A radioactive material detection system includes a plurality of radioactive material detection apparatuses and a master unit/master module. Each apparatus is disposed in or on a cargo receptacle and each apparatus has a wireless transmitter, a radiation sensor and a detection controller. The master unit/master module has a receiver configured to receive the wirelessly transmitted information from each of the wireless transmitters and a master controller. The system detects fissile or nuclear material that emits radiation by (i) calculating and storing at the master unit/master module an initial average measured radiation level at each radioactive material detection apparatus location throughout the entire array of radioactive material detection apparatuses and (ii) comparing the current measured radiation at each radioactive material detection apparatus location to the initial radiation level at each location in order to identify an anomaly amongst the plurality of cargo receptacles.