Activity estimation of shielded or hidden radionuclides in emergency conditions
H. Toivonen, M. Granström, G. Ågren, G. Jónsson, B. Møller, P. Roos, H. Ramebäck,
To perform a threat or risk estimation related to an unknown source, the following tasks need to be performed: detection of the source, identification of the nuclides involved, source localization and shield analysis around the source (attenuation). The present study focused on the shield analysis showing that the spectrum contains enough information to determine the attenuation of the photons in a material between the source and the detector.
The research brought together Nordic experts to use different gamma spectrometers in field conditions for improving readiness in a radiological or nuclear emergency. The field campaign was carried out in the FOI premises, Umeå, in August 2017 using HPGe, LaBr3 and NaI spectrometers. For the attenuation calculation, the spectra were analysed in two ways: step analysis underneath a peak for single line emitters and peak area ratio analysis for multi-line emitters.
Careful calibrations were performed with Cs-137, Co-60 and Eu-152 sources for different attenuating materials (Pb, Fe, water and concrete) at a distance of 5 m and 10 m. Excellent data sets were generated. The results showed that in all cases the step response was linear. The peak ratio method worked well too, but the uncertainty analysis is a challenge.
The environment, the source-detector distance in particular, seems to have an impact on the step ratio (scattering). Furthermore, the comparison of the step analysis between Cs-137 and Co-60 showed that the parameters of the model have an energy dependency. These issues require more detailed studies, simulations and experimental work, before the adaption of the method for routine field work.
The measurement campaign was a great success showing that the properties of an unknown source in an unknown location in an unknown shield can be revealed in the field conditions. The results pave the way for realistic activity calculations which are the basis of risk estimation and well-justified countermeasures in emergency conditions.