The influence of shielding - measurements of nuclear material

If suspicious objects are found and suspected to be improvised nuclear devices (IND) or radioactive dispersal devices (RDD) radioactive or nuclear material may be involved. In this case it is necessary to gain information about the composition of those materials. Fast and reliable measurements have to be performed and the material has to be identified especially in the case of nuclear material. This will be done by analysis of high resolution gamma spectra. A number of portable high energy resolution detection systems using electrical cooling and implemented automatic analysis routines are nowadays available. The obtained spectra and therefore the analysis results are strongly influenced by the geometry of the measurement setup. In the aforementioned scenario the radioactive or nuclear material would be surrounded by other components of the IND or RDD like steel or explosives. If the radiation is intended to be reduced lead might be present as well. In previous campaigns measurements using various sources of nuclear material were performed and the results obtained by the automatic analysis routines were analyzed. In the present paper similar measurements are presented also investigating the quality of the outcome of automatic analysis routines. The main focus of the present measurements is the influence of different shielding on the measurement results. Measurements were obtained with four different detectors. Those are three out of the Detective series (Detective EX, Micro-Detective and Detective 200) from ORTEC and the Falcon 5000 from Canberra. The Detectives differ in the crystal size, electronic components, and the firmware version. Therefore the comparison of the three different Detectives is meaningful. For shielding we used 1.5 cm of stainless steel, a 2 mm lead sheet, and 5 cm of explosive simulate; both in combination and by itself. The measurements were performed using in total eight uranium and plutonium sources with different enrichment respectively isotopic composition. They took place at the Institute for Transuranium Elements (ITU) at the Joint Research Center (JRC) in Karlsruhe, Germany.