Integration of ground–penetrating radar and gamma–ray detectors for non–intrusive localisation of buried radioactive sources
Lancaster University, Doctor of Philosophy, 2019
Supervisors: K. Gamage, M. Aspinall, D. Offin and C.J. Taylor
This thesis reports on the integration of ground-penetrating radar (GPR) and gamma-ray detectors to improve the non-intrusive localisation of radioactive wastes buried in porous materials such as soil and concrete.
The research was undertaken in two phases. In the first phase, a new non-intrusive technique for retrieving the depth of a buried radioactive source from two-dimensional raster radiation images was developed. The images were obtained by moving a gamma-ray detector in discrete steps on the surface of the material volume in which the source is buried and measuring the gamma spectrum at each step. The depth of the source was then estimated by fitting the intensity values from the measured spectra to an approximate three-dimensional gamma-ray attenuation model. This procedure was first optimised using Monte Carlo simulations and then validated using experiments. The results showed that this method is able to estimate the depth of a 658 kBq caesium-137 point source buried up to 18 cm in each of sand, soil and gravel.
However, the use of only gamma-ray data to estimate the depth of the sources requires foreknowledge of the density of the embedding material. This is usually difficult without having recourse to intrusive density estimation methods or historical density values. Therefore, the second phase of the research employed integrated GPR and gamma ray detection to solve this density requirement problem. Firstly, four density models were investigated using a suite of materials and the best model was then used to develop the integration method. Results from numerical simulations showed that the developed integration method can simultaneously retrieve the soil density and the depth and radius of disk-shaped radioactive objects buried up to 20 cm in soil of varying conditions with a relative error of less than 10%. Therefore, the integration method eliminates the need for prior knowledge of the density of the embedding material.
This work represents the first time data from these two systems, i.e. GPR and gamma-ray detector, will be integrated for the detection and localisation of radioactive sources. Furthermore, the results from the developed methods confirm that an integrated GPR and gamma-ray detector system is a viable tool for non-intrusive localisation of buried radioactive sources. This will enable improved characterisation of buried radioactive wastes encountered during the decommissioning of nuclear sites and facilities.