New isotopes push out the drip-line
Oct 25, 2007
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.
About the author
Michael Banks is news editor of Physics World