National Superconducting
Cyclotron Laboratory

Sean  Liddick
Sean Liddick
Associate Professor of Chemistry
Nuclear Chemistry
PhD, Chemical Physics, Michigan State University, 2004
Joined NSCL in December 2009
Phone (517) 908-7690
Fax (517) 353-5967
Office 1006
liddick at nscl.msu.edu

Sean Liddick

The ease of transitions between different states of the atomic nucleus carry a wealth of information and can be used in a variety of applications ranging from describing the basic configuration of the nucleus’ constituent protons and neutrons to constraining the synthesis of heavy elements in energetic astrophysical events. Nuclear properties are expected to vary significantly as a function of proton or neutron number as departure is made from stable nuclei. My group focuses on characterizing transition rates between nuclear states as a function of proton and neutron number and, from this information, inferring properties of the nucleus such as its shape or the cross section for neutron capture.

One focus of the group is on transitions between states with spin and parity of 0+. These transitions proceed are forbidden to occur through photon emission and instead occur through the emission of an electron leading to a characteristic signature in our detection system. The transition rate and excitation energy between the 0+ states can be related to the difference in mean square charge radius of the nucleus and the amount of mixing between the two states.

The other focus of the group lies in inferring the photon strength functions (related to the photon transition rates) of highly-excited states. The photon strength function combined with a knowledge of the number of nuclear states as a function of energy can be used to predict reaction cross sections such as neutron capture. Neutron capture rates are a necessary ingredient to predict elemental abundances produced in energy astrophysical events, such as supernovae and neutron star mergers, which are expected to lead to the synthesis of a significant amount of the elements heavier than iron. Abundance predictions require neutron capture rate uncertainties of roughly a factor of two while current constraints can reach over two orders of magnitude. The resulting impact on abundance predictions is shown in the figure.

Radioactive nuclei are produced, isolated, and delivered into an active detector and their subsequent decay radiations are monitored using charged particle and photon detectors. Decay spectroscopy provides a sensitive and selective means to populate and study low-energy excited states of daughter nuclei and a variety of different decay modes can be exploited. All detectors are instrumented using modern digital pulse processing systems and the group is pursuing advanced analysis techniques including the application of machine learning to nuclear science data.
• Compelling science program aligned with national priorities to develop a predictive model of the atomic nucleus and determine how heavy elements are made described in recent long range plans
• Ability to work with state-of-the-art instrumentation and looking forward to the completion of the FRIB decay station
• Development of advanced analysis techniques including the potential to apply machine learning
• Significant engagement with the national laboratories through the Nuclear Science and Security Consortium

sean-liddick.JPG

Predicted abundances as a function of mass number compared to solar r-process residuals (black dots). The shaded bands show the variances in a large number of predicted abundance patterns taken from network calculations.  In each calculation a variation of all neutron capture rates is applied. The shaded bands correspond to neutron capture rate uncertainties of a factor of 100 (light), 10 (middle), and 2 (dark). All but the largest abundance pattern features are obscured by the rate uncertainties at a factor of 100. Only with uncertainties smaller than a factor of 10 can fine features be observed.

Selected Publications

Novel techniques for constraining neutron-capture rates relevant for r-process heavy-element nucleosynthesis, A.C. Larsen, A. Spyrou, S.N. Liddick, M. Guttormsen, Prog. Part. Nucl, Phys, 107, 69 (2019)

Experimental neutron capture rate constraint far from stability, S.N. Liddick, et al., Phys. Rev. Lett 116, 242502 (2016). (Editor’s Suggestion)

Shape coexistence from lifetime and branching-ratio measurements in 68,70Ni, B.P. Crider, C.J. Prokop, S. N. Liddick, et al., Phys. Lett. B, 763, 108 (2016)

Shape coexistence in neutron-rich nuclei, A. Gade, S.N. Liddick, J. Phys (London) G43, 024001 (2016)