Muon tomography represents a new type of imaging technique that can be used in detecting high-Z materials (such as shielded HEU). As muons pass through materials, they continuously lose energy via ionisation and can stop within a material. Upon stopping secondary processes can occur, which result in the production of excess neutrons and gamma-rays. Due to their high energies most muons can pass through large depths of different materials. These muons will also undergo multiple scattering. Previous radiographic methods that looks to differentiate high-Z materials from low-Z clutter tend to be based on multiple scattering of muons only.
Presented is the development of two new types of analysis algorithm, which makes use of the different interactions cosmic-ray muons undergo when passing through a material. The first algorithm developed makes use of the multiple Coulomb scattering that a muon will undergo. This involves using a novel density based clustering approach that ascertains regions of high-Z material placed within an inspected volume. The capability of this new type of algorithm has been tested under realistic scenarios. These scenarios involve placing shielded HEU in cargo containers filled with a variety of different clutter. The algorithm has been shown to be efficient in detecting shielded HEU amongst low-Z clutter, but struggles upon the introduction of blocks of aluminium, or materials with densities higher than this.
Whereas previous radiographic methods were based on multiple scattering of muons only, the second developed algorithm uses muon absorption on nuclei to enhance the detection capabilities of the scattering technique. In particular, the goal is to improve on the distinction between high-density materials and low-density clutter. Muons will more readily lose energy in higher density materials. Therefore multiple muon disappearances within a localised volume may signal the presence of high-density materials. Muons that disappear have their track evaluated using a 3D line extrapolation algorithm, which is in turn used to construct a 3D tomographic image of the inspected volume. The ability to differentiate between materials using the 3D line extrapolation algorithm is established.
The technique of muon disappearance has been applied to identifying shielded HEU in realistic scenarios. Despite being capable of identifying shielded HEU in otherwise empty cargo containers, multiple additional regions (due to the lorry and container itself) were misclassified as possible threat materials. This makes accurate detection systems that solely use this technique for shielded HEU unlikely. However, since the build-up of nuclear materials was apparent, we have demonstrated how this technique can be used as a supplementary technique to help enhance muon scattering tomography capabilities. This is done through first identifying `areas of interest' using muon scattering tomography before confirming whether they are threats or other materials with the muon disappearance algorithm. It is concluded that while muon disappearance can slightly enhance the capabilities of muon scattering tomography, muon scattering still remains the most efficient method for detection of nuclear materials.
