Ni are the last ones with properties yet to be experimentally measured. Now the results from NSCL demonstrate that experiments with 78Ni are finally feasible.
In this experiment, a secondary beam comprised of a mix of several neutron-rich nuclei near 78Ni was produced by the fragmentation of a 86Kr34+ primary beam with and energy of 140 MeV per nucleon on a beryllium target at the NSCL Coupled Cyclotron Facility. A total of 11 78Ni events were identified over a total beam-time of 104 h. The half-life obtained, 110 + 100 – 60 ms, is lower than models predict. The measurement provides a first constraint for nuclear models and valuable experimental input to the understanding of the r process.
Further reading
P T Hosmer et al. 2005 Phys. Rev. Lett. 94 112501.
