Analysis of Na(TI) Scintillator Using Spectroscopy

Neutron-capture cross sections on unstable nuclei are key in the creation of heavy elements in our universe. These elements are created through rapid neutron capture nucleosynthesis, called the r - process, believed to occur in neutron star mergers or core-collapse supernovae. The (d,p) reaction has recently been validated for determining (n,g) cross sections, using the Surrogate Reaction Method (SRM). Using arrays such as ORRUBA/GODDESS, the SRM can be applied with radioactive beams to determine (n,g) cross sections on unstable nuclei that can not be measured directly. In order to increase sensitivity for these measurements, high efficiency for gamma ray detection is desired. A large array of 2"x4"x16" NaI(Tl) detectors is being considered for this purpose. A few detectors for this array have been instrumented using the ORRUBA data acquisition system, and their performance evaluated using standard gamma ray sources. The individual Na(TI) detector response has been studied as a function of PMT voltage, gain, shaping time, and position of illumination by a collimated source. An overview of the conceptual array design, and details of the detector characterization, will be presented.