Two research teams at Michigan State University’s National Superconducting Cyclotron Laboratory (NSCL) have reported fresh findings about neutron-rich nuclei. In separate experiments, groups measured a critical energy gap in oxygen nuclei and achieved their first-ever success using a new technique for finding isomers.
One important area of study with these nuclei focuses on the neutron drip line – the limit in the number of neutrons (N) that can bind to a given number of protons. For oxygen, that line was known to lie at 16 neutrons, and indeed indicated a new shell closure at N=16 in neutron-rich nuclei. However, theoretical calculations disagreed on the difference in binding energy between 24O, with a closed shell of 16 neutrons, and 25O, the first isotope beyond the drip line – in other words, the binding energy of the 17th neutron.
Calem Hoffman from Florida State University and colleagues have now pinpointed this quantity. The group used the NSCL’s coupled cyclotrons to accelerate a beam of 26F onto a fixed target, where they observed 25O for the first time. The 25O decays too quickly for direct detection, but the group was able instead to track its decay products: 24O and a single free neutron, measured with the Modular Neutron Array. The team then used the angles, energies and momenta of the decay products to calculate the mass of the 25O, which in turn allowed them to infer the difference in binding energy from 24O, and ultimately the N=16 shell gap, which they find to be 4.86(13) MeV (Hoffman et al. 2008).
The second experiment, conducted by NSCL’s Georg Bollen and colleagues, focused on nuclear isomers, in which neutrons are excited to a higher-energy arrangement for anywhere from fractions of a second to years. The team has discovered a previously unknown isomer of 65Fe, a nucleus that is intriguing for its proximity in terms of proton and neutron numbers to 68Ni, a particularly enigmatic isotope. 68Ni displays some characteristics of doubly magic nuclei, but nuclei with slightly fewer protons and neutrons than 68Ni reveal pronounced changes in structure – which generally is not the case for isotopes near others that are doubly magic. Researchers have little idea what is happening in this nuclear region, and so are keen to make more measurements.
These nuclei are a target for the Low Energy Beam and Ion Trap (LEBIT), which experimenters at NSCL use to collect high-speed products of cyclotron-spawned collisions. After firing a beam of germanium nuclei into a thin target, Bollen’s team captured the products in LEBIT and directed them into a Penning trap, allowing them to make very precise mass measurements of the particles caught. The team measured two distinct masses for 65Fe, indicating nuclei with different energy states – one the ground state and one a novel isomer at an excitation energy of 402(5) keV (Block et al. 2008) This is the first use of Penning trap mass spectrometry of this kind. Previous isomer studies have instead employed gamma-ray spectroscopy.
Further reading
M Block et al. 2008 Phys. Rev. Lett. 100 132501.
C R Hoffman et al. 2008 Phys. Rev. Lett. 100 152502.
