William G. Lynch (Bill)

Research Programs

Nuclear matter is the material from which nuclei and neutron stars are composed. In most nuclei, the density of neutrons is somewhat larger than the density of protons. In the interiors of neutron stars, there are regions where the neutrons comprise more than 90% if the matter and protons, electrons and muons comprise the remainder. The symmetry energy governs how the energy and pressure of such matter differs from symmetric matter composed of equal numbers of neutrons and protons.

One main research effort of our group is focused on experimentally constraining the symmetry energy of nuclear matter as a function of density. With the SpRIT TPC, we have compared pion production in ¹³²Sn+¹²⁴Sn collisions to that for ¹⁰⁸Sn+¹¹²Sn collisions at an incident energy of E/A=270 MeV and have constrained the symmetry energy at about 1.5 r₀ where r₀ = 0.16 nucleon/fm³ is the saturation density of nuclear matter. Within the next few years, we will extend these measurement to a higher incident energy of 335 MeV, which will probe the symmetry energy at higher densities. This additional measurement is part of a broader program including constraints on the isovector effective nucleonic mass, which is needed to constrain uncertainties the theoretical description of these collisions. We have combined this new measurement with existing constraints at sub-saturation density to determine the density dependence of the symmetry energy at densities ranging from 0.25r₀ to 1.5r_0.

We have also begun a novel program to measure light ion induced fission of rare isotope nuclei. These measurements involve collisions of rare isotope beams in the region of ¹⁹²Hg with a helium target nuclei in the counter gas of the Active Target Time Projection Chamber (ATTPC). This forms compound nuclei in the region of ¹⁹⁶Pb, which has recently been shown to decay by asymmetric fission decay branches. This new technique show allow the exploration of a wide range of fissionable isotopes off the valley of stability.