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Sean Liddick Assistant 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 W104   liddick at nsclmsuedu

My research focuses on experimentally identifying changes in nuclear structure far from the valley of beta stability. The changes are the result of evolving single-particle level configurations as a progression is made from stability towards more exotic nuclei and leads to specific observables in the low-energy level structure of a nucleus. Decay spectroscopy provides a sensitive and selective means to populate and study low-energy excited states of daughter nuclei looking for the signatures of changing shell structure. A variety of different decay modes can be employed depending on the nucleus of interest and the experimental setup and can include beta, alpha, and proton decay. Experiments have been performed both on the neutron-deficient nuclei near ¹⁰⁰Sn and neutron-rich nuclei near ⁷⁸Ni.

Alpha decay experiments near ¹⁰⁰Sn, originally intended to look for fast alpha decays, have instead demonstrated an unexpected inversion in the ordering of the single-particle states along the Sn isotonic chain between ¹⁰¹Sn and ¹⁰³Sn in comparison to theoretical predictions. The inversion indicates a rapid transition between a nucleus which can be described by single-particle excitations (¹⁰¹Sn) and a nucleus which displays a large amount of collectivity (¹⁰³Sn). That such a discrepancy occurred demonstrated the danger of extrapolating nuclear structure information from nuclei closer to stability and the need for continued experimental investigation.


In the neutron-rich region near ⁷⁸Ni, in additional to exploring the evolution of nuclear structure, decay spectroscopy is also useful in trying to understand explosive astrophysical processes responsible for the synthesis of elements heavier than Fe. The pattern of nuclear abundances following an r-process is significantly influenced by beta-decay half-lives most of which are unknown and extracted from theoretical calculations. Further, the abundances of stable nuclei produced from the r-process is altered as the nuclei produced decay back to stability due to the possibility of beta-delayed neutron emission which shifts abundances to lower mass chains. The probability of beta-delayed neutron emission in neutron-rich nuclei can be large and investigations along the Cu isotonic chain have identified large discrepancies between current and previous experimental values for the neutron emission probabilities.

The development of new detectors and techniques is critical to improving the sensitivity of the experimental system enabling access to increasingly exotic nuclei. The implementation of a digital acquisition system in the detection of alpha decays was critical for the detection of the single-particle level inversion near ¹⁰⁰Sn. The digital electronics enabled the capture of two sequential alpha decays from ¹⁰⁹Xe and ¹⁰⁵Te. In addition to digital electronics, a new planar Ge detector is being developed for decay spectroscopy experiments and the two improvements combined offer both increased detection efficiencies and the ability for novel experiments.