Michael R Thoennessen

University Distinguished Professor, Department of Physics & Astronomy
Location: 4208 Biomed Phys Sci
Profile photo of  Michael R Thoennessen
Photo of: Michael R Thoennessen


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 collaboration which specializes 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 first observed 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 inside the target into 24O and a neutron. Due to the large incoming velocity, 24O and the neutron will leave the target at very forward angles so that they are possible to be detected with high efficiency.

The detection is done with 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 the MoNA-LISA array which is a highly efficient large area neutron detector designed to measure the energy and angle of the emitted neutrons. MoNA and LISA were constructed by a collaboration of primarily undergraduate institutions, and undergraduates continue to participate in the experiments and data analyses. 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 we have recently expanded our experiments to study even more exotic nuclei which decay by the emission of two neutrons. The combination of MoNA-LISA 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 discovering more new unbound nuclides, we continuously develop new experimental capabilities, for example we are currently installing a liquid deuterium target for (d,p) reactions and are designing an active target to improve the overall resolution of the setup.

# Education:
* 1988: Ph.D., State University of New York, Stony Brook
* 1985: Diplom, University of Cologne, Germany

Selected Publications

  • Discovery of exotic nuclides 40Mg and 42Al suggests neutron dripline slant towards heavier isotopes, T. Baumann et al., Nature 449, 1022 (2007)
  • Evidence for a change in the nuclear mass surface with the discovery of the most neutron-rich nuclei with 17 ? Z ? 25, O.B. Tarasov et al., Phys. Rev. Lett. 102, 142501 (2009)
  • First Evidence for a Virtual 18B Ground State, A. Spyrou et al., Phys. Lett. B 683, 129 (2010)
  • Neutron Correlations in the decay of the first excited state of 11Li, JK Smith, et al., Nuc. Phys. A 955, pp. 27-40 (2016) View Publication
  • Observation of a low-lying neutron-unbound state in 19C, M. Thoennessen, et al., Nuc. Phys. A 912, pp. 1-6. (2013)
  • Observation of a two-neutron cascade from a resonance in 24O, C.R. Hoffman et al., Phys. Rev. C 83, 031303(R) (2011)
  • Reaching the Limits of Nuclear Stability, M. Thoennessen, Rep. Prog. Phys. 67, 1187 (2004)