Nuclear physicists have long been producing new isotopes by trying to cram as many neutrons as possible into them. But there is a limit to how many neutrons that can be squeezed into a nucleus with a certain number of protons. While this limit, or “drip-line”, is well known for smaller nuclei, it becomes less certain once you get to proton numbers of 8 (oxygen) and above. Now researchers in the US have discovered two neutron-rich isotopes of magnesium and aluminium that push the known drip-line out even further (Nature 449 1022).
The work gives the first firm experimental evidence for the discovery of the bound Mg isotope, magnesium-40, a so called even-even isotope containing 12 protons and 28 neutrons, and the odd-odd Al isotope aluminuim-42 containing 13 protons and 29 neutrons. The experiments were carried out by researchers at the National Superconducting Cyclotron Laboratory (NSCL) at Michigan State University by firing calcium-48 nuclei into a tungsten target. The resulting fragments could be identified by separating them first according to their momentum-to-charge ratio and then stopped in detectors which measured their energy.
Although magnesium-40 had been predicted to exist from the two best “global” theoretical models, the semi-classical finite range droplet model (FRDM) which uses a semi-classical description of the macroscopic contributions to the nuclear binding energy and the Hartree-Fock-Bogoliubov (HFB) model; a quantum mechanical model. However, both models do not predict the existence of a bound aluminuim-42 isotope and the mechanism of binding a aluminuim-42 isotope is currently not understood. “Seeing [aluminuim-42] is surprising, as usually this close to the drip-line binding occurs for an even number of neutrons”, says Michael Thoennessen, a Associate Director for Nuclear Science at NSCL.
The team also detected hints of an even heavier aluminium-43 isotope. Indeed, a more recent variation of the HFB model, named HFB-9, suggests that an aluminium-48 isotope could exist, with 13 protons and 35 neutrons. However, current experimental facilities may not be able to probe the limits of this modified drip-line towards heavy nuclei such as aluminuim-48, and heavier neutron-rich elements need to be used in the projectile fragmentation method instead of calcium-48, so that the nuclei that break off from the fragmentation contain more neutrons. “We need the next generation accelerators to determine the drip-line for aluminium and beyond to other elements like silicon and phosphorous,” says Thoennessen. Such a chance could come from the Radio Isotope Beam Factory (RIBF) at RIKEN in Japan, which just started initial operations earlier this year.
