A recent experiment performed at the National Superconducting Cyclotron Laboratory (NSCL) provided important insight into one of the significant questions about the structure of the most neutron-rich nuclei. It has been known for several years that the largest number of neutrons that an oxygen nucleus can hold is 16 while the neighboring element fluorine can hold at least 22. This surprising difference is somewhat unexpected based on extrapolations of the nuclear structure observed for the stable oxygen and other light isotopes. In the new work, the experimenters showed that the excitation energy of the first excited state in 24O was surprisingly high, indicative of a large energy gap between nuclear orbitals or that 16 represents a so-called closed shell of neutrons. The new work combined with the well-known feature of the nuclear shell model that the binding energy of nucleons drops significantly for nucleons added to a nucleus with a completed filled shell (so-called magic or closed-shell nuclei) leads immediately to loss of stability for oxygen isotopes heavier than 24O.
A researcher is adjusting aconnection to the MONA detector that is used to detection high energy neutronsin coincidence with reaction products from rare-isotope beams at the NSCL.
The world-unique combination of a fast rare-isotope beam from the NSCL with the high-efficiency MONA detector for neutrons made this study possible. The experimenters produced the excited 24O nuclei by the fragmentation of 26F nuclei, themselves rather exotic, and detecting 23O product nuclei in coincidence with a neutron. Both of these oxygen nuclei are at the limit of stability and only exist in the ground state, even the slightest excitation energy causes each of them to fall apart. The group was able to extract the energy of the first excited state of 24O for the first time and obtained a value that was significantly higher than expected.
The importance of the present work was highlighted by a News & Views article in Nature and the quality of the work is evidenced by the garnering of the 2010 Dissertation Award in Nuclear Physics from the APS/DNP by the graduate student, C. Hoffman from Florida State Univ., who performed the experiment with the MONA group.