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NKS Programme Area:NKS-B
Research Area:Emergency preparedness
Report Number:NKS-448
Report Title:Detection of orphan gamma radiation sources in shielded building geometries by mobile gamma spectrometry. A method of using Compton scattered radiation to estimate the shielding mass thickness of common building material
Activity Acronym:SHIELDMORC
Authors:Christopher L. Rääf, Robert R. Finck, Vikas C. Baranwal, Jonas Boson, Antanas Bukartas, Marie-Andrée Dumais, Kjartan Guðnason, Marjan Ilkov, Gísli Jónsson, Mattias Jönsson, Simon Karlsson, Marie Lundgaard Davidsdóttir, Frode Ofstad, Jan Steinar Rønning, Petri Smolander, Mikael Westin,
Abstract:If nuclear or radioactive materials are out of control (materials out of regulatory control, MORC), authorities must deal with the threatening situation. Gamma-ray sources out of control can have various degrees of shielding. When located in a building the building material shields the source. Information on the shielding of an identified source will provide data to estimate the radiation hazard and assist in securing the source. In a Nordic joint project, a method to assess shielding mass thickness of common building materials in cases with hidden Cs-137 point sources have been studied and experimentally investigated using mobile gamma spectrometry. The method uses detection of Compton scatted photons of different energies in relation to primary photons to obtain approximate information about the mass thickness of shielding material between the source and the detector. These ratios follow the expression of exponential functions of shield mass thickness, provided the shield is composed mainly of materials where Compton interaction dominates. The shielding thickness assessment method is based on previous detector response calibration measurements for well-known shielding geometries, thus determining function parameters. In the experiments, attempts to determine shield thickness from a measured pulse height distribution using various NaI(Tl) spectrometers and applying the exponential function produced values within ±50% of the true value. When applying the method to a HPGe spectrometer, the difference between measured and actual values was less than ±10%.
Keywords:Mobile gamma spectrometry, orphan sources, shielding
Publication date:04 Jun 2021
Number of downloads:798
Download:pdf 111010214697391.pdf
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