Previously we developed methods to predict maximum detection distances for unshielded sources by using their primary radiation. Detection distances for different detector sizes, vehicle speeds, acquisition times etc were determined and the results verified in joint field experiments. Because lost radiation sources can be shielded in buildings, it can lead to lower detection probability, shorter detection distances and lower apparent activity than if the source were unshielded. In the NKS funded project SHIELDMORC 2019-2020 it was found that the ratio of Compton scattered photons in the pulse height distribution in relation to primary photons could give information on the shield thickness for common building materials. This was experimentally investigated in built-up shielding geometries, resulting in a “knowledge library” for various detector types and volumes. Whether the method derived from the "knowledge library" also works for the greater variation in geometry and distance in real buildings of various kinds remains still to be investigated. In the SHIELDMORC project it was shown that a pulse height distribution from a mobile gamma spectrometer contains more useful information than just the primary radiation. This can be used not only to determine shield thickness but also to increase the detection distance of shielded sources. An unshielded Cs-137 source produces in a 4 liter NaI (TL) crystal a ratio of about 5 to 1 of detected photons with lower energy (75-580 keV) compared to photons with primary energy (662 keV). A source behind a shield of 300 kg/m2 (13 cm concrete) produces a corresponding ratio of about 20 to 1, while the primary fluence is reduced to about 1/10. Hence, there is a gain in detection probability and thus in the detection distance as well, provided that also registrations of scattered photons are included in the analysis. This effect will be explored in the proposed COMBMORC activity. Using combined analysis of scattered and primary photons must be radionuclide specific. In COMBMORC primarily Cs-137 sources in realistic environments will be used. The aim of COMBMORC is to investigate whether methods of combined analysis of primary and scattered components registered by mobile gamma spectrometry can be used to: - increase detection distances of shielded sources compared to using primary photon assessments only - determine the distance to the source and the thickness of shielding when composed of common building material - determine the real activity of the shielded source. This should be done using the method of regions of interest and "knowledge libraries" from which data are extracted into relatively simple algorithms. Both theoretical calculations and experimental verifications are needed to develop the method. Lund University will carry out the theoretical calculations and prepare for the experimental verifications. Participants from the Nordic countries (radiation safety authorities) will take part in the experimental verifications in 2021 and 2022.
Medical Physics, Dept. of Translational Medicine, Lund University, Sweden