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Time-of-Flight X-Ray Compton Scatter Imaging for Cargo Security: A Preliminary Study

22 March 2013


Nick Calvert

Cargo security is of increasing global concern due to the large volume of sea based freight that requires securing. Inefficient screening processes leads to delays in the delivery of goods leading to significant losses. Current screening methods rely on low energy X-Ray backscatter and high energy transmission imaging systems, restricted to providing two dimensional images. This thesis introduces the concept of Time-of-Flight (ToF) x-ray Compton scatter imaging (CSI), an imaging modality that utilises ToF information to acquire three dimensional scatter images. A mixture of simulation and experimental work was undertaken in order to assess the feasibility of such an imaging system by considering four key aspects required for a ToF CSI system. Monte Carlo simulations were performed to generate 4 MeV and 6 MeV spectra and to suggest optimal target design for a possible x-ray source. Optimal target design was found to be dependent on a number of factors relating to photon and electron yield, and photon energy. The problems of attenuation and multiple scatter were considered by performing Monte Carlo simulations of a cargo container and measuring the ToF of photons undergoing Compton scattering. ToF simulations suggested multiple scatter reached 30% for the geometry simulated and led to blurring in the ToF measured. Attenuation led to fewer counts from more distant objects, but penetration was calculated to be at least 20 cm in plastic. The timing response of a new silicon detector was experimentally compared to that of a traditional detector ,and was found to be up to 4 times slower and therefore not suitable for ToF imaging. This thesis shows that ToF CSI has potential, however future experimental work is required to verify the Monte Carlo simulations undertaken.