Radiological materials emit gamma photons, which gamma radiation detectors, also called radiation portal monitors (RPM), are good at detecting. Systems currently used in US ports (and steel mills) use several (usually 4) large PVT panels as scintillators and can be used on vehicles moving up to 16 km/h.
They provide very little information on energy of detected photons, and as a result, they were criticized for their inability to distinguish gammas originating from nuclear sources from gammas originating from a large variety of benign cargo types that naturally emit radioactivity, including bananas, cat litter, granite, porcelain, stoneware, etc. Those naturally occurring radioactive materials, called NORMs account for 99% of nuisance alarms. Some radiation, like in the case of large loads of bananas is due to potassium and its rarely occurring (0.0117%) radioactive isotope potassium-40, other is due to radium or uranium that occur naturally in earth and rock, and cargo types made out of them, like cat litter or porcelain.
Radiation originating from earth is also a major contributor to background radiation.
Another limitation of gamma radiation detectors is that gamma photons can be easily suppressed by high-density shields made from lead or steel, preventing detection of nuclear sources. Those types of shields do not stop fission neutrons produced by plutonium sources, however. As a result, radiation detectors usually combine gamma and neutron detectors, making shielding only effective for certain uranium sources.