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Michael Thoennessen Professor of Physics, Associate Director for Education Experimental Nuclear Physics   PhD, Physics, SUNY Stony Brook 1988 Joined NSCL in November 1990 Phone (517) 908-7323 Fax (517) 353-5967 Office W209   thoennessen at nsclmsuedu Professional homepage

My research begins where the nuclear chart ends. While normal neutron-rich nuclei decay by converting a neutron into a proton (β decay) on a time scale of milliseconds or longer, nuclei beyond the end of the nuclear chart, or neutron-unbound nuclei, contain so many neutrons that they decay by emitting one or two of the excess neutrons on a time scale of 10^-21s. I am part of the MoNA/Sweeper collaborations which specialize in the study of these neutron-unbound nuclei. The masses and lifetimes of these extremely short-lived nuclei cannot be measured with standard techniques. The availability of fast radioactive ion beams at NSCL gives us the opportunity to create neutron-unbound nuclei and study them by detecting their decay products. For example ²⁵O, the first neutron-unbound oxygen nucleus, was recently studied by our group. A primary beam of ⁴⁸Ca was accelerated to about 50% of the speed of light with the Coupled Cyclotron Facility and a secondary beam of ²⁶F was selected by the A1900 fragment separator. The ²⁶F interacted with a target where we were specifically interested in the one-proton stripping reaction which leads to ²⁵O. Instantaneously, ²⁵O then decays in the target into ²⁴O and a neutron. Due to the large incoming energy from both, ²⁴O and the neutron will leave the target essentially at beam velocity. Following the target are two devices which were specifically designed for these studies. The 4 Tesla superconducting "Sweeper" magnet deflects the charged decay fragment into a set of particle detectors that identify the ²⁴O fragments and measure their energies and angles. The Sweeper magnet was built at the National High Magnetic Field Laboratory at Florida State University in collaboration with NSCL.

The second device is MoNA, the Modular Neutron Array. It is a highly efficient large area neutron detector which measures the energy and angle of the emitted neutrons. It was constructed by a collaboration of primarily undergraduate institutions, and undergraduates continue to participate in the experiments and data analysis. From the energies and angles of the fragments and neutrons, it is possible to reconstruct the mass of the neutron-unbound nuclei. ²⁵O is only one example of the many neutron-unbound nuclei at the limit of nuclear existence, and our experiment was the first to identify this exotic nucleus. With the exception of the lightest elements (up to boron - and now ²⁵O), none of these nuclei have been explored. The combination of MoNA and the Sweeper with the fast radioactive beams is one of the few facilities in the world where these nuclei can be explored. In addition to the measurement of neutron-unbound nuclei, the setup is also ideal to search for more exotic decay modes for example di-neutron emission.