NSCL Directory Profile

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
Photograph of Sean Liddick

Selected Publications:
Low-energy structure of 2766Co39 and 2768Co41 populated through β decay, S.N.Liddick et al., Phys.Rev. C 85, 014328 (2012).

Algorithms for pulse shape analysis using silicon detectors, S.N. Liddick, I.G. Darby, R.K. Grzywacz, Nucl. Instrum. and Meth. Phys. Res. A 669, 70 (2012).

Shape coexistence along N=40, S.N.Liddick et al., hys.Rev. C 84, 061305 (2011).

Orbital Dependent Nucleonic Pairing in the Lightest Known Isotopes of Tin, I.G. Darby et al., Phys.Rev.Lett. 105, 162502 (2010).

Discovery of 109Xe and 105Te: Superallowed α Decay near Doubly Magic 100Sn, S.N. Liddick et al., Phys. Rev. Lett. 97, 082501 (2006).

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 68Ni.

In the neutron-rich region near 68Ni, decay spectroscopy is used to understand the rapid disappearance of the spectroscopic features that indicate a shell closure as protons are removed from 68Ni along the N = 40 and neighboring isotones. Long-lived excited nuclear states have been taken as evidence for the existence of competing nuclear shapes, prolate and spherical, in the low-energy level schemes of the odd-A 67Co nucleus and the odd-odd 66,68Co and 64,66Mn nuclei. Investigations into the even-even Fe nuclei have led to the tentative identification of excited 0+ states which, when compared to theoretical calculations, demonstrate the inversion between spherical and deformed configurations below 68Ni with approximately one proton and two neutrons excited across the Z = 28 and N = 40 shell closures, respectively.

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 that can be described by single-particle excitations (101Sn) and a nucleus that 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.

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 with an extraordinary low dead time was critical for the observation of isomeric states in 64,66Mn. The newest hardware development in the group has been the commissioning of a planar Ge detector for beta-decay spectroscopy experiments. The main goal of the detector is to increase the detection efficiency for beta-decay electrons.

Gamma rays detected within 5 ms of the arrival of either a 64Mn or 66Mn ion to the experimental station. Transitions attributed to either 64Mn or 66Mn are indicated by their energies and background transitions are marked with black circles. Coincidence spectra are shown as insets.