First generation RPMs often rely on PVT scintillators for gamma counting. They provide limited information on energy of detected photons, and as a result, they were criticized for their inability to distinguish gamma rays originating from nuclear sources from gamma rays originating from a large variety of benign cargo types that naturally emit radioactivity, including cat litter, granite, porcelain, stoneware, bananas etc. Those Naturally Occurring Radioactive Materials, called NORMs account for 99% of nuisance alarms.
It is worth noting that bananas have erroneously been reported as the source of radiation alarms; they are not. Most produce contains potassium-40, but packing density of fruits and vegetables is too low to produce a significant signal. PVT does have the ability to provide some energy discrimination, which can be exploited to limit nuisance alarms from NORM.