First experience with a HPGe telescope detector

For detection and identification of nuclear material we equipped a transportable container with neutron and gamma measurement systems. The container itself provides the necessary infrastructure, e.g., energy supply, communication means, and work places. In case of a threat by nuclear terrorist there is urgent need for information on type and quantity of the radioactive respectively nuclear material inside the object. Only with this knowledge it is possible to estimate the potential risk and to take adequate protective measures. Therefore we improved the system by a new telescope detector. This detector consists of two different HPGe Germanium detectors which are placed one after the other in one detector end-cap. Both detectors are cooled by one small liquid nitrogen dewar. Seen from the window the first detector is a planar germanium detector with high resolution suitable to detect the energy lines of uranium and plutonium gamma rays in the 100 keV range. The signals of this detector are processed by a digital spectrum analyzer. The spectrum is then evaluated with the MGA/MGAU code to determine the uranium and/or plutonium isotope vector. The second detector is a coaxial detector with high efficiency for high energy gammas. Although the efficiency of this detector is considerably reduced by the presence of the planar detector in front, this detector is well suited to detect gamma radiation in the Cs/Co energy range. The signals of the detector are processed by another digital spectrum analyzer and evaluated by standard nuclide identification software. The main advantage of this system in the field of nuclear terrorism is that one mobile device is used for two measurements: uranium and plutonium identification and the identification of industrial and medical isotopes. This is especially of great use if the system has to be transported to the suspect object with a robot. The paper presents first experiences with the device. This technical improvement may ease the task of identifying suspect objects containing radioactive or nuclear material and thus may help to combat nuclear terrorism. Keywords: in-situ measurements, germanium detector, uranium/plutonium analysis