Gamma-ray spectroscopy of neutron-rich fission fragments

Abstract : In a nuclear fission reaction, a large number of fission fragments are produced in various discrete quantum excited states. The characterization of these excited states along different isotopic chains with increasing mass numbers, for various elements, is necessary for understanding, microscopically, the evolution of the individual and collective motion of the nucleons (the intrinsic structure) in a complex nuclear many-body system as a function of its temperature (excitation energy), rotation (spin), and neutron-proton asymmetry (isospin). However, such a characterization for each set of isotopes is experimentally very challenging. The traditional approach uses correlations among the coincident characteristic gamma rays emitted by a given fission fragment, or correlations with the gamma rays emitted by its complementary partner (or partners), to identify the excited states of interest. In the experiments that will be discussed in this article, the sensitivity of the method of using prompt gamma rays for the spectroscopy of fission fragments is extended toward more neutron-rich exotic nuclei (nuclei with ratios of neutron number N to atomic number Z much larger than those found in nature), lying farther away from the valley of stability. This is done by measuring gamma rays in coincidence with the detection in a large-acceptance spectrometer of fission fragments that are isotopically identified by their atomic (Z) and mass (A) numbers.