As an experimentalist, I study collisions of nuclei at energies at approximately half the speed of light. From the collisions of nuclei, we can create environments that resemble the first moments of the universe after the big bang. Properties of extra-terrestrial objects such as neutron stars can be obtained from studying collisions of a variety of nuclei with different compositions of protons and neutrons. One important research area of current interest is the density dependence of the symmetry energy, which governs the stability as well as other properties of neutron stars. Symmetry energy also determines the degree of stability in nuclei.
We have an active program to study the symmetry energy term in the nuclear equation of state. In a series of experiments at NSCL and in Catania, Italy, we measured the isotope yields from the collisions of different tin isotopes, 112Sn+112Sn (light tin systems), 124Sn+124Sn (heavy tin systems with more neutrons) as well as the crossed reactions of 124Sn+112Sn, and 112Sn+124Sn. We measure isospin diffusions, which is related to the symmetry energy as the degree of isospin transferred in violent encounters of the projectile and target depends on the symmetry energy potentials. In an experiment at MSU, we also measured the neutron to proton ratios that are directly related to the symmetry energy. In addition to experiments, we carry out Transport simulations of nuclear collisions at the super-computing center at Austin, Texas and MSU in our quest to understand the role of symmetry energy in nuclear collisions, nuclear structure and neutron stars. Through measurements and comparisons to the transport model simulations, we are able to obtain a constraint on the density dependence of the symmetry energy below normal nuclear matter density (which is the density of the nucleus you encounter everyday, 0.26fm-3 or 2.04x1017 kg/m3) as shown in the blue shaded region in the figure. The curves are various theoretical predictions showing the large uncertainties in theory regarding the properties of symmetry energy.
To explore the density region above normal nuclear matter density - marked with question marks in the figure - experiments are planned at NSCL, as well as RIKEN, Japan. Currently, our group is building a Time Projection Chamber that will detect charged particles as well as pions when the TPC is placed inside a dipole magnet. When completed, the TPC will be installed in the SAMURAI magnet in RIKEN, Japan and will allow us to study the symmetry energy at twice of the nuclear matter density.
By studying particles emitted in nuclear collisions, we also gain knowledge about the structure of nuclei. Single nucleon transfer reactions, when either a proton or neutron is transferred from the projectile to the target or vice versa, have been used successfully in the study of nuclear structure. This type of reaction is especially useful in understanding the single particle states in a nucleus, such as 56Ni, which is a double magic nucleus but is unstable. 56NI is a nucleus of astrophysical interest as it is the end product in many astrophysical reactions. Accurate descriptions of single particle states are of fundamental importance to check the validity of the nuclear shell models that predict properties of exotic nuclei relevant in our understanding of nucleosynthesis of elements when the early universe was formed. Our group has an active experimental program in transfer reactions using a state of the art high resolution detector array (HiRA). Experiments are being planned for using radioactive beams at NSCL.
Constraints on the Density Dependence of the Symmetry Energy, M.B. Tsang, Y. Zhang, P. Danielewicz, M. Famiano, Z. Li, W.G. Lynch, A.W. Steiner, Phys. Rev. Lett. 102, 122701 (2009)
Isospin Diffusion in Heavy Ion Reactions, M.B. Tsang et al., Phys. Rev. Lett. 92, 062701 (2004).
Neutron-Proton Asymmetry Dependence of Spectroscopic Factors in Ar Isotopes, Jenny Lee, M.B. Tsang, et al., Phys. Rev. Lett. 104, 112701 (2010)
Survey of Excited State Neutron Spectroscopic Factors for Z=8-28 Nuclei, M.B. Tsang, Jenny Lee, S.C. Su, J.. Dai, M. Hori, H. Liu, W.G. Lynch, S. Warren, Phys. Rev. Lett. 102, 062501 (2009)
Constraints on the symmetry energy and neutron skins from experiments and theory, M.B. Tsang et al., Phys. Rev. C 86, 015803 (2012)