The energy resolution of scintillation counters like those is rubbish, and spectroscopic results obtained with them can be misleading as a result. We initially used NaI just to surround high resolution Ge detectors to reject Compton scattering events which deposit only part of the gamma energy in the Ge. However, the lumps of hyper-pure Ge you need to do high resolution spectroscopy are expensive and need to be cooled with liquid nitrogen.