Detection and Identification of Radioactive Sources Covert Under Water

Conventional measuring equipment for radioactive material is predominantly constructed for stationary application in a laboratory. There are only a few systems for use in the field. In the context of a terrorist threat it might be of great importance to localize and identify radioactive or nuclear material underwater. Up to now only very few measurement systems are available which can be used in such environments. The complexity of the situation follows from the shielding effect of the water. Depending on the source strength a depth of the water of few meters is sufficient to make the detection of the radioactive material above the water surface impossible. Another consequence of the underwater situation is a change in the shape of the measured gamma spectra. Possibilities for measurements with radioactive sources placed in a pond or lake are limited. Therefore only a few experiments in real water environment have been carried out. A 60Co source with an activity of 50 MBq was placed in a pond. From a bridge above that pond the detector was lowered in the water and placed at different positions relative to the source. For this investigation we used a detector system based on a Sodium-Iodide (NaI) scintillator. Both operating modes PHA (Pulse Height Analysis) and MCS (Multi Channel Scaling) were used for obtaining gamma spectra or measuring the total count rate in search mode, respectively. The aim was a feasibility test for locating and identifying radioactive material hidden in the pond. It was shown that localization as well as identification was basically successful. Several improvements are planned because of this exceptional environment. This includes an appliance for pulling the detector fast and safe through the water, for example while crossing a lake with a boat. The paper presents first experiences with measurements carried out under water. This technical improvement may ease the task of inspecting special environments where objects containing radioactive or nuclear material are presumed and thus may help to combat nuclear terrorism.