The structure of the atomic nucleus at the extremes of neutron-proton asymmetry is presently the focus of my research interest. Short-lived, radioactive nuclei that are composed of many more neutrons than protons, for example, often reveal surprising properties. The shape, the excitation pattern as well as the energy and occupation of the nucleus’ quantum mechanical orbits by protons and neutrons may be significantly altered compared to expectations that are based on the well-known properties of stable nuclei.
My group performs scattering experiments to characterize the bulk effects of these changes by assessing the deformation of a nucleus and its excitation pattern. Nucleon knockout or pickup experiments track these exciting modifications of the nuclear structure on the level of the neutron and proton orbits that make up the nucleus on a microscopic level.
In the grazing collision of an exotic projectile beam with a light target one or two protons or neutrons can be removed in direct, so-called knockout reaction. The heavy residue of this reaction is identified and its kinematics measured with NSCL’s large-acceptance S800 spectrograph. Spectroscopy of de-excitation γ-rays performed with NSCL’s segmented germanium array SeGA around the target then tells us if the reaction led to an excited state. The energy of the detected γ-rays measures the energy difference between two nuclear states and its intensity tells us how likely the state was actually populated in such a knockout reaction.
In intermediate-energy Coulomb excitation, the exotic nuclei are scattered off a stable gold target and excited in the electromagnetic Coulomb field of the target nuclei. Excited energy levels decay back by the emission of γ radiation, which photon detectors surrounding the target register. The energy of the γ-ray reveals the energy of the excited state and its intensity relates to the probability of the excitation process. This probability increases with increased deformation of the nucleus and thus provides a method to characterize the nuclear shape.
The results from those experiments are often surprising and reveal that exciting changes take place in the structure of exotic nuclei compared to stable species. We work in close collaboration with nuclear structure theorists and reaction theorists. Our experimental input helps to unravel the driving forces behind the often spectacular modifications in nuclear structure and adds to the improvement of modern theories that are aimed to provide a model of the atomic nucleus with predictive power also in the exotic regime.
Nuclear Structure Towards N=40 60Ca: In-Beam γ-Ray Spectroscopy of 58,60Ti, A. Gade et al., Phys. Rev. Lett. 112, 112503 (2014)
Quadrupole collectivity beyond N=28: Intermediate-energy Coulomb excitation of 47,48Ar, R. Winkler et al., Phys. Rev. Lett. 108, 182501 (2012).
Intermediate-energy inverse-kinematics one-proton pickup reactions on neutron-deficient fp-shell nuclei. S. McDaniel et al., Phys. Rev. C 85,014610 (2012)
Collectivity at N=40 in neutron-rich 64Cr. A. Gade et al., Phys. Rev. C 81, 051304(R) (2010)
In-beam nuclear spectroscopy of bound states with fast exotic ion beams. A. Gade and T. Glasmacher, Prog. Part. Nucl. Phys. 60, 161 (2008)