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NKS Programme Area:NKS-B
Research Area:Emergency preparedness
Report Number:NKS-421
Report Title:Mobile search of material out of regulatory control (MORC) – Detection limits assessed by field experiments
Activity Acronym:MOMORC
Authors:R. Finck, T. Geber-Bergstrand, J. Jarneborn, G. Jónsson, M. Jönsson, S Juul Krogh, S. Karlsson, J. Nilsson, M. Persson, P. Reppenhagen Grim, C.L. Rääf, M. Sickel, P. Smolander, R. Watson, K. Östlund,
Abstract:Searching for lost nuclear or radioactive sources (Material Out of Regulatory Control, MORC) is a necessary capability for radiation protection response organizations. Searching along roads with mobile gamma spectrometers is a common method. In order for the search effort to be effective within a limited time, it is important to choose instruments and methods that will be sensitive enough to detect the radiation from a possible the source. The aim of the MOMORC-project was to increase the knowledge of these settings by (1) developing a theoretical model for calculating detection distances, (2) testing the results of the model through experimental measurements and (3) making the model and calculation results available to the Nordic participants in the project. Based on the experiments the theortical model predicted the maximum detection distances within 30 m. For a 1 GBq Cs-137 point source in a natural background of 0.08 µSv/h, the detection distance with vehicle speed 50 km/h and 1 s acquisition time intervals is about 80 m for a 3"x 3" NaI(T1)-spectrometer, about 105 m for a 123% HPGe-spectrometer and about 135 m for a 2x4 litre NaI(T1)-spectrometer. An important observation in the model calculations was that the maximum detection distances were depending on the acquisition time. Using 1 s acquisition time intervals at the speed of 50 km/h is only beneficial if the source activity is below 100 MBq and located near the road. When searching for higher activities (from unshielded radiation sources) it is advantageous to increase the acquisition time to 5 or 10 s for a speed of 50 km/h. Hence, selecting an optimal acquisition time interval based on the assumption of source activity is important.
Keywords:Mobile gamma spectrometry, orphan source search, detection distance, acquisition times, NaI(Tl), HPGe
Publication date:13 Mar 2019
ISBN:978-87-7893-510-6
Number of downloads:17
Download:pdf NKS-421.pdf
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