Researchers from the National Superconducting Cyclotron Laboratory (NSCL) on the campus of Michigan State University have developed an important new method to study charge-exchange reactions at intermediate energies using rare isotopes. Charge-exchange nuclear reactions are those in which a proton in one nucleus is exchanged for a neutron in the other nucleus during a fast collision. This nuclear reaction is particularly simple and had been used as an important tool in nuclear structure studies of stable nuclei. In particular the reaction provides a unique insight into the weak force responsible for radioactive decay by allowing measurements of weak transition strengths (so called Gamow-Teller transitions) that cannot be measured otherwise and the extraction of the transition strengths from the data is model-independent. The reaction can provide a wealth of information about the spin-isospin response of nuclei that provides critical input to diverse topics such as late stellar evolution and neutrino nucleosynthesis, as well as in the design of neutrino detectors and (neutrinoless) double beta-decay studies. In spite of the strong motivation to extend this work to unstable nuclei, precise identification of the reaction with rare isotope beams has proven to be a major challenge. The new technique is the first to allow the measurement of the Gamow-Teller strength in a rare isotope with a charge-exchange reaction where a proton in the nucleus of interest is exchanged for a neutron.
A researcher stands near the target position of the S800 magnetic spectrograph at the NSCL. The S800 is a large device used to measure the momentum of the rare isotope products at intermediate energies with high resolution that was combined with a photon detector to establish a new technique to study charge exchange reactions.
The key to the new technique is the combination of the fast rare-isotope beams available at the NSCL with the detection of the final state of a particularly simple nuclear target. The rare-isotope beam to be studied strikes a thin 7Li target and some fraction of the time exchanges one of its protons with a neutron originally in the Lithium nucleus turning the target into a 7Be nucleus. This nucleus can only survive in the ground state or one excited state that decays by photon emission. The detection of the decay of the 7Be excited state provides a clear signal of the relatively rare charge-exchange reaction and discriminates it from other reactions that take place. The reaction partner from the rare isotope beam is also measured and the data is used to deduce the Gamow-Teller strength function of the exotic nuclei.
The first demonstration of this technique used a beam of short-lived 34P nuclei produced at the NSCL that, after the charge-exchange reaction with the 7Li target, became 34Si. The product, 34Si, is an important nucleus that neighbors the so-called island of inversion where the exotic nuclei have been found to exhibit an unexpected reordering of the normal shell-model particle levels. Understanding the evolution of nuclear structure starting from the well-known region is crucial to understand the reasons for the shell-inversion. The Gamow-Teller strengths extracted from the new experiment provided novel data to test theoretical models that attempt to describe this structural evolution.