202 FRIB Graduate Brochure

Sean Liddick
Associate Professor of Chemistry, Associate Director
of Experimental Science
Keywords: Gamma-ray, Beta Decay, Machine Learning Nuclear Structure,
Nucleosynthesis
Nuclear Chemistry
About
• BS, Chemistry, Texas A&M University, 2001
• PhD, Chemical Physics, Michigan State University,
2004
• Joined the laboratory in December 2009
• liddick@frib.msu.edu
Research
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.
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
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)
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