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To prevent the misuse of nuclear or radioactive material for malicious acts sensitive and reliable detection systems are mandatory. In addition to fixed monitors installed at border crossings, at the gateways of nuclear and industrial facilities or at the entrance of a military camp mobile handheld systems are needed to detect the dangerous material on-site as early as possible with high reliability excluding false alarms caused by natural radioactivity (like in potash fertilizer) or by patients which have undergone a nuclear medical therapy respectively examination. We investigated highly sensitive passive detection systems for gamma and neutron radiation. In a static situation an overview of the situation is gained by scanning of buildings or objects with a gamma camera. Here gamma intensity is converted into colors and then superimposed on the optical picture in order to get a visual overview. Terrorist attacks using nuclear material may have far reaching consequences compared to an attack with pure Gamma-sources. Nuclear material can be detected by neutron emissions from spontaneous fission as well as by the neutrons produced by (alpha, n) reactions in surrounding material. Therefore we operate a sensitive neutron detection system, fitted into a suitcase. With a special analysis procedure it is possible to categorize the substance in fissionable and non-fissionable material. In addition, the system can discover the existence of shielding material located around the source material. The presence of shielding material gives important information for the evaluation of the possible risk and appropriate protection measures.

In the context of the possible threat of proliferation of nuclear material by groups of terrorist and non-state actors reliable detection techniques of such material are of great importance at points of interest such as airports, harbors, railway stations, and border crossing points. The opportunity to detect nuclear material is given at border stations in particular because vehicles and pedestrians potentially carrying such material are forced to drive or walk slowly in lines, thus providing well-defined measuring conditions. Gamma and neutron detectors with a high efficiency are required for this task. As consortium leader of the current TACIS (Technical Assistance to the Commonwealth of Independent States) project "Ukrainian Border Security" of the European Union our institute is involved in setting up modern stationary portal monitor systems at selected Ukrainian border stations in order to provide adequate detection techniques for the prevention of illicit trafficking of nuclear material at these borders. The current status of the project will be presented. Furthermore, Fraunhofer-INT operates a mobile portal monitor system manufactured by Thermo based upon our institute's requirements and specifications. It comprises two pillars containing large-volume NaI crystals which can be used separately or combined, then forming a portal of variable width. Besides providing the option to localize radioactive or nuclear material, the system also allows for the discrimination between naturally occurring radiation and artificial material by means of the implemented NBR method. The measured count rate data are transferred to a PC by radio transmission. A series of measurements was performed with this portal monitor system to test its ability to detect both radioactive and nuclear material. An overview of the measurement results are presented as an example of the capabilities and performance of current portal monitor systems.