Development of a forward-angle gamma-ray detector array for MoNA-LISA

In recent years invariant mass spectroscopy has been successfully applied to measure neutron-unbound states. In this method neutrons are measured in coincidence with~charged fragments following reactions with radioactive beams produced in projectile fragmentation reactions. When the final nucleus has bound excited states it is necessary to include gamma-ray detection in order to extract the excitation energy of the initial state. Because the MoNA-LISA setup at NSCL uses a large-gap Sweeper magnet to deflect the charged particles, conventional gamma-ray scintillation arrays cannot be used efficiently because of the large fringe field of the magnet. Thus we are developing a small cesium iodide (CsI) array using silicon photomultipliers (SiPMs) which are agnostic to the presence of a magnetic field. Using GEANT4 simulations the parameters of the array will be optimized to achieve the required efficiency and energy resolution of the Doppler-corrected energy spectra, necessary to extract the gamma-ray transitions in the final nucleus.