Imaging large vessels using cosmic-ray muon energy-loss techniques

Imaging the internal structure of large vessels (2–20 m in diameter) is not possible with most traditional imaging methods. The sheer size renders gamma-ray and other high-energy photon, neutron, electrical and acoustic techniques useless, whilst the use of high-energy accelerators required to produce charged-particles of sufficient energy are impractical in most industrial situations. The use of naturally occurring high-energy (∼GeV) cosmic-ray mu-mesons (muons) provides an effective solution to the penetration problem. The problems of low intensity at near-horizontal angles with the cosmic-ray muon flux are addressed by using energy-loss imaging methods. In other methodologies, using charge-particle energy-loss imaging techniques, only a few events are needed compared to many thousands required if attenuation measurements were to be employed. The energies of horizontal cosmic-ray muons are distributed largely between 0.1 and 1000 GeV with a mean energy of about 50 GeV. Radiation Transport Monte-Carlo methods (GEANT4) have been used to calculate the energy loss for a selection of industrial materials in the energy range of interest. The energy loss of the muons along a ray-sum are modelled and compared to attenuation losses along the ray-sum using energy resolving detectors in coincidence before and after the sample. The energy-loss spectra across different samples are measured, demonstrating that embedded materials can be identified with as few as 10 muons passing through the sample. It is proposed that the imaging modality can be extended into a full tomographic modality allowing material identification within each voxel.