From Nuclei to Supernovae

Animation of Core-collapse Supernova SN1998s in the NGC 3877 galaxy (by Pedro Re).

What makes a star explode and eject its material into space to create planets like earth? What is the mass of the neutrino? And what forces govern the properties of nuclei? These are some of the questions our group is trying to address in a variety of experiments performed at FRIB and previously at NSCL. Although these questions seem rather different in nature, elements of the underlying physics can be studied by using a particular class of nuclear reactions: charge-exchange reactions.

In charge-exchange reactions, a proton in a target nucleus is exchanged for a neutron in the projectile nucleus, or vice-versa, thereby transferring charge between the target and the projectile. Although such reactions are governed by the strong nuclear force, they are closely connected to electron-capture and beta-decay, which are transitions governed by the weak nuclear force. These weak interactions play an important role in the evolution of stars just before they become supernovae and in physics related to neutrinos, such as (neutrinoless) double-beta decay. Charge-exchange reactions are also very useful for probing specific properties of nuclei, especially those related to spin and isospin and are, therefore, used to improve and test our fundamental understanding of nuclear structure.

Low-Energy Neutron Detector Array (LENDA) constructed for the purpose of (proton, neutron) experiments in inverse kinematics with unstable nuclei, such as 56Ni.

An important component of our research is focused on implementing techniques for performing charge-exchange reactions on unstable nuclei. Although such nuclei only exist for a short amount of time, they can play an important role in stellar environments where temperatures and densities are high. In addition, by performing charge-exchange experiments on unstable nuclei, one can probe properties of nuclei that are not (easily) accessible when studying stable nuclei. Nevertheless, we also perform experiments in which we study charge-exchange reactions on stable nuclei, especially because new tools and reactions have become available that provide avenues to learn new properties.

Performing charge-exchange studies in which unstable nuclei are probed requires that experiments are performed in inverse kinematics. We have developed a low energy neutron detector array (LENDA) for performing (proton, neutron) charge-exchange reactions in inverse kinematics. We also carry out experiments that use the (Lithium-7, Beryllium-7) reaction in inverse kinematics, which rely on measuring gamma rays in high-resolution gamma ray detectors, such as GRETINA. Most recently, we developed the (deuteron,Helium-2) reaction in inverse kinematics as an alternative probe by using an Active Target Time Projection Chamber. All these experiment use the S800 magnetic spectrograph.

NSCL now, FRIB in the future!

The Facility for Rare Isotope Beams (FRIB) is a new national user facility for nuclear science, funded by the Department of Energy Office of Science and operated by Michigan State University (MSU). FRIB will house a very powerful linear accelerator, whose beams are used to make unstable nuclei at unprecedented intensities, which will unlock a wide variety of exciting new opportunities, including for studies with charge-exchange reactions. The charge-exchange group is developing new techniques and associated equipment to perform novel experiments at FRIB.