The RS-400 system is a 4PMT based supercoincidence fully digital system specially designed for the difficult operating conditions of many Steel and Scrap processing plants. These advanced technology systems offer very high sensitivity performance with minimum false and nuisance alarms through advanced digital design and spectral analysis. This system uses large volume detectors for optimum vehicle coverage. Typically, long detectors suffer substantially from signal loss using conventional technology, as a result RSI has developed a 4PMT technology utilizing PMTs at both ends of the detector for greatly improved signal collection and spectral shape.